3007 lines
156 KiB
Plaintext
Executable File
3007 lines
156 KiB
Plaintext
Executable File
USING THE IJG JPEG LIBRARY
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Copyright (C) 1994-1998, Thomas G. Lane.
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This file is part of the Independent JPEG Group's software.
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For conditions of distribution and use, see the accompanying README file.
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This file describes how to use the IJG JPEG library within an application
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program. Read it if you want to write a program that uses the library.
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The file example.c provides heavily commented skeleton code for calling the
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JPEG library. Also see jpeglib.h (the include file to be used by application
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programs) for full details about data structures and function parameter lists.
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The library source code, of course, is the ultimate reference.
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Note that there have been *major* changes from the application interface
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presented by IJG version 4 and earlier versions. The old design had several
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inherent limitations, and it had accumulated a lot of cruft as we added
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features while trying to minimize application-interface changes. We have
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sacrificed backward compatibility in the version 5 rewrite, but we think the
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improvements justify this.
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TABLE OF CONTENTS
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-----------------
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Overview:
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Functions provided by the library
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Outline of typical usage
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Basic library usage:
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Data formats
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Compression details
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Decompression details
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Mechanics of usage: include files, linking, etc
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Advanced features:
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Compression parameter selection
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Decompression parameter selection
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Special color spaces
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Error handling
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Compressed data handling (source and destination managers)
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I/O suspension
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Progressive JPEG support
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Buffered-image mode
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Abbreviated datastreams and multiple images
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Special markers
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Raw (downsampled) image data
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Really raw data: DCT coefficients
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Progress monitoring
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Memory management
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Memory usage
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Library compile-time options
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Portability considerations
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Notes for MS-DOS implementors
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You should read at least the overview and basic usage sections before trying
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to program with the library. The sections on advanced features can be read
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if and when you need them.
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OVERVIEW
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========
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Functions provided by the library
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---------------------------------
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The IJG JPEG library provides C code to read and write JPEG-compressed image
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files. The surrounding application program receives or supplies image data a
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scanline at a time, using a straightforward uncompressed image format. All
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details of color conversion and other preprocessing/postprocessing can be
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handled by the library.
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The library includes a substantial amount of code that is not covered by the
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JPEG standard but is necessary for typical applications of JPEG. These
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functions preprocess the image before JPEG compression or postprocess it after
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decompression. They include colorspace conversion, downsampling/upsampling,
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and color quantization. The application indirectly selects use of this code
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by specifying the format in which it wishes to supply or receive image data.
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For example, if colormapped output is requested, then the decompression
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library automatically invokes color quantization.
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A wide range of quality vs. speed tradeoffs are possible in JPEG processing,
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and even more so in decompression postprocessing. The decompression library
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provides multiple implementations that cover most of the useful tradeoffs,
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ranging from very-high-quality down to fast-preview operation. On the
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compression side we have generally not provided low-quality choices, since
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compression is normally less time-critical. It should be understood that the
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low-quality modes may not meet the JPEG standard's accuracy requirements;
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nonetheless, they are useful for viewers.
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A word about functions *not* provided by the library. We handle a subset of
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the ISO JPEG standard; most baseline, extended-sequential, and progressive
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JPEG processes are supported. (Our subset includes all features now in common
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use.) Unsupported ISO options include:
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* Hierarchical storage
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* Lossless JPEG
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* Arithmetic entropy coding (unsupported for legal reasons)
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* DNL marker
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* Nonintegral subsampling ratios
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We support both 8- and 12-bit data precision, but this is a compile-time
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choice rather than a run-time choice; hence it is difficult to use both
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precisions in a single application.
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By itself, the library handles only interchange JPEG datastreams --- in
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particular the widely used JFIF file format. The library can be used by
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surrounding code to process interchange or abbreviated JPEG datastreams that
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are embedded in more complex file formats. (For example, this library is
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used by the free LIBTIFF library to support JPEG compression in TIFF.)
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Outline of typical usage
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------------------------
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The rough outline of a JPEG compression operation is:
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Allocate and initialize a JPEG compression object
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Specify the destination for the compressed data (eg, a file)
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Set parameters for compression, including image size & colorspace
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jpeg_start_compress(...);
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while (scan lines remain to be written)
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jpeg_write_scanlines(...);
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jpeg_finish_compress(...);
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Release the JPEG compression object
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A JPEG compression object holds parameters and working state for the JPEG
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library. We make creation/destruction of the object separate from starting
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or finishing compression of an image; the same object can be re-used for a
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series of image compression operations. This makes it easy to re-use the
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same parameter settings for a sequence of images. Re-use of a JPEG object
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also has important implications for processing abbreviated JPEG datastreams,
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as discussed later.
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The image data to be compressed is supplied to jpeg_write_scanlines() from
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in-memory buffers. If the application is doing file-to-file compression,
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reading image data from the source file is the application's responsibility.
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The library emits compressed data by calling a "data destination manager",
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which typically will write the data into a file; but the application can
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provide its own destination manager to do something else.
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Similarly, the rough outline of a JPEG decompression operation is:
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Allocate and initialize a JPEG decompression object
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Specify the source of the compressed data (eg, a file)
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Call jpeg_read_header() to obtain image info
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Set parameters for decompression
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jpeg_start_decompress(...);
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while (scan lines remain to be read)
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jpeg_read_scanlines(...);
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jpeg_finish_decompress(...);
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Release the JPEG decompression object
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This is comparable to the compression outline except that reading the
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datastream header is a separate step. This is helpful because information
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about the image's size, colorspace, etc is available when the application
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selects decompression parameters. For example, the application can choose an
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output scaling ratio that will fit the image into the available screen size.
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The decompression library obtains compressed data by calling a data source
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manager, which typically will read the data from a file; but other behaviors
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can be obtained with a custom source manager. Decompressed data is delivered
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into in-memory buffers passed to jpeg_read_scanlines().
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It is possible to abort an incomplete compression or decompression operation
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by calling jpeg_abort(); or, if you do not need to retain the JPEG object,
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simply release it by calling jpeg_destroy().
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JPEG compression and decompression objects are two separate struct types.
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However, they share some common fields, and certain routines such as
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jpeg_destroy() can work on either type of object.
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The JPEG library has no static variables: all state is in the compression
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or decompression object. Therefore it is possible to process multiple
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compression and decompression operations concurrently, using multiple JPEG
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objects.
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Both compression and decompression can be done in an incremental memory-to-
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memory fashion, if suitable source/destination managers are used. See the
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section on "I/O suspension" for more details.
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BASIC LIBRARY USAGE
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===================
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Data formats
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------------
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Before diving into procedural details, it is helpful to understand the
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image data format that the JPEG library expects or returns.
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The standard input image format is a rectangular array of pixels, with each
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pixel having the same number of "component" or "sample" values (color
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channels). You must specify how many components there are and the colorspace
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interpretation of the components. Most applications will use RGB data
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(three components per pixel) or grayscale data (one component per pixel).
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PLEASE NOTE THAT RGB DATA IS THREE SAMPLES PER PIXEL, GRAYSCALE ONLY ONE.
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A remarkable number of people manage to miss this, only to find that their
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programs don't work with grayscale JPEG files.
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There is no provision for colormapped input. JPEG files are always full-color
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or full grayscale (or sometimes another colorspace such as CMYK). You can
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feed in a colormapped image by expanding it to full-color format. However
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JPEG often doesn't work very well with source data that has been colormapped,
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because of dithering noise. This is discussed in more detail in the JPEG FAQ
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and the other references mentioned in the README file.
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Pixels are stored by scanlines, with each scanline running from left to
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right. The component values for each pixel are adjacent in the row; for
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example, R,G,B,R,G,B,R,G,B,... for 24-bit RGB color. Each scanline is an
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array of data type JSAMPLE --- which is typically "unsigned char", unless
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you've changed jmorecfg.h. (You can also change the RGB pixel layout, say
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to B,G,R order, by modifying jmorecfg.h. But see the restrictions listed in
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that file before doing so.)
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A 2-D array of pixels is formed by making a list of pointers to the starts of
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scanlines; so the scanlines need not be physically adjacent in memory. Even
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if you process just one scanline at a time, you must make a one-element
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pointer array to conform to this structure. Pointers to JSAMPLE rows are of
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type JSAMPROW, and the pointer to the pointer array is of type JSAMPARRAY.
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The library accepts or supplies one or more complete scanlines per call.
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It is not possible to process part of a row at a time. Scanlines are always
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processed top-to-bottom. You can process an entire image in one call if you
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have it all in memory, but usually it's simplest to process one scanline at
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a time.
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For best results, source data values should have the precision specified by
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BITS_IN_JSAMPLE (normally 8 bits). For instance, if you choose to compress
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data that's only 6 bits/channel, you should left-justify each value in a
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byte before passing it to the compressor. If you need to compress data
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that has more than 8 bits/channel, compile with BITS_IN_JSAMPLE = 12.
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(See "Library compile-time options", later.)
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The data format returned by the decompressor is the same in all details,
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except that colormapped output is supported. (Again, a JPEG file is never
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colormapped. But you can ask the decompressor to perform on-the-fly color
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quantization to deliver colormapped output.) If you request colormapped
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output then the returned data array contains a single JSAMPLE per pixel;
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its value is an index into a color map. The color map is represented as
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a 2-D JSAMPARRAY in which each row holds the values of one color component,
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that is, colormap[i][j] is the value of the i'th color component for pixel
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value (map index) j. Note that since the colormap indexes are stored in
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JSAMPLEs, the maximum number of colors is limited by the size of JSAMPLE
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(ie, at most 256 colors for an 8-bit JPEG library).
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Compression details
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-------------------
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Here we revisit the JPEG compression outline given in the overview.
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1. Allocate and initialize a JPEG compression object.
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A JPEG compression object is a "struct jpeg_compress_struct". (It also has
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a bunch of subsidiary structures which are allocated via malloc(), but the
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application doesn't control those directly.) This struct can be just a local
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variable in the calling routine, if a single routine is going to execute the
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whole JPEG compression sequence. Otherwise it can be static or allocated
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from malloc().
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You will also need a structure representing a JPEG error handler. The part
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of this that the library cares about is a "struct jpeg_error_mgr". If you
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are providing your own error handler, you'll typically want to embed the
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jpeg_error_mgr struct in a larger structure; this is discussed later under
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"Error handling". For now we'll assume you are just using the default error
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handler. The default error handler will print JPEG error/warning messages
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on stderr, and it will call exit() if a fatal error occurs.
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You must initialize the error handler structure, store a pointer to it into
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the JPEG object's "err" field, and then call jpeg_create_compress() to
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initialize the rest of the JPEG object.
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Typical code for this step, if you are using the default error handler, is
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struct jpeg_compress_struct cinfo;
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struct jpeg_error_mgr jerr;
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...
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cinfo.err = jpeg_std_error(&jerr);
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jpeg_create_compress(&cinfo);
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jpeg_create_compress allocates a small amount of memory, so it could fail
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if you are out of memory. In that case it will exit via the error handler;
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that's why the error handler must be initialized first.
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2. Specify the destination for the compressed data (eg, a file).
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As previously mentioned, the JPEG library delivers compressed data to a
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"data destination" module. The library includes one data destination
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module which knows how to write to a stdio stream. You can use your own
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destination module if you want to do something else, as discussed later.
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If you use the standard destination module, you must open the target stdio
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stream beforehand. Typical code for this step looks like:
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FILE * outfile;
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...
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if ((outfile = fopen(filename, "wb")) == NULL) {
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fprintf(stderr, "can't open %s\n", filename);
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exit(1);
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}
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jpeg_stdio_dest(&cinfo, outfile);
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where the last line invokes the standard destination module.
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WARNING: it is critical that the binary compressed data be delivered to the
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output file unchanged. On non-Unix systems the stdio library may perform
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newline translation or otherwise corrupt binary data. To suppress this
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behavior, you may need to use a "b" option to fopen (as shown above), or use
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setmode() or another routine to put the stdio stream in binary mode. See
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cjpeg.c and djpeg.c for code that has been found to work on many systems.
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You can select the data destination after setting other parameters (step 3),
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if that's more convenient. You may not change the destination between
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calling jpeg_start_compress() and jpeg_finish_compress().
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3. Set parameters for compression, including image size & colorspace.
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You must supply information about the source image by setting the following
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fields in the JPEG object (cinfo structure):
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image_width Width of image, in pixels
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image_height Height of image, in pixels
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input_components Number of color channels (samples per pixel)
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in_color_space Color space of source image
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The image dimensions are, hopefully, obvious. JPEG supports image dimensions
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of 1 to 64K pixels in either direction. The input color space is typically
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RGB or grayscale, and input_components is 3 or 1 accordingly. (See "Special
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color spaces", later, for more info.) The in_color_space field must be
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assigned one of the J_COLOR_SPACE enum constants, typically JCS_RGB or
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JCS_GRAYSCALE.
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JPEG has a large number of compression parameters that determine how the
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image is encoded. Most applications don't need or want to know about all
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these parameters. You can set all the parameters to reasonable defaults by
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calling jpeg_set_defaults(); then, if there are particular values you want
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to change, you can do so after that. The "Compression parameter selection"
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section tells about all the parameters.
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You must set in_color_space correctly before calling jpeg_set_defaults(),
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because the defaults depend on the source image colorspace. However the
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other three source image parameters need not be valid until you call
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jpeg_start_compress(). There's no harm in calling jpeg_set_defaults() more
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than once, if that happens to be convenient.
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Typical code for a 24-bit RGB source image is
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cinfo.image_width = Width; /* image width and height, in pixels */
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cinfo.image_height = Height;
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cinfo.input_components = 3; /* # of color components per pixel */
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cinfo.in_color_space = JCS_RGB; /* colorspace of input image */
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jpeg_set_defaults(&cinfo);
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/* Make optional parameter settings here */
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4. jpeg_start_compress(...);
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After you have established the data destination and set all the necessary
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source image info and other parameters, call jpeg_start_compress() to begin
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a compression cycle. This will initialize internal state, allocate working
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storage, and emit the first few bytes of the JPEG datastream header.
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Typical code:
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jpeg_start_compress(&cinfo, TRUE);
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The "TRUE" parameter ensures that a complete JPEG interchange datastream
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will be written. This is appropriate in most cases. If you think you might
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want to use an abbreviated datastream, read the section on abbreviated
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datastreams, below.
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Once you have called jpeg_start_compress(), you may not alter any JPEG
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parameters or other fields of the JPEG object until you have completed
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the compression cycle.
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5. while (scan lines remain to be written)
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jpeg_write_scanlines(...);
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Now write all the required image data by calling jpeg_write_scanlines()
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one or more times. You can pass one or more scanlines in each call, up
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to the total image height. In most applications it is convenient to pass
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just one or a few scanlines at a time. The expected format for the passed
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data is discussed under "Data formats", above.
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Image data should be written in top-to-bottom scanline order. The JPEG spec
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contains some weasel wording about how top and bottom are application-defined
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terms (a curious interpretation of the English language...) but if you want
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your files to be compatible with everyone else's, you WILL use top-to-bottom
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order. If the source data must be read in bottom-to-top order, you can use
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the JPEG library's virtual array mechanism to invert the data efficiently.
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Examples of this can be found in the sample application cjpeg.
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The library maintains a count of the number of scanlines written so far
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in the next_scanline field of the JPEG object. Usually you can just use
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this variable as the loop counter, so that the loop test looks like
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"while (cinfo.next_scanline < cinfo.image_height)".
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Code for this step depends heavily on the way that you store the source data.
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example.c shows the following code for the case of a full-size 2-D source
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array containing 3-byte RGB pixels:
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JSAMPROW row_pointer[1]; /* pointer to a single row */
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int row_stride; /* physical row width in buffer */
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row_stride = image_width * 3; /* JSAMPLEs per row in image_buffer */
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while (cinfo.next_scanline < cinfo.image_height) {
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row_pointer[0] = & image_buffer[cinfo.next_scanline * row_stride];
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jpeg_write_scanlines(&cinfo, row_pointer, 1);
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}
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jpeg_write_scanlines() returns the number of scanlines actually written.
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This will normally be equal to the number passed in, so you can usually
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ignore the return value. It is different in just two cases:
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* If you try to write more scanlines than the declared image height,
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the additional scanlines are ignored.
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* If you use a suspending data destination manager, output buffer overrun
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will cause the compressor to return before accepting all the passed lines.
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This feature is discussed under "I/O suspension", below. The normal
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stdio destination manager will NOT cause this to happen.
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In any case, the return value is the same as the change in the value of
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next_scanline.
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6. jpeg_finish_compress(...);
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After all the image data has been written, call jpeg_finish_compress() to
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complete the compression cycle. This step is ESSENTIAL to ensure that the
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last bufferload of data is written to the data destination.
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jpeg_finish_compress() also releases working memory associated with the JPEG
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object.
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Typical code:
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jpeg_finish_compress(&cinfo);
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If using the stdio destination manager, don't forget to close the output
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stdio stream (if necessary) afterwards.
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If you have requested a multi-pass operating mode, such as Huffman code
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optimization, jpeg_finish_compress() will perform the additional passes using
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data buffered by the first pass. In this case jpeg_finish_compress() may take
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quite a while to complete. With the default compression parameters, this will
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not happen.
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It is an error to call jpeg_finish_compress() before writing the necessary
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total number of scanlines. If you wish to abort compression, call
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jpeg_abort() as discussed below.
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After completing a compression cycle, you may dispose of the JPEG object
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as discussed next, or you may use it to compress another image. In that case
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return to step 2, 3, or 4 as appropriate. If you do not change the
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destination manager, the new datastream will be written to the same target.
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If you do not change any JPEG parameters, the new datastream will be written
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with the same parameters as before. Note that you can change the input image
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dimensions freely between cycles, but if you change the input colorspace, you
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should call jpeg_set_defaults() to adjust for the new colorspace; and then
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you'll need to repeat all of step 3.
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7. Release the JPEG compression object.
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When you are done with a JPEG compression object, destroy it by calling
|
|
jpeg_destroy_compress(). This will free all subsidiary memory (regardless of
|
|
the previous state of the object). Or you can call jpeg_destroy(), which
|
|
works for either compression or decompression objects --- this may be more
|
|
convenient if you are sharing code between compression and decompression
|
|
cases. (Actually, these routines are equivalent except for the declared type
|
|
of the passed pointer. To avoid gripes from ANSI C compilers, jpeg_destroy()
|
|
should be passed a j_common_ptr.)
|
|
|
|
If you allocated the jpeg_compress_struct structure from malloc(), freeing
|
|
it is your responsibility --- jpeg_destroy() won't. Ditto for the error
|
|
handler structure.
|
|
|
|
Typical code:
|
|
|
|
jpeg_destroy_compress(&cinfo);
|
|
|
|
|
|
8. Aborting.
|
|
|
|
If you decide to abort a compression cycle before finishing, you can clean up
|
|
in either of two ways:
|
|
|
|
* If you don't need the JPEG object any more, just call
|
|
jpeg_destroy_compress() or jpeg_destroy() to release memory. This is
|
|
legitimate at any point after calling jpeg_create_compress() --- in fact,
|
|
it's safe even if jpeg_create_compress() fails.
|
|
|
|
* If you want to re-use the JPEG object, call jpeg_abort_compress(), or call
|
|
jpeg_abort() which works on both compression and decompression objects.
|
|
This will return the object to an idle state, releasing any working memory.
|
|
jpeg_abort() is allowed at any time after successful object creation.
|
|
|
|
Note that cleaning up the data destination, if required, is your
|
|
responsibility; neither of these routines will call term_destination().
|
|
(See "Compressed data handling", below, for more about that.)
|
|
|
|
jpeg_destroy() and jpeg_abort() are the only safe calls to make on a JPEG
|
|
object that has reported an error by calling error_exit (see "Error handling"
|
|
for more info). The internal state of such an object is likely to be out of
|
|
whack. Either of these two routines will return the object to a known state.
|
|
|
|
|
|
Decompression details
|
|
---------------------
|
|
|
|
Here we revisit the JPEG decompression outline given in the overview.
|
|
|
|
1. Allocate and initialize a JPEG decompression object.
|
|
|
|
This is just like initialization for compression, as discussed above,
|
|
except that the object is a "struct jpeg_decompress_struct" and you
|
|
call jpeg_create_decompress(). Error handling is exactly the same.
|
|
|
|
Typical code:
|
|
|
|
struct jpeg_decompress_struct cinfo;
|
|
struct jpeg_error_mgr jerr;
|
|
...
|
|
cinfo.err = jpeg_std_error(&jerr);
|
|
jpeg_create_decompress(&cinfo);
|
|
|
|
(Both here and in the IJG code, we usually use variable name "cinfo" for
|
|
both compression and decompression objects.)
|
|
|
|
|
|
2. Specify the source of the compressed data (eg, a file).
|
|
|
|
As previously mentioned, the JPEG library reads compressed data from a "data
|
|
source" module. The library includes one data source module which knows how
|
|
to read from a stdio stream. You can use your own source module if you want
|
|
to do something else, as discussed later.
|
|
|
|
If you use the standard source module, you must open the source stdio stream
|
|
beforehand. Typical code for this step looks like:
|
|
|
|
FILE * infile;
|
|
...
|
|
if ((infile = fopen(filename, "rb")) == NULL) {
|
|
fprintf(stderr, "can't open %s\n", filename);
|
|
exit(1);
|
|
}
|
|
jpeg_stdio_src(&cinfo, infile);
|
|
|
|
where the last line invokes the standard source module.
|
|
|
|
WARNING: it is critical that the binary compressed data be read unchanged.
|
|
On non-Unix systems the stdio library may perform newline translation or
|
|
otherwise corrupt binary data. To suppress this behavior, you may need to use
|
|
a "b" option to fopen (as shown above), or use setmode() or another routine to
|
|
put the stdio stream in binary mode. See cjpeg.c and djpeg.c for code that
|
|
has been found to work on many systems.
|
|
|
|
You may not change the data source between calling jpeg_read_header() and
|
|
jpeg_finish_decompress(). If you wish to read a series of JPEG images from
|
|
a single source file, you should repeat the jpeg_read_header() to
|
|
jpeg_finish_decompress() sequence without reinitializing either the JPEG
|
|
object or the data source module; this prevents buffered input data from
|
|
being discarded.
|
|
|
|
|
|
3. Call jpeg_read_header() to obtain image info.
|
|
|
|
Typical code for this step is just
|
|
|
|
jpeg_read_header(&cinfo, TRUE);
|
|
|
|
This will read the source datastream header markers, up to the beginning
|
|
of the compressed data proper. On return, the image dimensions and other
|
|
info have been stored in the JPEG object. The application may wish to
|
|
consult this information before selecting decompression parameters.
|
|
|
|
More complex code is necessary if
|
|
* A suspending data source is used --- in that case jpeg_read_header()
|
|
may return before it has read all the header data. See "I/O suspension",
|
|
below. The normal stdio source manager will NOT cause this to happen.
|
|
* Abbreviated JPEG files are to be processed --- see the section on
|
|
abbreviated datastreams. Standard applications that deal only in
|
|
interchange JPEG files need not be concerned with this case either.
|
|
|
|
It is permissible to stop at this point if you just wanted to find out the
|
|
image dimensions and other header info for a JPEG file. In that case,
|
|
call jpeg_destroy() when you are done with the JPEG object, or call
|
|
jpeg_abort() to return it to an idle state before selecting a new data
|
|
source and reading another header.
|
|
|
|
|
|
4. Set parameters for decompression.
|
|
|
|
jpeg_read_header() sets appropriate default decompression parameters based on
|
|
the properties of the image (in particular, its colorspace). However, you
|
|
may well want to alter these defaults before beginning the decompression.
|
|
For example, the default is to produce full color output from a color file.
|
|
If you want colormapped output you must ask for it. Other options allow the
|
|
returned image to be scaled and allow various speed/quality tradeoffs to be
|
|
selected. "Decompression parameter selection", below, gives details.
|
|
|
|
If the defaults are appropriate, nothing need be done at this step.
|
|
|
|
Note that all default values are set by each call to jpeg_read_header().
|
|
If you reuse a decompression object, you cannot expect your parameter
|
|
settings to be preserved across cycles, as you can for compression.
|
|
You must set desired parameter values each time.
|
|
|
|
|
|
5. jpeg_start_decompress(...);
|
|
|
|
Once the parameter values are satisfactory, call jpeg_start_decompress() to
|
|
begin decompression. This will initialize internal state, allocate working
|
|
memory, and prepare for returning data.
|
|
|
|
Typical code is just
|
|
|
|
jpeg_start_decompress(&cinfo);
|
|
|
|
If you have requested a multi-pass operating mode, such as 2-pass color
|
|
quantization, jpeg_start_decompress() will do everything needed before data
|
|
output can begin. In this case jpeg_start_decompress() may take quite a while
|
|
to complete. With a single-scan (non progressive) JPEG file and default
|
|
decompression parameters, this will not happen; jpeg_start_decompress() will
|
|
return quickly.
|
|
|
|
After this call, the final output image dimensions, including any requested
|
|
scaling, are available in the JPEG object; so is the selected colormap, if
|
|
colormapped output has been requested. Useful fields include
|
|
|
|
output_width image width and height, as scaled
|
|
output_height
|
|
out_color_components # of color components in out_color_space
|
|
output_components # of color components returned per pixel
|
|
colormap the selected colormap, if any
|
|
actual_number_of_colors number of entries in colormap
|
|
|
|
output_components is 1 (a colormap index) when quantizing colors; otherwise it
|
|
equals out_color_components. It is the number of JSAMPLE values that will be
|
|
emitted per pixel in the output arrays.
|
|
|
|
Typically you will need to allocate data buffers to hold the incoming image.
|
|
You will need output_width * output_components JSAMPLEs per scanline in your
|
|
output buffer, and a total of output_height scanlines will be returned.
|
|
|
|
Note: if you are using the JPEG library's internal memory manager to allocate
|
|
data buffers (as djpeg does), then the manager's protocol requires that you
|
|
request large buffers *before* calling jpeg_start_decompress(). This is a
|
|
little tricky since the output_XXX fields are not normally valid then. You
|
|
can make them valid by calling jpeg_calc_output_dimensions() after setting the
|
|
relevant parameters (scaling, output color space, and quantization flag).
|
|
|
|
|
|
6. while (scan lines remain to be read)
|
|
jpeg_read_scanlines(...);
|
|
|
|
Now you can read the decompressed image data by calling jpeg_read_scanlines()
|
|
one or more times. At each call, you pass in the maximum number of scanlines
|
|
to be read (ie, the height of your working buffer); jpeg_read_scanlines()
|
|
will return up to that many lines. The return value is the number of lines
|
|
actually read. The format of the returned data is discussed under "Data
|
|
formats", above. Don't forget that grayscale and color JPEGs will return
|
|
different data formats!
|
|
|
|
Image data is returned in top-to-bottom scanline order. If you must write
|
|
out the image in bottom-to-top order, you can use the JPEG library's virtual
|
|
array mechanism to invert the data efficiently. Examples of this can be
|
|
found in the sample application djpeg.
|
|
|
|
The library maintains a count of the number of scanlines returned so far
|
|
in the output_scanline field of the JPEG object. Usually you can just use
|
|
this variable as the loop counter, so that the loop test looks like
|
|
"while (cinfo.output_scanline < cinfo.output_height)". (Note that the test
|
|
should NOT be against image_height, unless you never use scaling. The
|
|
image_height field is the height of the original unscaled image.)
|
|
The return value always equals the change in the value of output_scanline.
|
|
|
|
If you don't use a suspending data source, it is safe to assume that
|
|
jpeg_read_scanlines() reads at least one scanline per call, until the
|
|
bottom of the image has been reached.
|
|
|
|
If you use a buffer larger than one scanline, it is NOT safe to assume that
|
|
jpeg_read_scanlines() fills it. (The current implementation returns only a
|
|
few scanlines per call, no matter how large a buffer you pass.) So you must
|
|
always provide a loop that calls jpeg_read_scanlines() repeatedly until the
|
|
whole image has been read.
|
|
|
|
|
|
7. jpeg_finish_decompress(...);
|
|
|
|
After all the image data has been read, call jpeg_finish_decompress() to
|
|
complete the decompression cycle. This causes working memory associated
|
|
with the JPEG object to be released.
|
|
|
|
Typical code:
|
|
|
|
jpeg_finish_decompress(&cinfo);
|
|
|
|
If using the stdio source manager, don't forget to close the source stdio
|
|
stream if necessary.
|
|
|
|
It is an error to call jpeg_finish_decompress() before reading the correct
|
|
total number of scanlines. If you wish to abort decompression, call
|
|
jpeg_abort() as discussed below.
|
|
|
|
After completing a decompression cycle, you may dispose of the JPEG object as
|
|
discussed next, or you may use it to decompress another image. In that case
|
|
return to step 2 or 3 as appropriate. If you do not change the source
|
|
manager, the next image will be read from the same source.
|
|
|
|
|
|
8. Release the JPEG decompression object.
|
|
|
|
When you are done with a JPEG decompression object, destroy it by calling
|
|
jpeg_destroy_decompress() or jpeg_destroy(). The previous discussion of
|
|
destroying compression objects applies here too.
|
|
|
|
Typical code:
|
|
|
|
jpeg_destroy_decompress(&cinfo);
|
|
|
|
|
|
9. Aborting.
|
|
|
|
You can abort a decompression cycle by calling jpeg_destroy_decompress() or
|
|
jpeg_destroy() if you don't need the JPEG object any more, or
|
|
jpeg_abort_decompress() or jpeg_abort() if you want to reuse the object.
|
|
The previous discussion of aborting compression cycles applies here too.
|
|
|
|
|
|
Mechanics of usage: include files, linking, etc
|
|
-----------------------------------------------
|
|
|
|
Applications using the JPEG library should include the header file jpeglib.h
|
|
to obtain declarations of data types and routines. Before including
|
|
jpeglib.h, include system headers that define at least the typedefs FILE and
|
|
size_t. On ANSI-conforming systems, including <stdio.h> is sufficient; on
|
|
older Unix systems, you may need <sys/types.h> to define size_t.
|
|
|
|
If the application needs to refer to individual JPEG library error codes, also
|
|
include jerror.h to define those symbols.
|
|
|
|
jpeglib.h indirectly includes the files jconfig.h and jmorecfg.h. If you are
|
|
installing the JPEG header files in a system directory, you will want to
|
|
install all four files: jpeglib.h, jerror.h, jconfig.h, jmorecfg.h.
|
|
|
|
The most convenient way to include the JPEG code into your executable program
|
|
is to prepare a library file ("libjpeg.a", or a corresponding name on non-Unix
|
|
machines) and reference it at your link step. If you use only half of the
|
|
library (only compression or only decompression), only that much code will be
|
|
included from the library, unless your linker is hopelessly brain-damaged.
|
|
The supplied makefiles build libjpeg.a automatically (see install.doc).
|
|
|
|
While you can build the JPEG library as a shared library if the whim strikes
|
|
you, we don't really recommend it. The trouble with shared libraries is that
|
|
at some point you'll probably try to substitute a new version of the library
|
|
without recompiling the calling applications. That generally doesn't work
|
|
because the parameter struct declarations usually change with each new
|
|
version. In other words, the library's API is *not* guaranteed binary
|
|
compatible across versions; we only try to ensure source-code compatibility.
|
|
(In hindsight, it might have been smarter to hide the parameter structs from
|
|
applications and introduce a ton of access functions instead. Too late now,
|
|
however.)
|
|
|
|
On some systems your application may need to set up a signal handler to ensure
|
|
that temporary files are deleted if the program is interrupted. This is most
|
|
critical if you are on MS-DOS and use the jmemdos.c memory manager back end;
|
|
it will try to grab extended memory for temp files, and that space will NOT be
|
|
freed automatically. See cjpeg.c or djpeg.c for an example signal handler.
|
|
|
|
It may be worth pointing out that the core JPEG library does not actually
|
|
require the stdio library: only the default source/destination managers and
|
|
error handler need it. You can use the library in a stdio-less environment
|
|
if you replace those modules and use jmemnobs.c (or another memory manager of
|
|
your own devising). More info about the minimum system library requirements
|
|
may be found in jinclude.h.
|
|
|
|
|
|
ADVANCED FEATURES
|
|
=================
|
|
|
|
Compression parameter selection
|
|
-------------------------------
|
|
|
|
This section describes all the optional parameters you can set for JPEG
|
|
compression, as well as the "helper" routines provided to assist in this
|
|
task. Proper setting of some parameters requires detailed understanding
|
|
of the JPEG standard; if you don't know what a parameter is for, it's best
|
|
not to mess with it! See REFERENCES in the README file for pointers to
|
|
more info about JPEG.
|
|
|
|
It's a good idea to call jpeg_set_defaults() first, even if you plan to set
|
|
all the parameters; that way your code is more likely to work with future JPEG
|
|
libraries that have additional parameters. For the same reason, we recommend
|
|
you use a helper routine where one is provided, in preference to twiddling
|
|
cinfo fields directly.
|
|
|
|
The helper routines are:
|
|
|
|
jpeg_set_defaults (j_compress_ptr cinfo)
|
|
This routine sets all JPEG parameters to reasonable defaults, using
|
|
only the input image's color space (field in_color_space, which must
|
|
already be set in cinfo). Many applications will only need to use
|
|
this routine and perhaps jpeg_set_quality().
|
|
|
|
jpeg_set_colorspace (j_compress_ptr cinfo, J_COLOR_SPACE colorspace)
|
|
Sets the JPEG file's colorspace (field jpeg_color_space) as specified,
|
|
and sets other color-space-dependent parameters appropriately. See
|
|
"Special color spaces", below, before using this. A large number of
|
|
parameters, including all per-component parameters, are set by this
|
|
routine; if you want to twiddle individual parameters you should call
|
|
jpeg_set_colorspace() before rather than after.
|
|
|
|
jpeg_default_colorspace (j_compress_ptr cinfo)
|
|
Selects an appropriate JPEG colorspace based on cinfo->in_color_space,
|
|
and calls jpeg_set_colorspace(). This is actually a subroutine of
|
|
jpeg_set_defaults(). It's broken out in case you want to change
|
|
just the colorspace-dependent JPEG parameters.
|
|
|
|
jpeg_set_quality (j_compress_ptr cinfo, int quality, boolean force_baseline)
|
|
Constructs JPEG quantization tables appropriate for the indicated
|
|
quality setting. The quality value is expressed on the 0..100 scale
|
|
recommended by IJG (cjpeg's "-quality" switch uses this routine).
|
|
Note that the exact mapping from quality values to tables may change
|
|
in future IJG releases as more is learned about DCT quantization.
|
|
If the force_baseline parameter is TRUE, then the quantization table
|
|
entries are constrained to the range 1..255 for full JPEG baseline
|
|
compatibility. In the current implementation, this only makes a
|
|
difference for quality settings below 25, and it effectively prevents
|
|
very small/low quality files from being generated. The IJG decoder
|
|
is capable of reading the non-baseline files generated at low quality
|
|
settings when force_baseline is FALSE, but other decoders may not be.
|
|
|
|
jpeg_set_linear_quality (j_compress_ptr cinfo, int scale_factor,
|
|
boolean force_baseline)
|
|
Same as jpeg_set_quality() except that the generated tables are the
|
|
sample tables given in the JPEC spec section K.1, multiplied by the
|
|
specified scale factor (which is expressed as a percentage; thus
|
|
scale_factor = 100 reproduces the spec's tables). Note that larger
|
|
scale factors give lower quality. This entry point is useful for
|
|
conforming to the Adobe PostScript DCT conventions, but we do not
|
|
recommend linear scaling as a user-visible quality scale otherwise.
|
|
force_baseline again constrains the computed table entries to 1..255.
|
|
|
|
int jpeg_quality_scaling (int quality)
|
|
Converts a value on the IJG-recommended quality scale to a linear
|
|
scaling percentage. Note that this routine may change or go away
|
|
in future releases --- IJG may choose to adopt a scaling method that
|
|
can't be expressed as a simple scalar multiplier, in which case the
|
|
premise of this routine collapses. Caveat user.
|
|
|
|
jpeg_add_quant_table (j_compress_ptr cinfo, int which_tbl,
|
|
const unsigned int *basic_table,
|
|
int scale_factor, boolean force_baseline)
|
|
Allows an arbitrary quantization table to be created. which_tbl
|
|
indicates which table slot to fill. basic_table points to an array
|
|
of 64 unsigned ints given in normal array order. These values are
|
|
multiplied by scale_factor/100 and then clamped to the range 1..65535
|
|
(or to 1..255 if force_baseline is TRUE).
|
|
CAUTION: prior to library version 6a, jpeg_add_quant_table expected
|
|
the basic table to be given in JPEG zigzag order. If you need to
|
|
write code that works with either older or newer versions of this
|
|
routine, you must check the library version number. Something like
|
|
"#if JPEG_LIB_VERSION >= 61" is the right test.
|
|
|
|
jpeg_simple_progression (j_compress_ptr cinfo)
|
|
Generates a default scan script for writing a progressive-JPEG file.
|
|
This is the recommended method of creating a progressive file,
|
|
unless you want to make a custom scan sequence. You must ensure that
|
|
the JPEG color space is set correctly before calling this routine.
|
|
|
|
|
|
Compression parameters (cinfo fields) include:
|
|
|
|
J_DCT_METHOD dct_method
|
|
Selects the algorithm used for the DCT step. Choices are:
|
|
JDCT_ISLOW: slow but accurate integer algorithm
|
|
JDCT_IFAST: faster, less accurate integer method
|
|
JDCT_FLOAT: floating-point method
|
|
JDCT_DEFAULT: default method (normally JDCT_ISLOW)
|
|
JDCT_FASTEST: fastest method (normally JDCT_IFAST)
|
|
The FLOAT method is very slightly more accurate than the ISLOW method,
|
|
but may give different results on different machines due to varying
|
|
roundoff behavior. The integer methods should give the same results
|
|
on all machines. On machines with sufficiently fast FP hardware, the
|
|
floating-point method may also be the fastest. The IFAST method is
|
|
considerably less accurate than the other two; its use is not
|
|
recommended if high quality is a concern. JDCT_DEFAULT and
|
|
JDCT_FASTEST are macros configurable by each installation.
|
|
|
|
J_COLOR_SPACE jpeg_color_space
|
|
int num_components
|
|
The JPEG color space and corresponding number of components; see
|
|
"Special color spaces", below, for more info. We recommend using
|
|
jpeg_set_color_space() if you want to change these.
|
|
|
|
boolean optimize_coding
|
|
TRUE causes the compressor to compute optimal Huffman coding tables
|
|
for the image. This requires an extra pass over the data and
|
|
therefore costs a good deal of space and time. The default is
|
|
FALSE, which tells the compressor to use the supplied or default
|
|
Huffman tables. In most cases optimal tables save only a few percent
|
|
of file size compared to the default tables. Note that when this is
|
|
TRUE, you need not supply Huffman tables at all, and any you do
|
|
supply will be overwritten.
|
|
|
|
unsigned int restart_interval
|
|
int restart_in_rows
|
|
To emit restart markers in the JPEG file, set one of these nonzero.
|
|
Set restart_interval to specify the exact interval in MCU blocks.
|
|
Set restart_in_rows to specify the interval in MCU rows. (If
|
|
restart_in_rows is not 0, then restart_interval is set after the
|
|
image width in MCUs is computed.) Defaults are zero (no restarts).
|
|
One restart marker per MCU row is often a good choice.
|
|
NOTE: the overhead of restart markers is higher in grayscale JPEG
|
|
files than in color files, and MUCH higher in progressive JPEGs.
|
|
If you use restarts, you may want to use larger intervals in those
|
|
cases.
|
|
|
|
const jpeg_scan_info * scan_info
|
|
int num_scans
|
|
By default, scan_info is NULL; this causes the compressor to write a
|
|
single-scan sequential JPEG file. If not NULL, scan_info points to
|
|
an array of scan definition records of length num_scans. The
|
|
compressor will then write a JPEG file having one scan for each scan
|
|
definition record. This is used to generate noninterleaved or
|
|
progressive JPEG files. The library checks that the scan array
|
|
defines a valid JPEG scan sequence. (jpeg_simple_progression creates
|
|
a suitable scan definition array for progressive JPEG.) This is
|
|
discussed further under "Progressive JPEG support".
|
|
|
|
int smoothing_factor
|
|
If non-zero, the input image is smoothed; the value should be 1 for
|
|
minimal smoothing to 100 for maximum smoothing. Consult jcsample.c
|
|
for details of the smoothing algorithm. The default is zero.
|
|
|
|
boolean write_JFIF_header
|
|
If TRUE, a JFIF APP0 marker is emitted. jpeg_set_defaults() and
|
|
jpeg_set_colorspace() set this TRUE if a JFIF-legal JPEG color space
|
|
(ie, YCbCr or grayscale) is selected, otherwise FALSE.
|
|
|
|
UINT8 JFIF_major_version
|
|
UINT8 JFIF_minor_version
|
|
The version number to be written into the JFIF marker.
|
|
jpeg_set_defaults() initializes the version to 1.01 (major=minor=1).
|
|
You should set it to 1.02 (major=1, minor=2) if you plan to write
|
|
any JFIF 1.02 extension markers.
|
|
|
|
UINT8 density_unit
|
|
UINT16 X_density
|
|
UINT16 Y_density
|
|
The resolution information to be written into the JFIF marker;
|
|
not used otherwise. density_unit may be 0 for unknown,
|
|
1 for dots/inch, or 2 for dots/cm. The default values are 0,1,1
|
|
indicating square pixels of unknown size.
|
|
|
|
boolean write_Adobe_marker
|
|
If TRUE, an Adobe APP14 marker is emitted. jpeg_set_defaults() and
|
|
jpeg_set_colorspace() set this TRUE if JPEG color space RGB, CMYK,
|
|
or YCCK is selected, otherwise FALSE. It is generally a bad idea
|
|
to set both write_JFIF_header and write_Adobe_marker. In fact,
|
|
you probably shouldn't change the default settings at all --- the
|
|
default behavior ensures that the JPEG file's color space can be
|
|
recognized by the decoder.
|
|
|
|
JQUANT_TBL * quant_tbl_ptrs[NUM_QUANT_TBLS]
|
|
Pointers to coefficient quantization tables, one per table slot,
|
|
or NULL if no table is defined for a slot. Usually these should
|
|
be set via one of the above helper routines; jpeg_add_quant_table()
|
|
is general enough to define any quantization table. The other
|
|
routines will set up table slot 0 for luminance quality and table
|
|
slot 1 for chrominance.
|
|
|
|
JHUFF_TBL * dc_huff_tbl_ptrs[NUM_HUFF_TBLS]
|
|
JHUFF_TBL * ac_huff_tbl_ptrs[NUM_HUFF_TBLS]
|
|
Pointers to Huffman coding tables, one per table slot, or NULL if
|
|
no table is defined for a slot. Slots 0 and 1 are filled with the
|
|
JPEG sample tables by jpeg_set_defaults(). If you need to allocate
|
|
more table structures, jpeg_alloc_huff_table() may be used.
|
|
Note that optimal Huffman tables can be computed for an image
|
|
by setting optimize_coding, as discussed above; there's seldom
|
|
any need to mess with providing your own Huffman tables.
|
|
|
|
There are some additional cinfo fields which are not documented here
|
|
because you currently can't change them; for example, you can't set
|
|
arith_code TRUE because arithmetic coding is unsupported.
|
|
|
|
|
|
Per-component parameters are stored in the struct cinfo.comp_info[i] for
|
|
component number i. Note that components here refer to components of the
|
|
JPEG color space, *not* the source image color space. A suitably large
|
|
comp_info[] array is allocated by jpeg_set_defaults(); if you choose not
|
|
to use that routine, it's up to you to allocate the array.
|
|
|
|
int component_id
|
|
The one-byte identifier code to be recorded in the JPEG file for
|
|
this component. For the standard color spaces, we recommend you
|
|
leave the default values alone.
|
|
|
|
int h_samp_factor
|
|
int v_samp_factor
|
|
Horizontal and vertical sampling factors for the component; must
|
|
be 1..4 according to the JPEG standard. Note that larger sampling
|
|
factors indicate a higher-resolution component; many people find
|
|
this behavior quite unintuitive. The default values are 2,2 for
|
|
luminance components and 1,1 for chrominance components, except
|
|
for grayscale where 1,1 is used.
|
|
|
|
int quant_tbl_no
|
|
Quantization table number for component. The default value is
|
|
0 for luminance components and 1 for chrominance components.
|
|
|
|
int dc_tbl_no
|
|
int ac_tbl_no
|
|
DC and AC entropy coding table numbers. The default values are
|
|
0 for luminance components and 1 for chrominance components.
|
|
|
|
int component_index
|
|
Must equal the component's index in comp_info[]. (Beginning in
|
|
release v6, the compressor library will fill this in automatically;
|
|
you don't have to.)
|
|
|
|
|
|
Decompression parameter selection
|
|
---------------------------------
|
|
|
|
Decompression parameter selection is somewhat simpler than compression
|
|
parameter selection, since all of the JPEG internal parameters are
|
|
recorded in the source file and need not be supplied by the application.
|
|
(Unless you are working with abbreviated files, in which case see
|
|
"Abbreviated datastreams", below.) Decompression parameters control
|
|
the postprocessing done on the image to deliver it in a format suitable
|
|
for the application's use. Many of the parameters control speed/quality
|
|
tradeoffs, in which faster decompression may be obtained at the price of
|
|
a poorer-quality image. The defaults select the highest quality (slowest)
|
|
processing.
|
|
|
|
The following fields in the JPEG object are set by jpeg_read_header() and
|
|
may be useful to the application in choosing decompression parameters:
|
|
|
|
JDIMENSION image_width Width and height of image
|
|
JDIMENSION image_height
|
|
int num_components Number of color components
|
|
J_COLOR_SPACE jpeg_color_space Colorspace of image
|
|
boolean saw_JFIF_marker TRUE if a JFIF APP0 marker was seen
|
|
UINT8 JFIF_major_version Version information from JFIF marker
|
|
UINT8 JFIF_minor_version
|
|
UINT8 density_unit Resolution data from JFIF marker
|
|
UINT16 X_density
|
|
UINT16 Y_density
|
|
boolean saw_Adobe_marker TRUE if an Adobe APP14 marker was seen
|
|
UINT8 Adobe_transform Color transform code from Adobe marker
|
|
|
|
The JPEG color space, unfortunately, is something of a guess since the JPEG
|
|
standard proper does not provide a way to record it. In practice most files
|
|
adhere to the JFIF or Adobe conventions, and the decoder will recognize these
|
|
correctly. See "Special color spaces", below, for more info.
|
|
|
|
|
|
The decompression parameters that determine the basic properties of the
|
|
returned image are:
|
|
|
|
J_COLOR_SPACE out_color_space
|
|
Output color space. jpeg_read_header() sets an appropriate default
|
|
based on jpeg_color_space; typically it will be RGB or grayscale.
|
|
The application can change this field to request output in a different
|
|
colorspace. For example, set it to JCS_GRAYSCALE to get grayscale
|
|
output from a color file. (This is useful for previewing: grayscale
|
|
output is faster than full color since the color components need not
|
|
be processed.) Note that not all possible color space transforms are
|
|
currently implemented; you may need to extend jdcolor.c if you want an
|
|
unusual conversion.
|
|
|
|
unsigned int scale_num, scale_denom
|
|
Scale the image by the fraction scale_num/scale_denom. Default is
|
|
1/1, or no scaling. Currently, the only supported scaling ratios
|
|
are 1/1, 1/2, 1/4, and 1/8. (The library design allows for arbitrary
|
|
scaling ratios but this is not likely to be implemented any time soon.)
|
|
Smaller scaling ratios permit significantly faster decoding since
|
|
fewer pixels need be processed and a simpler IDCT method can be used.
|
|
|
|
boolean quantize_colors
|
|
If set TRUE, colormapped output will be delivered. Default is FALSE,
|
|
meaning that full-color output will be delivered.
|
|
|
|
The next three parameters are relevant only if quantize_colors is TRUE.
|
|
|
|
int desired_number_of_colors
|
|
Maximum number of colors to use in generating a library-supplied color
|
|
map (the actual number of colors is returned in a different field).
|
|
Default 256. Ignored when the application supplies its own color map.
|
|
|
|
boolean two_pass_quantize
|
|
If TRUE, an extra pass over the image is made to select a custom color
|
|
map for the image. This usually looks a lot better than the one-size-
|
|
fits-all colormap that is used otherwise. Default is TRUE. Ignored
|
|
when the application supplies its own color map.
|
|
|
|
J_DITHER_MODE dither_mode
|
|
Selects color dithering method. Supported values are:
|
|
JDITHER_NONE no dithering: fast, very low quality
|
|
JDITHER_ORDERED ordered dither: moderate speed and quality
|
|
JDITHER_FS Floyd-Steinberg dither: slow, high quality
|
|
Default is JDITHER_FS. (At present, ordered dither is implemented
|
|
only in the single-pass, standard-colormap case. If you ask for
|
|
ordered dither when two_pass_quantize is TRUE or when you supply
|
|
an external color map, you'll get F-S dithering.)
|
|
|
|
When quantize_colors is TRUE, the target color map is described by the next
|
|
two fields. colormap is set to NULL by jpeg_read_header(). The application
|
|
can supply a color map by setting colormap non-NULL and setting
|
|
actual_number_of_colors to the map size. Otherwise, jpeg_start_decompress()
|
|
selects a suitable color map and sets these two fields itself.
|
|
[Implementation restriction: at present, an externally supplied colormap is
|
|
only accepted for 3-component output color spaces.]
|
|
|
|
JSAMPARRAY colormap
|
|
The color map, represented as a 2-D pixel array of out_color_components
|
|
rows and actual_number_of_colors columns. Ignored if not quantizing.
|
|
CAUTION: if the JPEG library creates its own colormap, the storage
|
|
pointed to by this field is released by jpeg_finish_decompress().
|
|
Copy the colormap somewhere else first, if you want to save it.
|
|
|
|
int actual_number_of_colors
|
|
The number of colors in the color map.
|
|
|
|
Additional decompression parameters that the application may set include:
|
|
|
|
J_DCT_METHOD dct_method
|
|
Selects the algorithm used for the DCT step. Choices are the same
|
|
as described above for compression.
|
|
|
|
boolean do_fancy_upsampling
|
|
If TRUE, do careful upsampling of chroma components. If FALSE,
|
|
a faster but sloppier method is used. Default is TRUE. The visual
|
|
impact of the sloppier method is often very small.
|
|
|
|
boolean do_block_smoothing
|
|
If TRUE, interblock smoothing is applied in early stages of decoding
|
|
progressive JPEG files; if FALSE, not. Default is TRUE. Early
|
|
progression stages look "fuzzy" with smoothing, "blocky" without.
|
|
In any case, block smoothing ceases to be applied after the first few
|
|
AC coefficients are known to full accuracy, so it is relevant only
|
|
when using buffered-image mode for progressive images.
|
|
|
|
boolean enable_1pass_quant
|
|
boolean enable_external_quant
|
|
boolean enable_2pass_quant
|
|
These are significant only in buffered-image mode, which is
|
|
described in its own section below.
|
|
|
|
|
|
The output image dimensions are given by the following fields. These are
|
|
computed from the source image dimensions and the decompression parameters
|
|
by jpeg_start_decompress(). You can also call jpeg_calc_output_dimensions()
|
|
to obtain the values that will result from the current parameter settings.
|
|
This can be useful if you are trying to pick a scaling ratio that will get
|
|
close to a desired target size. It's also important if you are using the
|
|
JPEG library's memory manager to allocate output buffer space, because you
|
|
are supposed to request such buffers *before* jpeg_start_decompress().
|
|
|
|
JDIMENSION output_width Actual dimensions of output image.
|
|
JDIMENSION output_height
|
|
int out_color_components Number of color components in out_color_space.
|
|
int output_components Number of color components returned.
|
|
int rec_outbuf_height Recommended height of scanline buffer.
|
|
|
|
When quantizing colors, output_components is 1, indicating a single color map
|
|
index per pixel. Otherwise it equals out_color_components. The output arrays
|
|
are required to be output_width * output_components JSAMPLEs wide.
|
|
|
|
rec_outbuf_height is the recommended minimum height (in scanlines) of the
|
|
buffer passed to jpeg_read_scanlines(). If the buffer is smaller, the
|
|
library will still work, but time will be wasted due to unnecessary data
|
|
copying. In high-quality modes, rec_outbuf_height is always 1, but some
|
|
faster, lower-quality modes set it to larger values (typically 2 to 4).
|
|
If you are going to ask for a high-speed processing mode, you may as well
|
|
go to the trouble of honoring rec_outbuf_height so as to avoid data copying.
|
|
(An output buffer larger than rec_outbuf_height lines is OK, but won't
|
|
provide any material speed improvement over that height.)
|
|
|
|
|
|
Special color spaces
|
|
--------------------
|
|
|
|
The JPEG standard itself is "color blind" and doesn't specify any particular
|
|
color space. It is customary to convert color data to a luminance/chrominance
|
|
color space before compressing, since this permits greater compression. The
|
|
existing de-facto JPEG file format standards specify YCbCr or grayscale data
|
|
(JFIF), or grayscale, RGB, YCbCr, CMYK, or YCCK (Adobe). For special
|
|
applications such as multispectral images, other color spaces can be used,
|
|
but it must be understood that such files will be unportable.
|
|
|
|
The JPEG library can handle the most common colorspace conversions (namely
|
|
RGB <=> YCbCr and CMYK <=> YCCK). It can also deal with data of an unknown
|
|
color space, passing it through without conversion. If you deal extensively
|
|
with an unusual color space, you can easily extend the library to understand
|
|
additional color spaces and perform appropriate conversions.
|
|
|
|
For compression, the source data's color space is specified by field
|
|
in_color_space. This is transformed to the JPEG file's color space given
|
|
by jpeg_color_space. jpeg_set_defaults() chooses a reasonable JPEG color
|
|
space depending on in_color_space, but you can override this by calling
|
|
jpeg_set_colorspace(). Of course you must select a supported transformation.
|
|
jccolor.c currently supports the following transformations:
|
|
RGB => YCbCr
|
|
RGB => GRAYSCALE
|
|
YCbCr => GRAYSCALE
|
|
CMYK => YCCK
|
|
plus the null transforms: GRAYSCALE => GRAYSCALE, RGB => RGB,
|
|
YCbCr => YCbCr, CMYK => CMYK, YCCK => YCCK, and UNKNOWN => UNKNOWN.
|
|
|
|
The de-facto file format standards (JFIF and Adobe) specify APPn markers that
|
|
indicate the color space of the JPEG file. It is important to ensure that
|
|
these are written correctly, or omitted if the JPEG file's color space is not
|
|
one of the ones supported by the de-facto standards. jpeg_set_colorspace()
|
|
will set the compression parameters to include or omit the APPn markers
|
|
properly, so long as it is told the truth about the JPEG color space.
|
|
For example, if you are writing some random 3-component color space without
|
|
conversion, don't try to fake out the library by setting in_color_space and
|
|
jpeg_color_space to JCS_YCbCr; use JCS_UNKNOWN. You may want to write an
|
|
APPn marker of your own devising to identify the colorspace --- see "Special
|
|
markers", below.
|
|
|
|
When told that the color space is UNKNOWN, the library will default to using
|
|
luminance-quality compression parameters for all color components. You may
|
|
well want to change these parameters. See the source code for
|
|
jpeg_set_colorspace(), in jcparam.c, for details.
|
|
|
|
For decompression, the JPEG file's color space is given in jpeg_color_space,
|
|
and this is transformed to the output color space out_color_space.
|
|
jpeg_read_header's setting of jpeg_color_space can be relied on if the file
|
|
conforms to JFIF or Adobe conventions, but otherwise it is no better than a
|
|
guess. If you know the JPEG file's color space for certain, you can override
|
|
jpeg_read_header's guess by setting jpeg_color_space. jpeg_read_header also
|
|
selects a default output color space based on (its guess of) jpeg_color_space;
|
|
set out_color_space to override this. Again, you must select a supported
|
|
transformation. jdcolor.c currently supports
|
|
YCbCr => GRAYSCALE
|
|
YCbCr => RGB
|
|
GRAYSCALE => RGB
|
|
YCCK => CMYK
|
|
as well as the null transforms. (Since GRAYSCALE=>RGB is provided, an
|
|
application can force grayscale JPEGs to look like color JPEGs if it only
|
|
wants to handle one case.)
|
|
|
|
The two-pass color quantizer, jquant2.c, is specialized to handle RGB data
|
|
(it weights distances appropriately for RGB colors). You'll need to modify
|
|
the code if you want to use it for non-RGB output color spaces. Note that
|
|
jquant2.c is used to map to an application-supplied colormap as well as for
|
|
the normal two-pass colormap selection process.
|
|
|
|
CAUTION: it appears that Adobe Photoshop writes inverted data in CMYK JPEG
|
|
files: 0 represents 100% ink coverage, rather than 0% ink as you'd expect.
|
|
This is arguably a bug in Photoshop, but if you need to work with Photoshop
|
|
CMYK files, you will have to deal with it in your application. We cannot
|
|
"fix" this in the library by inverting the data during the CMYK<=>YCCK
|
|
transform, because that would break other applications, notably Ghostscript.
|
|
Photoshop versions prior to 3.0 write EPS files containing JPEG-encoded CMYK
|
|
data in the same inverted-YCCK representation used in bare JPEG files, but
|
|
the surrounding PostScript code performs an inversion using the PS image
|
|
operator. I am told that Photoshop 3.0 will write uninverted YCCK in
|
|
EPS/JPEG files, and will omit the PS-level inversion. (But the data
|
|
polarity used in bare JPEG files will not change in 3.0.) In either case,
|
|
the JPEG library must not invert the data itself, or else Ghostscript would
|
|
read these EPS files incorrectly.
|
|
|
|
|
|
Error handling
|
|
--------------
|
|
|
|
When the default error handler is used, any error detected inside the JPEG
|
|
routines will cause a message to be printed on stderr, followed by exit().
|
|
You can supply your own error handling routines to override this behavior
|
|
and to control the treatment of nonfatal warnings and trace/debug messages.
|
|
The file example.c illustrates the most common case, which is to have the
|
|
application regain control after an error rather than exiting.
|
|
|
|
The JPEG library never writes any message directly; it always goes through
|
|
the error handling routines. Three classes of messages are recognized:
|
|
* Fatal errors: the library cannot continue.
|
|
* Warnings: the library can continue, but the data is corrupt, and a
|
|
damaged output image is likely to result.
|
|
* Trace/informational messages. These come with a trace level indicating
|
|
the importance of the message; you can control the verbosity of the
|
|
program by adjusting the maximum trace level that will be displayed.
|
|
|
|
You may, if you wish, simply replace the entire JPEG error handling module
|
|
(jerror.c) with your own code. However, you can avoid code duplication by
|
|
only replacing some of the routines depending on the behavior you need.
|
|
This is accomplished by calling jpeg_std_error() as usual, but then overriding
|
|
some of the method pointers in the jpeg_error_mgr struct, as illustrated by
|
|
example.c.
|
|
|
|
All of the error handling routines will receive a pointer to the JPEG object
|
|
(a j_common_ptr which points to either a jpeg_compress_struct or a
|
|
jpeg_decompress_struct; if you need to tell which, test the is_decompressor
|
|
field). This struct includes a pointer to the error manager struct in its
|
|
"err" field. Frequently, custom error handler routines will need to access
|
|
additional data which is not known to the JPEG library or the standard error
|
|
handler. The most convenient way to do this is to embed either the JPEG
|
|
object or the jpeg_error_mgr struct in a larger structure that contains
|
|
additional fields; then casting the passed pointer provides access to the
|
|
additional fields. Again, see example.c for one way to do it. (Beginning
|
|
with IJG version 6b, there is also a void pointer "client_data" in each
|
|
JPEG object, which the application can also use to find related data.
|
|
The library does not touch client_data at all.)
|
|
|
|
The individual methods that you might wish to override are:
|
|
|
|
error_exit (j_common_ptr cinfo)
|
|
Receives control for a fatal error. Information sufficient to
|
|
generate the error message has been stored in cinfo->err; call
|
|
output_message to display it. Control must NOT return to the caller;
|
|
generally this routine will exit() or longjmp() somewhere.
|
|
Typically you would override this routine to get rid of the exit()
|
|
default behavior. Note that if you continue processing, you should
|
|
clean up the JPEG object with jpeg_abort() or jpeg_destroy().
|
|
|
|
output_message (j_common_ptr cinfo)
|
|
Actual output of any JPEG message. Override this to send messages
|
|
somewhere other than stderr. Note that this method does not know
|
|
how to generate a message, only where to send it.
|
|
|
|
format_message (j_common_ptr cinfo, char * buffer)
|
|
Constructs a readable error message string based on the error info
|
|
stored in cinfo->err. This method is called by output_message. Few
|
|
applications should need to override this method. One possible
|
|
reason for doing so is to implement dynamic switching of error message
|
|
language.
|
|
|
|
emit_message (j_common_ptr cinfo, int msg_level)
|
|
Decide whether or not to emit a warning or trace message; if so,
|
|
calls output_message. The main reason for overriding this method
|
|
would be to abort on warnings. msg_level is -1 for warnings,
|
|
0 and up for trace messages.
|
|
|
|
Only error_exit() and emit_message() are called from the rest of the JPEG
|
|
library; the other two are internal to the error handler.
|
|
|
|
The actual message texts are stored in an array of strings which is pointed to
|
|
by the field err->jpeg_message_table. The messages are numbered from 0 to
|
|
err->last_jpeg_message, and it is these code numbers that are used in the
|
|
JPEG library code. You could replace the message texts (for instance, with
|
|
messages in French or German) by changing the message table pointer. See
|
|
jerror.h for the default texts. CAUTION: this table will almost certainly
|
|
change or grow from one library version to the next.
|
|
|
|
It may be useful for an application to add its own message texts that are
|
|
handled by the same mechanism. The error handler supports a second "add-on"
|
|
message table for this purpose. To define an addon table, set the pointer
|
|
err->addon_message_table and the message numbers err->first_addon_message and
|
|
err->last_addon_message. If you number the addon messages beginning at 1000
|
|
or so, you won't have to worry about conflicts with the library's built-in
|
|
messages. See the sample applications cjpeg/djpeg for an example of using
|
|
addon messages (the addon messages are defined in cderror.h).
|
|
|
|
Actual invocation of the error handler is done via macros defined in jerror.h:
|
|
ERREXITn(...) for fatal errors
|
|
WARNMSn(...) for corrupt-data warnings
|
|
TRACEMSn(...) for trace and informational messages.
|
|
These macros store the message code and any additional parameters into the
|
|
error handler struct, then invoke the error_exit() or emit_message() method.
|
|
The variants of each macro are for varying numbers of additional parameters.
|
|
The additional parameters are inserted into the generated message using
|
|
standard printf() format codes.
|
|
|
|
See jerror.h and jerror.c for further details.
|
|
|
|
|
|
Compressed data handling (source and destination managers)
|
|
----------------------------------------------------------
|
|
|
|
The JPEG compression library sends its compressed data to a "destination
|
|
manager" module. The default destination manager just writes the data to a
|
|
stdio stream, but you can provide your own manager to do something else.
|
|
Similarly, the decompression library calls a "source manager" to obtain the
|
|
compressed data; you can provide your own source manager if you want the data
|
|
to come from somewhere other than a stdio stream.
|
|
|
|
In both cases, compressed data is processed a bufferload at a time: the
|
|
destination or source manager provides a work buffer, and the library invokes
|
|
the manager only when the buffer is filled or emptied. (You could define a
|
|
one-character buffer to force the manager to be invoked for each byte, but
|
|
that would be rather inefficient.) The buffer's size and location are
|
|
controlled by the manager, not by the library. For example, if you desired to
|
|
decompress a JPEG datastream that was all in memory, you could just make the
|
|
buffer pointer and length point to the original data in memory. Then the
|
|
buffer-reload procedure would be invoked only if the decompressor ran off the
|
|
end of the datastream, which would indicate an erroneous datastream.
|
|
|
|
The work buffer is defined as an array of datatype JOCTET, which is generally
|
|
"char" or "unsigned char". On a machine where char is not exactly 8 bits
|
|
wide, you must define JOCTET as a wider data type and then modify the data
|
|
source and destination modules to transcribe the work arrays into 8-bit units
|
|
on external storage.
|
|
|
|
A data destination manager struct contains a pointer and count defining the
|
|
next byte to write in the work buffer and the remaining free space:
|
|
|
|
JOCTET * next_output_byte; /* => next byte to write in buffer */
|
|
size_t free_in_buffer; /* # of byte spaces remaining in buffer */
|
|
|
|
The library increments the pointer and decrements the count until the buffer
|
|
is filled. The manager's empty_output_buffer method must reset the pointer
|
|
and count. The manager is expected to remember the buffer's starting address
|
|
and total size in private fields not visible to the library.
|
|
|
|
A data destination manager provides three methods:
|
|
|
|
init_destination (j_compress_ptr cinfo)
|
|
Initialize destination. This is called by jpeg_start_compress()
|
|
before any data is actually written. It must initialize
|
|
next_output_byte and free_in_buffer. free_in_buffer must be
|
|
initialized to a positive value.
|
|
|
|
empty_output_buffer (j_compress_ptr cinfo)
|
|
This is called whenever the buffer has filled (free_in_buffer
|
|
reaches zero). In typical applications, it should write out the
|
|
*entire* buffer (use the saved start address and buffer length;
|
|
ignore the current state of next_output_byte and free_in_buffer).
|
|
Then reset the pointer & count to the start of the buffer, and
|
|
return TRUE indicating that the buffer has been dumped.
|
|
free_in_buffer must be set to a positive value when TRUE is
|
|
returned. A FALSE return should only be used when I/O suspension is
|
|
desired (this operating mode is discussed in the next section).
|
|
|
|
term_destination (j_compress_ptr cinfo)
|
|
Terminate destination --- called by jpeg_finish_compress() after all
|
|
data has been written. In most applications, this must flush any
|
|
data remaining in the buffer. Use either next_output_byte or
|
|
free_in_buffer to determine how much data is in the buffer.
|
|
|
|
term_destination() is NOT called by jpeg_abort() or jpeg_destroy(). If you
|
|
want the destination manager to be cleaned up during an abort, you must do it
|
|
yourself.
|
|
|
|
You will also need code to create a jpeg_destination_mgr struct, fill in its
|
|
method pointers, and insert a pointer to the struct into the "dest" field of
|
|
the JPEG compression object. This can be done in-line in your setup code if
|
|
you like, but it's probably cleaner to provide a separate routine similar to
|
|
the jpeg_stdio_dest() routine of the supplied destination manager.
|
|
|
|
Decompression source managers follow a parallel design, but with some
|
|
additional frammishes. The source manager struct contains a pointer and count
|
|
defining the next byte to read from the work buffer and the number of bytes
|
|
remaining:
|
|
|
|
const JOCTET * next_input_byte; /* => next byte to read from buffer */
|
|
size_t bytes_in_buffer; /* # of bytes remaining in buffer */
|
|
|
|
The library increments the pointer and decrements the count until the buffer
|
|
is emptied. The manager's fill_input_buffer method must reset the pointer and
|
|
count. In most applications, the manager must remember the buffer's starting
|
|
address and total size in private fields not visible to the library.
|
|
|
|
A data source manager provides five methods:
|
|
|
|
init_source (j_decompress_ptr cinfo)
|
|
Initialize source. This is called by jpeg_read_header() before any
|
|
data is actually read. Unlike init_destination(), it may leave
|
|
bytes_in_buffer set to 0 (in which case a fill_input_buffer() call
|
|
will occur immediately).
|
|
|
|
fill_input_buffer (j_decompress_ptr cinfo)
|
|
This is called whenever bytes_in_buffer has reached zero and more
|
|
data is wanted. In typical applications, it should read fresh data
|
|
into the buffer (ignoring the current state of next_input_byte and
|
|
bytes_in_buffer), reset the pointer & count to the start of the
|
|
buffer, and return TRUE indicating that the buffer has been reloaded.
|
|
It is not necessary to fill the buffer entirely, only to obtain at
|
|
least one more byte. bytes_in_buffer MUST be set to a positive value
|
|
if TRUE is returned. A FALSE return should only be used when I/O
|
|
suspension is desired (this mode is discussed in the next section).
|
|
|
|
skip_input_data (j_decompress_ptr cinfo, long num_bytes)
|
|
Skip num_bytes worth of data. The buffer pointer and count should
|
|
be advanced over num_bytes input bytes, refilling the buffer as
|
|
needed. This is used to skip over a potentially large amount of
|
|
uninteresting data (such as an APPn marker). In some applications
|
|
it may be possible to optimize away the reading of the skipped data,
|
|
but it's not clear that being smart is worth much trouble; large
|
|
skips are uncommon. bytes_in_buffer may be zero on return.
|
|
A zero or negative skip count should be treated as a no-op.
|
|
|
|
resync_to_restart (j_decompress_ptr cinfo, int desired)
|
|
This routine is called only when the decompressor has failed to find
|
|
a restart (RSTn) marker where one is expected. Its mission is to
|
|
find a suitable point for resuming decompression. For most
|
|
applications, we recommend that you just use the default resync
|
|
procedure, jpeg_resync_to_restart(). However, if you are able to back
|
|
up in the input data stream, or if you have a-priori knowledge about
|
|
the likely location of restart markers, you may be able to do better.
|
|
Read the read_restart_marker() and jpeg_resync_to_restart() routines
|
|
in jdmarker.c if you think you'd like to implement your own resync
|
|
procedure.
|
|
|
|
term_source (j_decompress_ptr cinfo)
|
|
Terminate source --- called by jpeg_finish_decompress() after all
|
|
data has been read. Often a no-op.
|
|
|
|
For both fill_input_buffer() and skip_input_data(), there is no such thing
|
|
as an EOF return. If the end of the file has been reached, the routine has
|
|
a choice of exiting via ERREXIT() or inserting fake data into the buffer.
|
|
In most cases, generating a warning message and inserting a fake EOI marker
|
|
is the best course of action --- this will allow the decompressor to output
|
|
however much of the image is there. In pathological cases, the decompressor
|
|
may swallow the EOI and again demand data ... just keep feeding it fake EOIs.
|
|
jdatasrc.c illustrates the recommended error recovery behavior.
|
|
|
|
term_source() is NOT called by jpeg_abort() or jpeg_destroy(). If you want
|
|
the source manager to be cleaned up during an abort, you must do it yourself.
|
|
|
|
You will also need code to create a jpeg_source_mgr struct, fill in its method
|
|
pointers, and insert a pointer to the struct into the "src" field of the JPEG
|
|
decompression object. This can be done in-line in your setup code if you
|
|
like, but it's probably cleaner to provide a separate routine similar to the
|
|
jpeg_stdio_src() routine of the supplied source manager.
|
|
|
|
For more information, consult the stdio source and destination managers
|
|
in jdatasrc.c and jdatadst.c.
|
|
|
|
|
|
I/O suspension
|
|
--------------
|
|
|
|
Some applications need to use the JPEG library as an incremental memory-to-
|
|
memory filter: when the compressed data buffer is filled or emptied, they want
|
|
control to return to the outer loop, rather than expecting that the buffer can
|
|
be emptied or reloaded within the data source/destination manager subroutine.
|
|
The library supports this need by providing an "I/O suspension" mode, which we
|
|
describe in this section.
|
|
|
|
The I/O suspension mode is not a panacea: nothing is guaranteed about the
|
|
maximum amount of time spent in any one call to the library, so it will not
|
|
eliminate response-time problems in single-threaded applications. If you
|
|
need guaranteed response time, we suggest you "bite the bullet" and implement
|
|
a real multi-tasking capability.
|
|
|
|
To use I/O suspension, cooperation is needed between the calling application
|
|
and the data source or destination manager; you will always need a custom
|
|
source/destination manager. (Please read the previous section if you haven't
|
|
already.) The basic idea is that the empty_output_buffer() or
|
|
fill_input_buffer() routine is a no-op, merely returning FALSE to indicate
|
|
that it has done nothing. Upon seeing this, the JPEG library suspends
|
|
operation and returns to its caller. The surrounding application is
|
|
responsible for emptying or refilling the work buffer before calling the
|
|
JPEG library again.
|
|
|
|
Compression suspension:
|
|
|
|
For compression suspension, use an empty_output_buffer() routine that returns
|
|
FALSE; typically it will not do anything else. This will cause the
|
|
compressor to return to the caller of jpeg_write_scanlines(), with the return
|
|
value indicating that not all the supplied scanlines have been accepted.
|
|
The application must make more room in the output buffer, adjust the output
|
|
buffer pointer/count appropriately, and then call jpeg_write_scanlines()
|
|
again, pointing to the first unconsumed scanline.
|
|
|
|
When forced to suspend, the compressor will backtrack to a convenient stopping
|
|
point (usually the start of the current MCU); it will regenerate some output
|
|
data when restarted. Therefore, although empty_output_buffer() is only
|
|
called when the buffer is filled, you should NOT write out the entire buffer
|
|
after a suspension. Write only the data up to the current position of
|
|
next_output_byte/free_in_buffer. The data beyond that point will be
|
|
regenerated after resumption.
|
|
|
|
Because of the backtracking behavior, a good-size output buffer is essential
|
|
for efficiency; you don't want the compressor to suspend often. (In fact, an
|
|
overly small buffer could lead to infinite looping, if a single MCU required
|
|
more data than would fit in the buffer.) We recommend a buffer of at least
|
|
several Kbytes. You may want to insert explicit code to ensure that you don't
|
|
call jpeg_write_scanlines() unless there is a reasonable amount of space in
|
|
the output buffer; in other words, flush the buffer before trying to compress
|
|
more data.
|
|
|
|
The compressor does not allow suspension while it is trying to write JPEG
|
|
markers at the beginning and end of the file. This means that:
|
|
* At the beginning of a compression operation, there must be enough free
|
|
space in the output buffer to hold the header markers (typically 600 or
|
|
so bytes). The recommended buffer size is bigger than this anyway, so
|
|
this is not a problem as long as you start with an empty buffer. However,
|
|
this restriction might catch you if you insert large special markers, such
|
|
as a JFIF thumbnail image, without flushing the buffer afterwards.
|
|
* When you call jpeg_finish_compress(), there must be enough space in the
|
|
output buffer to emit any buffered data and the final EOI marker. In the
|
|
current implementation, half a dozen bytes should suffice for this, but
|
|
for safety's sake we recommend ensuring that at least 100 bytes are free
|
|
before calling jpeg_finish_compress().
|
|
|
|
A more significant restriction is that jpeg_finish_compress() cannot suspend.
|
|
This means you cannot use suspension with multi-pass operating modes, namely
|
|
Huffman code optimization and multiple-scan output. Those modes write the
|
|
whole file during jpeg_finish_compress(), which will certainly result in
|
|
buffer overrun. (Note that this restriction applies only to compression,
|
|
not decompression. The decompressor supports input suspension in all of its
|
|
operating modes.)
|
|
|
|
Decompression suspension:
|
|
|
|
For decompression suspension, use a fill_input_buffer() routine that simply
|
|
returns FALSE (except perhaps during error recovery, as discussed below).
|
|
This will cause the decompressor to return to its caller with an indication
|
|
that suspension has occurred. This can happen at four places:
|
|
* jpeg_read_header(): will return JPEG_SUSPENDED.
|
|
* jpeg_start_decompress(): will return FALSE, rather than its usual TRUE.
|
|
* jpeg_read_scanlines(): will return the number of scanlines already
|
|
completed (possibly 0).
|
|
* jpeg_finish_decompress(): will return FALSE, rather than its usual TRUE.
|
|
The surrounding application must recognize these cases, load more data into
|
|
the input buffer, and repeat the call. In the case of jpeg_read_scanlines(),
|
|
increment the passed pointers past any scanlines successfully read.
|
|
|
|
Just as with compression, the decompressor will typically backtrack to a
|
|
convenient restart point before suspending. When fill_input_buffer() is
|
|
called, next_input_byte/bytes_in_buffer point to the current restart point,
|
|
which is where the decompressor will backtrack to if FALSE is returned.
|
|
The data beyond that position must NOT be discarded if you suspend; it needs
|
|
to be re-read upon resumption. In most implementations, you'll need to shift
|
|
this data down to the start of your work buffer and then load more data after
|
|
it. Again, this behavior means that a several-Kbyte work buffer is essential
|
|
for decent performance; furthermore, you should load a reasonable amount of
|
|
new data before resuming decompression. (If you loaded, say, only one new
|
|
byte each time around, you could waste a LOT of cycles.)
|
|
|
|
The skip_input_data() source manager routine requires special care in a
|
|
suspension scenario. This routine is NOT granted the ability to suspend the
|
|
decompressor; it can decrement bytes_in_buffer to zero, but no more. If the
|
|
requested skip distance exceeds the amount of data currently in the input
|
|
buffer, then skip_input_data() must set bytes_in_buffer to zero and record the
|
|
additional skip distance somewhere else. The decompressor will immediately
|
|
call fill_input_buffer(), which should return FALSE, which will cause a
|
|
suspension return. The surrounding application must then arrange to discard
|
|
the recorded number of bytes before it resumes loading the input buffer.
|
|
(Yes, this design is rather baroque, but it avoids complexity in the far more
|
|
common case where a non-suspending source manager is used.)
|
|
|
|
If the input data has been exhausted, we recommend that you emit a warning
|
|
and insert dummy EOI markers just as a non-suspending data source manager
|
|
would do. This can be handled either in the surrounding application logic or
|
|
within fill_input_buffer(); the latter is probably more efficient. If
|
|
fill_input_buffer() knows that no more data is available, it can set the
|
|
pointer/count to point to a dummy EOI marker and then return TRUE just as
|
|
though it had read more data in a non-suspending situation.
|
|
|
|
The decompressor does not attempt to suspend within standard JPEG markers;
|
|
instead it will backtrack to the start of the marker and reprocess the whole
|
|
marker next time. Hence the input buffer must be large enough to hold the
|
|
longest standard marker in the file. Standard JPEG markers should normally
|
|
not exceed a few hundred bytes each (DHT tables are typically the longest).
|
|
We recommend at least a 2K buffer for performance reasons, which is much
|
|
larger than any correct marker is likely to be. For robustness against
|
|
damaged marker length counts, you may wish to insert a test in your
|
|
application for the case that the input buffer is completely full and yet
|
|
the decoder has suspended without consuming any data --- otherwise, if this
|
|
situation did occur, it would lead to an endless loop. (The library can't
|
|
provide this test since it has no idea whether "the buffer is full", or
|
|
even whether there is a fixed-size input buffer.)
|
|
|
|
The input buffer would need to be 64K to allow for arbitrary COM or APPn
|
|
markers, but these are handled specially: they are either saved into allocated
|
|
memory, or skipped over by calling skip_input_data(). In the former case,
|
|
suspension is handled correctly, and in the latter case, the problem of
|
|
buffer overrun is placed on skip_input_data's shoulders, as explained above.
|
|
Note that if you provide your own marker handling routine for large markers,
|
|
you should consider how to deal with buffer overflow.
|
|
|
|
Multiple-buffer management:
|
|
|
|
In some applications it is desirable to store the compressed data in a linked
|
|
list of buffer areas, so as to avoid data copying. This can be handled by
|
|
having empty_output_buffer() or fill_input_buffer() set the pointer and count
|
|
to reference the next available buffer; FALSE is returned only if no more
|
|
buffers are available. Although seemingly straightforward, there is a
|
|
pitfall in this approach: the backtrack that occurs when FALSE is returned
|
|
could back up into an earlier buffer. For example, when fill_input_buffer()
|
|
is called, the current pointer & count indicate the backtrack restart point.
|
|
Since fill_input_buffer() will set the pointer and count to refer to a new
|
|
buffer, the restart position must be saved somewhere else. Suppose a second
|
|
call to fill_input_buffer() occurs in the same library call, and no
|
|
additional input data is available, so fill_input_buffer must return FALSE.
|
|
If the JPEG library has not moved the pointer/count forward in the current
|
|
buffer, then *the correct restart point is the saved position in the prior
|
|
buffer*. Prior buffers may be discarded only after the library establishes
|
|
a restart point within a later buffer. Similar remarks apply for output into
|
|
a chain of buffers.
|
|
|
|
The library will never attempt to backtrack over a skip_input_data() call,
|
|
so any skipped data can be permanently discarded. You still have to deal
|
|
with the case of skipping not-yet-received data, however.
|
|
|
|
It's much simpler to use only a single buffer; when fill_input_buffer() is
|
|
called, move any unconsumed data (beyond the current pointer/count) down to
|
|
the beginning of this buffer and then load new data into the remaining buffer
|
|
space. This approach requires a little more data copying but is far easier
|
|
to get right.
|
|
|
|
|
|
Progressive JPEG support
|
|
------------------------
|
|
|
|
Progressive JPEG rearranges the stored data into a series of scans of
|
|
increasing quality. In situations where a JPEG file is transmitted across a
|
|
slow communications link, a decoder can generate a low-quality image very
|
|
quickly from the first scan, then gradually improve the displayed quality as
|
|
more scans are received. The final image after all scans are complete is
|
|
identical to that of a regular (sequential) JPEG file of the same quality
|
|
setting. Progressive JPEG files are often slightly smaller than equivalent
|
|
sequential JPEG files, but the possibility of incremental display is the main
|
|
reason for using progressive JPEG.
|
|
|
|
The IJG encoder library generates progressive JPEG files when given a
|
|
suitable "scan script" defining how to divide the data into scans.
|
|
Creation of progressive JPEG files is otherwise transparent to the encoder.
|
|
Progressive JPEG files can also be read transparently by the decoder library.
|
|
If the decoding application simply uses the library as defined above, it
|
|
will receive a final decoded image without any indication that the file was
|
|
progressive. Of course, this approach does not allow incremental display.
|
|
To perform incremental display, an application needs to use the decoder
|
|
library's "buffered-image" mode, in which it receives a decoded image
|
|
multiple times.
|
|
|
|
Each displayed scan requires about as much work to decode as a full JPEG
|
|
image of the same size, so the decoder must be fairly fast in relation to the
|
|
data transmission rate in order to make incremental display useful. However,
|
|
it is possible to skip displaying the image and simply add the incoming bits
|
|
to the decoder's coefficient buffer. This is fast because only Huffman
|
|
decoding need be done, not IDCT, upsampling, colorspace conversion, etc.
|
|
The IJG decoder library allows the application to switch dynamically between
|
|
displaying the image and simply absorbing the incoming bits. A properly
|
|
coded application can automatically adapt the number of display passes to
|
|
suit the time available as the image is received. Also, a final
|
|
higher-quality display cycle can be performed from the buffered data after
|
|
the end of the file is reached.
|
|
|
|
Progressive compression:
|
|
|
|
To create a progressive JPEG file (or a multiple-scan sequential JPEG file),
|
|
set the scan_info cinfo field to point to an array of scan descriptors, and
|
|
perform compression as usual. Instead of constructing your own scan list,
|
|
you can call the jpeg_simple_progression() helper routine to create a
|
|
recommended progression sequence; this method should be used by all
|
|
applications that don't want to get involved in the nitty-gritty of
|
|
progressive scan sequence design. (If you want to provide user control of
|
|
scan sequences, you may wish to borrow the scan script reading code found
|
|
in rdswitch.c, so that you can read scan script files just like cjpeg's.)
|
|
When scan_info is not NULL, the compression library will store DCT'd data
|
|
into a buffer array as jpeg_write_scanlines() is called, and will emit all
|
|
the requested scans during jpeg_finish_compress(). This implies that
|
|
multiple-scan output cannot be created with a suspending data destination
|
|
manager, since jpeg_finish_compress() does not support suspension. We
|
|
should also note that the compressor currently forces Huffman optimization
|
|
mode when creating a progressive JPEG file, because the default Huffman
|
|
tables are unsuitable for progressive files.
|
|
|
|
Progressive decompression:
|
|
|
|
When buffered-image mode is not used, the decoder library will read all of
|
|
a multi-scan file during jpeg_start_decompress(), so that it can provide a
|
|
final decoded image. (Here "multi-scan" means either progressive or
|
|
multi-scan sequential.) This makes multi-scan files transparent to the
|
|
decoding application. However, existing applications that used suspending
|
|
input with version 5 of the IJG library will need to be modified to check
|
|
for a suspension return from jpeg_start_decompress().
|
|
|
|
To perform incremental display, an application must use the library's
|
|
buffered-image mode. This is described in the next section.
|
|
|
|
|
|
Buffered-image mode
|
|
-------------------
|
|
|
|
In buffered-image mode, the library stores the partially decoded image in a
|
|
coefficient buffer, from which it can be read out as many times as desired.
|
|
This mode is typically used for incremental display of progressive JPEG files,
|
|
but it can be used with any JPEG file. Each scan of a progressive JPEG file
|
|
adds more data (more detail) to the buffered image. The application can
|
|
display in lockstep with the source file (one display pass per input scan),
|
|
or it can allow input processing to outrun display processing. By making
|
|
input and display processing run independently, it is possible for the
|
|
application to adapt progressive display to a wide range of data transmission
|
|
rates.
|
|
|
|
The basic control flow for buffered-image decoding is
|
|
|
|
jpeg_create_decompress()
|
|
set data source
|
|
jpeg_read_header()
|
|
set overall decompression parameters
|
|
cinfo.buffered_image = TRUE; /* select buffered-image mode */
|
|
jpeg_start_decompress()
|
|
for (each output pass) {
|
|
adjust output decompression parameters if required
|
|
jpeg_start_output() /* start a new output pass */
|
|
for (all scanlines in image) {
|
|
jpeg_read_scanlines()
|
|
display scanlines
|
|
}
|
|
jpeg_finish_output() /* terminate output pass */
|
|
}
|
|
jpeg_finish_decompress()
|
|
jpeg_destroy_decompress()
|
|
|
|
This differs from ordinary unbuffered decoding in that there is an additional
|
|
level of looping. The application can choose how many output passes to make
|
|
and how to display each pass.
|
|
|
|
The simplest approach to displaying progressive images is to do one display
|
|
pass for each scan appearing in the input file. In this case the outer loop
|
|
condition is typically
|
|
while (! jpeg_input_complete(&cinfo))
|
|
and the start-output call should read
|
|
jpeg_start_output(&cinfo, cinfo.input_scan_number);
|
|
The second parameter to jpeg_start_output() indicates which scan of the input
|
|
file is to be displayed; the scans are numbered starting at 1 for this
|
|
purpose. (You can use a loop counter starting at 1 if you like, but using
|
|
the library's input scan counter is easier.) The library automatically reads
|
|
data as necessary to complete each requested scan, and jpeg_finish_output()
|
|
advances to the next scan or end-of-image marker (hence input_scan_number
|
|
will be incremented by the time control arrives back at jpeg_start_output()).
|
|
With this technique, data is read from the input file only as needed, and
|
|
input and output processing run in lockstep.
|
|
|
|
After reading the final scan and reaching the end of the input file, the
|
|
buffered image remains available; it can be read additional times by
|
|
repeating the jpeg_start_output()/jpeg_read_scanlines()/jpeg_finish_output()
|
|
sequence. For example, a useful technique is to use fast one-pass color
|
|
quantization for display passes made while the image is arriving, followed by
|
|
a final display pass using two-pass quantization for highest quality. This
|
|
is done by changing the library parameters before the final output pass.
|
|
Changing parameters between passes is discussed in detail below.
|
|
|
|
In general the last scan of a progressive file cannot be recognized as such
|
|
until after it is read, so a post-input display pass is the best approach if
|
|
you want special processing in the final pass.
|
|
|
|
When done with the image, be sure to call jpeg_finish_decompress() to release
|
|
the buffered image (or just use jpeg_destroy_decompress()).
|
|
|
|
If input data arrives faster than it can be displayed, the application can
|
|
cause the library to decode input data in advance of what's needed to produce
|
|
output. This is done by calling the routine jpeg_consume_input().
|
|
The return value is one of the following:
|
|
JPEG_REACHED_SOS: reached an SOS marker (the start of a new scan)
|
|
JPEG_REACHED_EOI: reached the EOI marker (end of image)
|
|
JPEG_ROW_COMPLETED: completed reading one MCU row of compressed data
|
|
JPEG_SCAN_COMPLETED: completed reading last MCU row of current scan
|
|
JPEG_SUSPENDED: suspended before completing any of the above
|
|
(JPEG_SUSPENDED can occur only if a suspending data source is used.) This
|
|
routine can be called at any time after initializing the JPEG object. It
|
|
reads some additional data and returns when one of the indicated significant
|
|
events occurs. (If called after the EOI marker is reached, it will
|
|
immediately return JPEG_REACHED_EOI without attempting to read more data.)
|
|
|
|
The library's output processing will automatically call jpeg_consume_input()
|
|
whenever the output processing overtakes the input; thus, simple lockstep
|
|
display requires no direct calls to jpeg_consume_input(). But by adding
|
|
calls to jpeg_consume_input(), you can absorb data in advance of what is
|
|
being displayed. This has two benefits:
|
|
* You can limit buildup of unprocessed data in your input buffer.
|
|
* You can eliminate extra display passes by paying attention to the
|
|
state of the library's input processing.
|
|
|
|
The first of these benefits only requires interspersing calls to
|
|
jpeg_consume_input() with your display operations and any other processing
|
|
you may be doing. To avoid wasting cycles due to backtracking, it's best to
|
|
call jpeg_consume_input() only after a hundred or so new bytes have arrived.
|
|
This is discussed further under "I/O suspension", above. (Note: the JPEG
|
|
library currently is not thread-safe. You must not call jpeg_consume_input()
|
|
from one thread of control if a different library routine is working on the
|
|
same JPEG object in another thread.)
|
|
|
|
When input arrives fast enough that more than one new scan is available
|
|
before you start a new output pass, you may as well skip the output pass
|
|
corresponding to the completed scan. This occurs for free if you pass
|
|
cinfo.input_scan_number as the target scan number to jpeg_start_output().
|
|
The input_scan_number field is simply the index of the scan currently being
|
|
consumed by the input processor. You can ensure that this is up-to-date by
|
|
emptying the input buffer just before calling jpeg_start_output(): call
|
|
jpeg_consume_input() repeatedly until it returns JPEG_SUSPENDED or
|
|
JPEG_REACHED_EOI.
|
|
|
|
The target scan number passed to jpeg_start_output() is saved in the
|
|
cinfo.output_scan_number field. The library's output processing calls
|
|
jpeg_consume_input() whenever the current input scan number and row within
|
|
that scan is less than or equal to the current output scan number and row.
|
|
Thus, input processing can "get ahead" of the output processing but is not
|
|
allowed to "fall behind". You can achieve several different effects by
|
|
manipulating this interlock rule. For example, if you pass a target scan
|
|
number greater than the current input scan number, the output processor will
|
|
wait until that scan starts to arrive before producing any output. (To avoid
|
|
an infinite loop, the target scan number is automatically reset to the last
|
|
scan number when the end of image is reached. Thus, if you specify a large
|
|
target scan number, the library will just absorb the entire input file and
|
|
then perform an output pass. This is effectively the same as what
|
|
jpeg_start_decompress() does when you don't select buffered-image mode.)
|
|
When you pass a target scan number equal to the current input scan number,
|
|
the image is displayed no faster than the current input scan arrives. The
|
|
final possibility is to pass a target scan number less than the current input
|
|
scan number; this disables the input/output interlock and causes the output
|
|
processor to simply display whatever it finds in the image buffer, without
|
|
waiting for input. (However, the library will not accept a target scan
|
|
number less than one, so you can't avoid waiting for the first scan.)
|
|
|
|
When data is arriving faster than the output display processing can advance
|
|
through the image, jpeg_consume_input() will store data into the buffered
|
|
image beyond the point at which the output processing is reading data out
|
|
again. If the input arrives fast enough, it may "wrap around" the buffer to
|
|
the point where the input is more than one whole scan ahead of the output.
|
|
If the output processing simply proceeds through its display pass without
|
|
paying attention to the input, the effect seen on-screen is that the lower
|
|
part of the image is one or more scans better in quality than the upper part.
|
|
Then, when the next output scan is started, you have a choice of what target
|
|
scan number to use. The recommended choice is to use the current input scan
|
|
number at that time, which implies that you've skipped the output scans
|
|
corresponding to the input scans that were completed while you processed the
|
|
previous output scan. In this way, the decoder automatically adapts its
|
|
speed to the arriving data, by skipping output scans as necessary to keep up
|
|
with the arriving data.
|
|
|
|
When using this strategy, you'll want to be sure that you perform a final
|
|
output pass after receiving all the data; otherwise your last display may not
|
|
be full quality across the whole screen. So the right outer loop logic is
|
|
something like this:
|
|
do {
|
|
absorb any waiting input by calling jpeg_consume_input()
|
|
final_pass = jpeg_input_complete(&cinfo);
|
|
adjust output decompression parameters if required
|
|
jpeg_start_output(&cinfo, cinfo.input_scan_number);
|
|
...
|
|
jpeg_finish_output()
|
|
} while (! final_pass);
|
|
rather than quitting as soon as jpeg_input_complete() returns TRUE. This
|
|
arrangement makes it simple to use higher-quality decoding parameters
|
|
for the final pass. But if you don't want to use special parameters for
|
|
the final pass, the right loop logic is like this:
|
|
for (;;) {
|
|
absorb any waiting input by calling jpeg_consume_input()
|
|
jpeg_start_output(&cinfo, cinfo.input_scan_number);
|
|
...
|
|
jpeg_finish_output()
|
|
if (jpeg_input_complete(&cinfo) &&
|
|
cinfo.input_scan_number == cinfo.output_scan_number)
|
|
break;
|
|
}
|
|
In this case you don't need to know in advance whether an output pass is to
|
|
be the last one, so it's not necessary to have reached EOF before starting
|
|
the final output pass; rather, what you want to test is whether the output
|
|
pass was performed in sync with the final input scan. This form of the loop
|
|
will avoid an extra output pass whenever the decoder is able (or nearly able)
|
|
to keep up with the incoming data.
|
|
|
|
When the data transmission speed is high, you might begin a display pass,
|
|
then find that much or all of the file has arrived before you can complete
|
|
the pass. (You can detect this by noting the JPEG_REACHED_EOI return code
|
|
from jpeg_consume_input(), or equivalently by testing jpeg_input_complete().)
|
|
In this situation you may wish to abort the current display pass and start a
|
|
new one using the newly arrived information. To do so, just call
|
|
jpeg_finish_output() and then start a new pass with jpeg_start_output().
|
|
|
|
A variant strategy is to abort and restart display if more than one complete
|
|
scan arrives during an output pass; this can be detected by noting
|
|
JPEG_REACHED_SOS returns and/or examining cinfo.input_scan_number. This
|
|
idea should be employed with caution, however, since the display process
|
|
might never get to the bottom of the image before being aborted, resulting
|
|
in the lower part of the screen being several passes worse than the upper.
|
|
In most cases it's probably best to abort an output pass only if the whole
|
|
file has arrived and you want to begin the final output pass immediately.
|
|
|
|
When receiving data across a communication link, we recommend always using
|
|
the current input scan number for the output target scan number; if a
|
|
higher-quality final pass is to be done, it should be started (aborting any
|
|
incomplete output pass) as soon as the end of file is received. However,
|
|
many other strategies are possible. For example, the application can examine
|
|
the parameters of the current input scan and decide whether to display it or
|
|
not. If the scan contains only chroma data, one might choose not to use it
|
|
as the target scan, expecting that the scan will be small and will arrive
|
|
quickly. To skip to the next scan, call jpeg_consume_input() until it
|
|
returns JPEG_REACHED_SOS or JPEG_REACHED_EOI. Or just use the next higher
|
|
number as the target scan for jpeg_start_output(); but that method doesn't
|
|
let you inspect the next scan's parameters before deciding to display it.
|
|
|
|
|
|
In buffered-image mode, jpeg_start_decompress() never performs input and
|
|
thus never suspends. An application that uses input suspension with
|
|
buffered-image mode must be prepared for suspension returns from these
|
|
routines:
|
|
* jpeg_start_output() performs input only if you request 2-pass quantization
|
|
and the target scan isn't fully read yet. (This is discussed below.)
|
|
* jpeg_read_scanlines(), as always, returns the number of scanlines that it
|
|
was able to produce before suspending.
|
|
* jpeg_finish_output() will read any markers following the target scan,
|
|
up to the end of the file or the SOS marker that begins another scan.
|
|
(But it reads no input if jpeg_consume_input() has already reached the
|
|
end of the file or a SOS marker beyond the target output scan.)
|
|
* jpeg_finish_decompress() will read until the end of file, and thus can
|
|
suspend if the end hasn't already been reached (as can be tested by
|
|
calling jpeg_input_complete()).
|
|
jpeg_start_output(), jpeg_finish_output(), and jpeg_finish_decompress()
|
|
all return TRUE if they completed their tasks, FALSE if they had to suspend.
|
|
In the event of a FALSE return, the application must load more input data
|
|
and repeat the call. Applications that use non-suspending data sources need
|
|
not check the return values of these three routines.
|
|
|
|
|
|
It is possible to change decoding parameters between output passes in the
|
|
buffered-image mode. The decoder library currently supports only very
|
|
limited changes of parameters. ONLY THE FOLLOWING parameter changes are
|
|
allowed after jpeg_start_decompress() is called:
|
|
* dct_method can be changed before each call to jpeg_start_output().
|
|
For example, one could use a fast DCT method for early scans, changing
|
|
to a higher quality method for the final scan.
|
|
* dither_mode can be changed before each call to jpeg_start_output();
|
|
of course this has no impact if not using color quantization. Typically
|
|
one would use ordered dither for initial passes, then switch to
|
|
Floyd-Steinberg dither for the final pass. Caution: changing dither mode
|
|
can cause more memory to be allocated by the library. Although the amount
|
|
of memory involved is not large (a scanline or so), it may cause the
|
|
initial max_memory_to_use specification to be exceeded, which in the worst
|
|
case would result in an out-of-memory failure.
|
|
* do_block_smoothing can be changed before each call to jpeg_start_output().
|
|
This setting is relevant only when decoding a progressive JPEG image.
|
|
During the first DC-only scan, block smoothing provides a very "fuzzy" look
|
|
instead of the very "blocky" look seen without it; which is better seems a
|
|
matter of personal taste. But block smoothing is nearly always a win
|
|
during later stages, especially when decoding a successive-approximation
|
|
image: smoothing helps to hide the slight blockiness that otherwise shows
|
|
up on smooth gradients until the lowest coefficient bits are sent.
|
|
* Color quantization mode can be changed under the rules described below.
|
|
You *cannot* change between full-color and quantized output (because that
|
|
would alter the required I/O buffer sizes), but you can change which
|
|
quantization method is used.
|
|
|
|
When generating color-quantized output, changing quantization method is a
|
|
very useful way of switching between high-speed and high-quality display.
|
|
The library allows you to change among its three quantization methods:
|
|
1. Single-pass quantization to a fixed color cube.
|
|
Selected by cinfo.two_pass_quantize = FALSE and cinfo.colormap = NULL.
|
|
2. Single-pass quantization to an application-supplied colormap.
|
|
Selected by setting cinfo.colormap to point to the colormap (the value of
|
|
two_pass_quantize is ignored); also set cinfo.actual_number_of_colors.
|
|
3. Two-pass quantization to a colormap chosen specifically for the image.
|
|
Selected by cinfo.two_pass_quantize = TRUE and cinfo.colormap = NULL.
|
|
(This is the default setting selected by jpeg_read_header, but it is
|
|
probably NOT what you want for the first pass of progressive display!)
|
|
These methods offer successively better quality and lesser speed. However,
|
|
only the first method is available for quantizing in non-RGB color spaces.
|
|
|
|
IMPORTANT: because the different quantizer methods have very different
|
|
working-storage requirements, the library requires you to indicate which
|
|
one(s) you intend to use before you call jpeg_start_decompress(). (If we did
|
|
not require this, the max_memory_to_use setting would be a complete fiction.)
|
|
You do this by setting one or more of these three cinfo fields to TRUE:
|
|
enable_1pass_quant Fixed color cube colormap
|
|
enable_external_quant Externally-supplied colormap
|
|
enable_2pass_quant Two-pass custom colormap
|
|
All three are initialized FALSE by jpeg_read_header(). But
|
|
jpeg_start_decompress() automatically sets TRUE the one selected by the
|
|
current two_pass_quantize and colormap settings, so you only need to set the
|
|
enable flags for any other quantization methods you plan to change to later.
|
|
|
|
After setting the enable flags correctly at jpeg_start_decompress() time, you
|
|
can change to any enabled quantization method by setting two_pass_quantize
|
|
and colormap properly just before calling jpeg_start_output(). The following
|
|
special rules apply:
|
|
1. You must explicitly set cinfo.colormap to NULL when switching to 1-pass
|
|
or 2-pass mode from a different mode, or when you want the 2-pass
|
|
quantizer to be re-run to generate a new colormap.
|
|
2. To switch to an external colormap, or to change to a different external
|
|
colormap than was used on the prior pass, you must call
|
|
jpeg_new_colormap() after setting cinfo.colormap.
|
|
NOTE: if you want to use the same colormap as was used in the prior pass,
|
|
you should not do either of these things. This will save some nontrivial
|
|
switchover costs.
|
|
(These requirements exist because cinfo.colormap will always be non-NULL
|
|
after completing a prior output pass, since both the 1-pass and 2-pass
|
|
quantizers set it to point to their output colormaps. Thus you have to
|
|
do one of these two things to notify the library that something has changed.
|
|
Yup, it's a bit klugy, but it's necessary to do it this way for backwards
|
|
compatibility.)
|
|
|
|
Note that in buffered-image mode, the library generates any requested colormap
|
|
during jpeg_start_output(), not during jpeg_start_decompress().
|
|
|
|
When using two-pass quantization, jpeg_start_output() makes a pass over the
|
|
buffered image to determine the optimum color map; it therefore may take a
|
|
significant amount of time, whereas ordinarily it does little work. The
|
|
progress monitor hook is called during this pass, if defined. It is also
|
|
important to realize that if the specified target scan number is greater than
|
|
or equal to the current input scan number, jpeg_start_output() will attempt
|
|
to consume input as it makes this pass. If you use a suspending data source,
|
|
you need to check for a FALSE return from jpeg_start_output() under these
|
|
conditions. The combination of 2-pass quantization and a not-yet-fully-read
|
|
target scan is the only case in which jpeg_start_output() will consume input.
|
|
|
|
|
|
Application authors who support buffered-image mode may be tempted to use it
|
|
for all JPEG images, even single-scan ones. This will work, but it is
|
|
inefficient: there is no need to create an image-sized coefficient buffer for
|
|
single-scan images. Requesting buffered-image mode for such an image wastes
|
|
memory. Worse, it can cost time on large images, since the buffered data has
|
|
to be swapped out or written to a temporary file. If you are concerned about
|
|
maximum performance on baseline JPEG files, you should use buffered-image
|
|
mode only when the incoming file actually has multiple scans. This can be
|
|
tested by calling jpeg_has_multiple_scans(), which will return a correct
|
|
result at any time after jpeg_read_header() completes.
|
|
|
|
It is also worth noting that when you use jpeg_consume_input() to let input
|
|
processing get ahead of output processing, the resulting pattern of access to
|
|
the coefficient buffer is quite nonsequential. It's best to use the memory
|
|
manager jmemnobs.c if you can (ie, if you have enough real or virtual main
|
|
memory). If not, at least make sure that max_memory_to_use is set as high as
|
|
possible. If the JPEG memory manager has to use a temporary file, you will
|
|
probably see a lot of disk traffic and poor performance. (This could be
|
|
improved with additional work on the memory manager, but we haven't gotten
|
|
around to it yet.)
|
|
|
|
In some applications it may be convenient to use jpeg_consume_input() for all
|
|
input processing, including reading the initial markers; that is, you may
|
|
wish to call jpeg_consume_input() instead of jpeg_read_header() during
|
|
startup. This works, but note that you must check for JPEG_REACHED_SOS and
|
|
JPEG_REACHED_EOI return codes as the equivalent of jpeg_read_header's codes.
|
|
Once the first SOS marker has been reached, you must call
|
|
jpeg_start_decompress() before jpeg_consume_input() will consume more input;
|
|
it'll just keep returning JPEG_REACHED_SOS until you do. If you read a
|
|
tables-only file this way, jpeg_consume_input() will return JPEG_REACHED_EOI
|
|
without ever returning JPEG_REACHED_SOS; be sure to check for this case.
|
|
If this happens, the decompressor will not read any more input until you call
|
|
jpeg_abort() to reset it. It is OK to call jpeg_consume_input() even when not
|
|
using buffered-image mode, but in that case it's basically a no-op after the
|
|
initial markers have been read: it will just return JPEG_SUSPENDED.
|
|
|
|
|
|
Abbreviated datastreams and multiple images
|
|
-------------------------------------------
|
|
|
|
A JPEG compression or decompression object can be reused to process multiple
|
|
images. This saves a small amount of time per image by eliminating the
|
|
"create" and "destroy" operations, but that isn't the real purpose of the
|
|
feature. Rather, reuse of an object provides support for abbreviated JPEG
|
|
datastreams. Object reuse can also simplify processing a series of images in
|
|
a single input or output file. This section explains these features.
|
|
|
|
A JPEG file normally contains several hundred bytes worth of quantization
|
|
and Huffman tables. In a situation where many images will be stored or
|
|
transmitted with identical tables, this may represent an annoying overhead.
|
|
The JPEG standard therefore permits tables to be omitted. The standard
|
|
defines three classes of JPEG datastreams:
|
|
* "Interchange" datastreams contain an image and all tables needed to decode
|
|
the image. These are the usual kind of JPEG file.
|
|
* "Abbreviated image" datastreams contain an image, but are missing some or
|
|
all of the tables needed to decode that image.
|
|
* "Abbreviated table specification" (henceforth "tables-only") datastreams
|
|
contain only table specifications.
|
|
To decode an abbreviated image, it is necessary to load the missing table(s)
|
|
into the decoder beforehand. This can be accomplished by reading a separate
|
|
tables-only file. A variant scheme uses a series of images in which the first
|
|
image is an interchange (complete) datastream, while subsequent ones are
|
|
abbreviated and rely on the tables loaded by the first image. It is assumed
|
|
that once the decoder has read a table, it will remember that table until a
|
|
new definition for the same table number is encountered.
|
|
|
|
It is the application designer's responsibility to figure out how to associate
|
|
the correct tables with an abbreviated image. While abbreviated datastreams
|
|
can be useful in a closed environment, their use is strongly discouraged in
|
|
any situation where data exchange with other applications might be needed.
|
|
Caveat designer.
|
|
|
|
The JPEG library provides support for reading and writing any combination of
|
|
tables-only datastreams and abbreviated images. In both compression and
|
|
decompression objects, a quantization or Huffman table will be retained for
|
|
the lifetime of the object, unless it is overwritten by a new table definition.
|
|
|
|
|
|
To create abbreviated image datastreams, it is only necessary to tell the
|
|
compressor not to emit some or all of the tables it is using. Each
|
|
quantization and Huffman table struct contains a boolean field "sent_table",
|
|
which normally is initialized to FALSE. For each table used by the image, the
|
|
header-writing process emits the table and sets sent_table = TRUE unless it is
|
|
already TRUE. (In normal usage, this prevents outputting the same table
|
|
definition multiple times, as would otherwise occur because the chroma
|
|
components typically share tables.) Thus, setting this field to TRUE before
|
|
calling jpeg_start_compress() will prevent the table from being written at
|
|
all.
|
|
|
|
If you want to create a "pure" abbreviated image file containing no tables,
|
|
just call "jpeg_suppress_tables(&cinfo, TRUE)" after constructing all the
|
|
tables. If you want to emit some but not all tables, you'll need to set the
|
|
individual sent_table fields directly.
|
|
|
|
To create an abbreviated image, you must also call jpeg_start_compress()
|
|
with a second parameter of FALSE, not TRUE. Otherwise jpeg_start_compress()
|
|
will force all the sent_table fields to FALSE. (This is a safety feature to
|
|
prevent abbreviated images from being created accidentally.)
|
|
|
|
To create a tables-only file, perform the same parameter setup that you
|
|
normally would, but instead of calling jpeg_start_compress() and so on, call
|
|
jpeg_write_tables(&cinfo). This will write an abbreviated datastream
|
|
containing only SOI, DQT and/or DHT markers, and EOI. All the quantization
|
|
and Huffman tables that are currently defined in the compression object will
|
|
be emitted unless their sent_tables flag is already TRUE, and then all the
|
|
sent_tables flags will be set TRUE.
|
|
|
|
A sure-fire way to create matching tables-only and abbreviated image files
|
|
is to proceed as follows:
|
|
|
|
create JPEG compression object
|
|
set JPEG parameters
|
|
set destination to tables-only file
|
|
jpeg_write_tables(&cinfo);
|
|
set destination to image file
|
|
jpeg_start_compress(&cinfo, FALSE);
|
|
write data...
|
|
jpeg_finish_compress(&cinfo);
|
|
|
|
Since the JPEG parameters are not altered between writing the table file and
|
|
the abbreviated image file, the same tables are sure to be used. Of course,
|
|
you can repeat the jpeg_start_compress() ... jpeg_finish_compress() sequence
|
|
many times to produce many abbreviated image files matching the table file.
|
|
|
|
You cannot suppress output of the computed Huffman tables when Huffman
|
|
optimization is selected. (If you could, there'd be no way to decode the
|
|
image...) Generally, you don't want to set optimize_coding = TRUE when
|
|
you are trying to produce abbreviated files.
|
|
|
|
In some cases you might want to compress an image using tables which are
|
|
not stored in the application, but are defined in an interchange or
|
|
tables-only file readable by the application. This can be done by setting up
|
|
a JPEG decompression object to read the specification file, then copying the
|
|
tables into your compression object. See jpeg_copy_critical_parameters()
|
|
for an example of copying quantization tables.
|
|
|
|
|
|
To read abbreviated image files, you simply need to load the proper tables
|
|
into the decompression object before trying to read the abbreviated image.
|
|
If the proper tables are stored in the application program, you can just
|
|
allocate the table structs and fill in their contents directly. For example,
|
|
to load a fixed quantization table into table slot "n":
|
|
|
|
if (cinfo.quant_tbl_ptrs[n] == NULL)
|
|
cinfo.quant_tbl_ptrs[n] = jpeg_alloc_quant_table((j_common_ptr) &cinfo);
|
|
quant_ptr = cinfo.quant_tbl_ptrs[n]; /* quant_ptr is JQUANT_TBL* */
|
|
for (i = 0; i < 64; i++) {
|
|
/* Qtable[] is desired quantization table, in natural array order */
|
|
quant_ptr->quantval[i] = Qtable[i];
|
|
}
|
|
|
|
Code to load a fixed Huffman table is typically (for AC table "n"):
|
|
|
|
if (cinfo.ac_huff_tbl_ptrs[n] == NULL)
|
|
cinfo.ac_huff_tbl_ptrs[n] = jpeg_alloc_huff_table((j_common_ptr) &cinfo);
|
|
huff_ptr = cinfo.ac_huff_tbl_ptrs[n]; /* huff_ptr is JHUFF_TBL* */
|
|
for (i = 1; i <= 16; i++) {
|
|
/* counts[i] is number of Huffman codes of length i bits, i=1..16 */
|
|
huff_ptr->bits[i] = counts[i];
|
|
}
|
|
for (i = 0; i < 256; i++) {
|
|
/* symbols[] is the list of Huffman symbols, in code-length order */
|
|
huff_ptr->huffval[i] = symbols[i];
|
|
}
|
|
|
|
(Note that trying to set cinfo.quant_tbl_ptrs[n] to point directly at a
|
|
constant JQUANT_TBL object is not safe. If the incoming file happened to
|
|
contain a quantization table definition, your master table would get
|
|
overwritten! Instead allocate a working table copy and copy the master table
|
|
into it, as illustrated above. Ditto for Huffman tables, of course.)
|
|
|
|
You might want to read the tables from a tables-only file, rather than
|
|
hard-wiring them into your application. The jpeg_read_header() call is
|
|
sufficient to read a tables-only file. You must pass a second parameter of
|
|
FALSE to indicate that you do not require an image to be present. Thus, the
|
|
typical scenario is
|
|
|
|
create JPEG decompression object
|
|
set source to tables-only file
|
|
jpeg_read_header(&cinfo, FALSE);
|
|
set source to abbreviated image file
|
|
jpeg_read_header(&cinfo, TRUE);
|
|
set decompression parameters
|
|
jpeg_start_decompress(&cinfo);
|
|
read data...
|
|
jpeg_finish_decompress(&cinfo);
|
|
|
|
In some cases, you may want to read a file without knowing whether it contains
|
|
an image or just tables. In that case, pass FALSE and check the return value
|
|
from jpeg_read_header(): it will be JPEG_HEADER_OK if an image was found,
|
|
JPEG_HEADER_TABLES_ONLY if only tables were found. (A third return value,
|
|
JPEG_SUSPENDED, is possible when using a suspending data source manager.)
|
|
Note that jpeg_read_header() will not complain if you read an abbreviated
|
|
image for which you haven't loaded the missing tables; the missing-table check
|
|
occurs later, in jpeg_start_decompress().
|
|
|
|
|
|
It is possible to read a series of images from a single source file by
|
|
repeating the jpeg_read_header() ... jpeg_finish_decompress() sequence,
|
|
without releasing/recreating the JPEG object or the data source module.
|
|
(If you did reinitialize, any partial bufferload left in the data source
|
|
buffer at the end of one image would be discarded, causing you to lose the
|
|
start of the next image.) When you use this method, stored tables are
|
|
automatically carried forward, so some of the images can be abbreviated images
|
|
that depend on tables from earlier images.
|
|
|
|
If you intend to write a series of images into a single destination file,
|
|
you might want to make a specialized data destination module that doesn't
|
|
flush the output buffer at term_destination() time. This would speed things
|
|
up by some trifling amount. Of course, you'd need to remember to flush the
|
|
buffer after the last image. You can make the later images be abbreviated
|
|
ones by passing FALSE to jpeg_start_compress().
|
|
|
|
|
|
Special markers
|
|
---------------
|
|
|
|
Some applications may need to insert or extract special data in the JPEG
|
|
datastream. The JPEG standard provides marker types "COM" (comment) and
|
|
"APP0" through "APP15" (application) to hold application-specific data.
|
|
Unfortunately, the use of these markers is not specified by the standard.
|
|
COM markers are fairly widely used to hold user-supplied text. The JFIF file
|
|
format spec uses APP0 markers with specified initial strings to hold certain
|
|
data. Adobe applications use APP14 markers beginning with the string "Adobe"
|
|
for miscellaneous data. Other APPn markers are rarely seen, but might
|
|
contain almost anything.
|
|
|
|
If you wish to store user-supplied text, we recommend you use COM markers
|
|
and place readable 7-bit ASCII text in them. Newline conventions are not
|
|
standardized --- expect to find LF (Unix style), CR/LF (DOS style), or CR
|
|
(Mac style). A robust COM reader should be able to cope with random binary
|
|
garbage, including nulls, since some applications generate COM markers
|
|
containing non-ASCII junk. (But yours should not be one of them.)
|
|
|
|
For program-supplied data, use an APPn marker, and be sure to begin it with an
|
|
identifying string so that you can tell whether the marker is actually yours.
|
|
It's probably best to avoid using APP0 or APP14 for any private markers.
|
|
(NOTE: the upcoming SPIFF standard will use APP8 markers; we recommend you
|
|
not use APP8 markers for any private purposes, either.)
|
|
|
|
Keep in mind that at most 65533 bytes can be put into one marker, but you
|
|
can have as many markers as you like.
|
|
|
|
By default, the IJG compression library will write a JFIF APP0 marker if the
|
|
selected JPEG colorspace is grayscale or YCbCr, or an Adobe APP14 marker if
|
|
the selected colorspace is RGB, CMYK, or YCCK. You can disable this, but
|
|
we don't recommend it. The decompression library will recognize JFIF and
|
|
Adobe markers and will set the JPEG colorspace properly when one is found.
|
|
|
|
|
|
You can write special markers immediately following the datastream header by
|
|
calling jpeg_write_marker() after jpeg_start_compress() and before the first
|
|
call to jpeg_write_scanlines(). When you do this, the markers appear after
|
|
the SOI and the JFIF APP0 and Adobe APP14 markers (if written), but before
|
|
all else. Specify the marker type parameter as "JPEG_COM" for COM or
|
|
"JPEG_APP0 + n" for APPn. (Actually, jpeg_write_marker will let you write
|
|
any marker type, but we don't recommend writing any other kinds of marker.)
|
|
For example, to write a user comment string pointed to by comment_text:
|
|
jpeg_write_marker(cinfo, JPEG_COM, comment_text, strlen(comment_text));
|
|
|
|
If it's not convenient to store all the marker data in memory at once,
|
|
you can instead call jpeg_write_m_header() followed by multiple calls to
|
|
jpeg_write_m_byte(). If you do it this way, it's your responsibility to
|
|
call jpeg_write_m_byte() exactly the number of times given in the length
|
|
parameter to jpeg_write_m_header(). (This method lets you empty the
|
|
output buffer partway through a marker, which might be important when
|
|
using a suspending data destination module. In any case, if you are using
|
|
a suspending destination, you should flush its buffer after inserting
|
|
any special markers. See "I/O suspension".)
|
|
|
|
Or, if you prefer to synthesize the marker byte sequence yourself,
|
|
you can just cram it straight into the data destination module.
|
|
|
|
If you are writing JFIF 1.02 extension markers (thumbnail images), don't
|
|
forget to set cinfo.JFIF_minor_version = 2 so that the encoder will write the
|
|
correct JFIF version number in the JFIF header marker. The library's default
|
|
is to write version 1.01, but that's wrong if you insert any 1.02 extension
|
|
markers. (We could probably get away with just defaulting to 1.02, but there
|
|
used to be broken decoders that would complain about unknown minor version
|
|
numbers. To reduce compatibility risks it's safest not to write 1.02 unless
|
|
you are actually using 1.02 extensions.)
|
|
|
|
|
|
When reading, two methods of handling special markers are available:
|
|
1. You can ask the library to save the contents of COM and/or APPn markers
|
|
into memory, and then examine them at your leisure afterwards.
|
|
2. You can supply your own routine to process COM and/or APPn markers
|
|
on-the-fly as they are read.
|
|
The first method is simpler to use, especially if you are using a suspending
|
|
data source; writing a marker processor that copes with input suspension is
|
|
not easy (consider what happens if the marker is longer than your available
|
|
input buffer). However, the second method conserves memory since the marker
|
|
data need not be kept around after it's been processed.
|
|
|
|
For either method, you'd normally set up marker handling after creating a
|
|
decompression object and before calling jpeg_read_header(), because the
|
|
markers of interest will typically be near the head of the file and so will
|
|
be scanned by jpeg_read_header. Once you've established a marker handling
|
|
method, it will be used for the life of that decompression object
|
|
(potentially many datastreams), unless you change it. Marker handling is
|
|
determined separately for COM markers and for each APPn marker code.
|
|
|
|
|
|
To save the contents of special markers in memory, call
|
|
jpeg_save_markers(cinfo, marker_code, length_limit)
|
|
where marker_code is the marker type to save, JPEG_COM or JPEG_APP0+n.
|
|
(To arrange to save all the special marker types, you need to call this
|
|
routine 17 times, for COM and APP0-APP15.) If the incoming marker is longer
|
|
than length_limit data bytes, only length_limit bytes will be saved; this
|
|
parameter allows you to avoid chewing up memory when you only need to see the
|
|
first few bytes of a potentially large marker. If you want to save all the
|
|
data, set length_limit to 0xFFFF; that is enough since marker lengths are only
|
|
16 bits. As a special case, setting length_limit to 0 prevents that marker
|
|
type from being saved at all. (That is the default behavior, in fact.)
|
|
|
|
After jpeg_read_header() completes, you can examine the special markers by
|
|
following the cinfo->marker_list pointer chain. All the special markers in
|
|
the file appear in this list, in order of their occurrence in the file (but
|
|
omitting any markers of types you didn't ask for). Both the original data
|
|
length and the saved data length are recorded for each list entry; the latter
|
|
will not exceed length_limit for the particular marker type. Note that these
|
|
lengths exclude the marker length word, whereas the stored representation
|
|
within the JPEG file includes it. (Hence the maximum data length is really
|
|
only 65533.)
|
|
|
|
It is possible that additional special markers appear in the file beyond the
|
|
SOS marker at which jpeg_read_header stops; if so, the marker list will be
|
|
extended during reading of the rest of the file. This is not expected to be
|
|
common, however. If you are short on memory you may want to reset the length
|
|
limit to zero for all marker types after finishing jpeg_read_header, to
|
|
ensure that the max_memory_to_use setting cannot be exceeded due to addition
|
|
of later markers.
|
|
|
|
The marker list remains stored until you call jpeg_finish_decompress or
|
|
jpeg_abort, at which point the memory is freed and the list is set to empty.
|
|
(jpeg_destroy also releases the storage, of course.)
|
|
|
|
Note that the library is internally interested in APP0 and APP14 markers;
|
|
if you try to set a small nonzero length limit on these types, the library
|
|
will silently force the length up to the minimum it wants. (But you can set
|
|
a zero length limit to prevent them from being saved at all.) Also, in a
|
|
16-bit environment, the maximum length limit may be constrained to less than
|
|
65533 by malloc() limitations. It is therefore best not to assume that the
|
|
effective length limit is exactly what you set it to be.
|
|
|
|
|
|
If you want to supply your own marker-reading routine, you do it by calling
|
|
jpeg_set_marker_processor(). A marker processor routine must have the
|
|
signature
|
|
boolean jpeg_marker_parser_method (j_decompress_ptr cinfo)
|
|
Although the marker code is not explicitly passed, the routine can find it
|
|
in cinfo->unread_marker. At the time of call, the marker proper has been
|
|
read from the data source module. The processor routine is responsible for
|
|
reading the marker length word and the remaining parameter bytes, if any.
|
|
Return TRUE to indicate success. (FALSE should be returned only if you are
|
|
using a suspending data source and it tells you to suspend. See the standard
|
|
marker processors in jdmarker.c for appropriate coding methods if you need to
|
|
use a suspending data source.)
|
|
|
|
If you override the default APP0 or APP14 processors, it is up to you to
|
|
recognize JFIF and Adobe markers if you want colorspace recognition to occur
|
|
properly. We recommend copying and extending the default processors if you
|
|
want to do that. (A better idea is to save these marker types for later
|
|
examination by calling jpeg_save_markers(); that method doesn't interfere
|
|
with the library's own processing of these markers.)
|
|
|
|
jpeg_set_marker_processor() and jpeg_save_markers() are mutually exclusive
|
|
--- if you call one it overrides any previous call to the other, for the
|
|
particular marker type specified.
|
|
|
|
A simple example of an external COM processor can be found in djpeg.c.
|
|
Also, see jpegtran.c for an example of using jpeg_save_markers.
|
|
|
|
|
|
Raw (downsampled) image data
|
|
----------------------------
|
|
|
|
Some applications need to supply already-downsampled image data to the JPEG
|
|
compressor, or to receive raw downsampled data from the decompressor. The
|
|
library supports this requirement by allowing the application to write or
|
|
read raw data, bypassing the normal preprocessing or postprocessing steps.
|
|
The interface is different from the standard one and is somewhat harder to
|
|
use. If your interest is merely in bypassing color conversion, we recommend
|
|
that you use the standard interface and simply set jpeg_color_space =
|
|
in_color_space (or jpeg_color_space = out_color_space for decompression).
|
|
The mechanism described in this section is necessary only to supply or
|
|
receive downsampled image data, in which not all components have the same
|
|
dimensions.
|
|
|
|
|
|
To compress raw data, you must supply the data in the colorspace to be used
|
|
in the JPEG file (please read the earlier section on Special color spaces)
|
|
and downsampled to the sampling factors specified in the JPEG parameters.
|
|
You must supply the data in the format used internally by the JPEG library,
|
|
namely a JSAMPIMAGE array. This is an array of pointers to two-dimensional
|
|
arrays, each of type JSAMPARRAY. Each 2-D array holds the values for one
|
|
color component. This structure is necessary since the components are of
|
|
different sizes. If the image dimensions are not a multiple of the MCU size,
|
|
you must also pad the data correctly (usually, this is done by replicating
|
|
the last column and/or row). The data must be padded to a multiple of a DCT
|
|
block in each component: that is, each downsampled row must contain a
|
|
multiple of 8 valid samples, and there must be a multiple of 8 sample rows
|
|
for each component. (For applications such as conversion of digital TV
|
|
images, the standard image size is usually a multiple of the DCT block size,
|
|
so that no padding need actually be done.)
|
|
|
|
The procedure for compression of raw data is basically the same as normal
|
|
compression, except that you call jpeg_write_raw_data() in place of
|
|
jpeg_write_scanlines(). Before calling jpeg_start_compress(), you must do
|
|
the following:
|
|
* Set cinfo->raw_data_in to TRUE. (It is set FALSE by jpeg_set_defaults().)
|
|
This notifies the library that you will be supplying raw data.
|
|
* Ensure jpeg_color_space is correct --- an explicit jpeg_set_colorspace()
|
|
call is a good idea. Note that since color conversion is bypassed,
|
|
in_color_space is ignored, except that jpeg_set_defaults() uses it to
|
|
choose the default jpeg_color_space setting.
|
|
* Ensure the sampling factors, cinfo->comp_info[i].h_samp_factor and
|
|
cinfo->comp_info[i].v_samp_factor, are correct. Since these indicate the
|
|
dimensions of the data you are supplying, it's wise to set them
|
|
explicitly, rather than assuming the library's defaults are what you want.
|
|
|
|
To pass raw data to the library, call jpeg_write_raw_data() in place of
|
|
jpeg_write_scanlines(). The two routines work similarly except that
|
|
jpeg_write_raw_data takes a JSAMPIMAGE data array rather than JSAMPARRAY.
|
|
The scanlines count passed to and returned from jpeg_write_raw_data is
|
|
measured in terms of the component with the largest v_samp_factor.
|
|
|
|
jpeg_write_raw_data() processes one MCU row per call, which is to say
|
|
v_samp_factor*DCTSIZE sample rows of each component. The passed num_lines
|
|
value must be at least max_v_samp_factor*DCTSIZE, and the return value will
|
|
be exactly that amount (or possibly some multiple of that amount, in future
|
|
library versions). This is true even on the last call at the bottom of the
|
|
image; don't forget to pad your data as necessary.
|
|
|
|
The required dimensions of the supplied data can be computed for each
|
|
component as
|
|
cinfo->comp_info[i].width_in_blocks*DCTSIZE samples per row
|
|
cinfo->comp_info[i].height_in_blocks*DCTSIZE rows in image
|
|
after jpeg_start_compress() has initialized those fields. If the valid data
|
|
is smaller than this, it must be padded appropriately. For some sampling
|
|
factors and image sizes, additional dummy DCT blocks are inserted to make
|
|
the image a multiple of the MCU dimensions. The library creates such dummy
|
|
blocks itself; it does not read them from your supplied data. Therefore you
|
|
need never pad by more than DCTSIZE samples. An example may help here.
|
|
Assume 2h2v downsampling of YCbCr data, that is
|
|
cinfo->comp_info[0].h_samp_factor = 2 for Y
|
|
cinfo->comp_info[0].v_samp_factor = 2
|
|
cinfo->comp_info[1].h_samp_factor = 1 for Cb
|
|
cinfo->comp_info[1].v_samp_factor = 1
|
|
cinfo->comp_info[2].h_samp_factor = 1 for Cr
|
|
cinfo->comp_info[2].v_samp_factor = 1
|
|
and suppose that the nominal image dimensions (cinfo->image_width and
|
|
cinfo->image_height) are 101x101 pixels. Then jpeg_start_compress() will
|
|
compute downsampled_width = 101 and width_in_blocks = 13 for Y,
|
|
downsampled_width = 51 and width_in_blocks = 7 for Cb and Cr (and the same
|
|
for the height fields). You must pad the Y data to at least 13*8 = 104
|
|
columns and rows, the Cb/Cr data to at least 7*8 = 56 columns and rows. The
|
|
MCU height is max_v_samp_factor = 2 DCT rows so you must pass at least 16
|
|
scanlines on each call to jpeg_write_raw_data(), which is to say 16 actual
|
|
sample rows of Y and 8 each of Cb and Cr. A total of 7 MCU rows are needed,
|
|
so you must pass a total of 7*16 = 112 "scanlines". The last DCT block row
|
|
of Y data is dummy, so it doesn't matter what you pass for it in the data
|
|
arrays, but the scanlines count must total up to 112 so that all of the Cb
|
|
and Cr data gets passed.
|
|
|
|
Output suspension is supported with raw-data compression: if the data
|
|
destination module suspends, jpeg_write_raw_data() will return 0.
|
|
In this case the same data rows must be passed again on the next call.
|
|
|
|
|
|
Decompression with raw data output implies bypassing all postprocessing:
|
|
you cannot ask for rescaling or color quantization, for instance. More
|
|
seriously, you must deal with the color space and sampling factors present in
|
|
the incoming file. If your application only handles, say, 2h1v YCbCr data,
|
|
you must check for and fail on other color spaces or other sampling factors.
|
|
The library will not convert to a different color space for you.
|
|
|
|
To obtain raw data output, set cinfo->raw_data_out = TRUE before
|
|
jpeg_start_decompress() (it is set FALSE by jpeg_read_header()). Be sure to
|
|
verify that the color space and sampling factors are ones you can handle.
|
|
Then call jpeg_read_raw_data() in place of jpeg_read_scanlines(). The
|
|
decompression process is otherwise the same as usual.
|
|
|
|
jpeg_read_raw_data() returns one MCU row per call, and thus you must pass a
|
|
buffer of at least max_v_samp_factor*DCTSIZE scanlines (scanline counting is
|
|
the same as for raw-data compression). The buffer you pass must be large
|
|
enough to hold the actual data plus padding to DCT-block boundaries. As with
|
|
compression, any entirely dummy DCT blocks are not processed so you need not
|
|
allocate space for them, but the total scanline count includes them. The
|
|
above example of computing buffer dimensions for raw-data compression is
|
|
equally valid for decompression.
|
|
|
|
Input suspension is supported with raw-data decompression: if the data source
|
|
module suspends, jpeg_read_raw_data() will return 0. You can also use
|
|
buffered-image mode to read raw data in multiple passes.
|
|
|
|
|
|
Really raw data: DCT coefficients
|
|
---------------------------------
|
|
|
|
It is possible to read or write the contents of a JPEG file as raw DCT
|
|
coefficients. This facility is mainly intended for use in lossless
|
|
transcoding between different JPEG file formats. Other possible applications
|
|
include lossless cropping of a JPEG image, lossless reassembly of a
|
|
multi-strip or multi-tile TIFF/JPEG file into a single JPEG datastream, etc.
|
|
|
|
To read the contents of a JPEG file as DCT coefficients, open the file and do
|
|
jpeg_read_header() as usual. But instead of calling jpeg_start_decompress()
|
|
and jpeg_read_scanlines(), call jpeg_read_coefficients(). This will read the
|
|
entire image into a set of virtual coefficient-block arrays, one array per
|
|
component. The return value is a pointer to an array of virtual-array
|
|
descriptors. Each virtual array can be accessed directly using the JPEG
|
|
memory manager's access_virt_barray method (see Memory management, below,
|
|
and also read structure.doc's discussion of virtual array handling). Or,
|
|
for simple transcoding to a different JPEG file format, the array list can
|
|
just be handed directly to jpeg_write_coefficients().
|
|
|
|
Each block in the block arrays contains quantized coefficient values in
|
|
normal array order (not JPEG zigzag order). The block arrays contain only
|
|
DCT blocks containing real data; any entirely-dummy blocks added to fill out
|
|
interleaved MCUs at the right or bottom edges of the image are discarded
|
|
during reading and are not stored in the block arrays. (The size of each
|
|
block array can be determined from the width_in_blocks and height_in_blocks
|
|
fields of the component's comp_info entry.) This is also the data format
|
|
expected by jpeg_write_coefficients().
|
|
|
|
When you are done using the virtual arrays, call jpeg_finish_decompress()
|
|
to release the array storage and return the decompression object to an idle
|
|
state; or just call jpeg_destroy() if you don't need to reuse the object.
|
|
|
|
If you use a suspending data source, jpeg_read_coefficients() will return
|
|
NULL if it is forced to suspend; a non-NULL return value indicates successful
|
|
completion. You need not test for a NULL return value when using a
|
|
non-suspending data source.
|
|
|
|
It is also possible to call jpeg_read_coefficients() to obtain access to the
|
|
decoder's coefficient arrays during a normal decode cycle in buffered-image
|
|
mode. This frammish might be useful for progressively displaying an incoming
|
|
image and then re-encoding it without loss. To do this, decode in buffered-
|
|
image mode as discussed previously, then call jpeg_read_coefficients() after
|
|
the last jpeg_finish_output() call. The arrays will be available for your use
|
|
until you call jpeg_finish_decompress().
|
|
|
|
|
|
To write the contents of a JPEG file as DCT coefficients, you must provide
|
|
the DCT coefficients stored in virtual block arrays. You can either pass
|
|
block arrays read from an input JPEG file by jpeg_read_coefficients(), or
|
|
allocate virtual arrays from the JPEG compression object and fill them
|
|
yourself. In either case, jpeg_write_coefficients() is substituted for
|
|
jpeg_start_compress() and jpeg_write_scanlines(). Thus the sequence is
|
|
* Create compression object
|
|
* Set all compression parameters as necessary
|
|
* Request virtual arrays if needed
|
|
* jpeg_write_coefficients()
|
|
* jpeg_finish_compress()
|
|
* Destroy or re-use compression object
|
|
jpeg_write_coefficients() is passed a pointer to an array of virtual block
|
|
array descriptors; the number of arrays is equal to cinfo.num_components.
|
|
|
|
The virtual arrays need only have been requested, not realized, before
|
|
jpeg_write_coefficients() is called. A side-effect of
|
|
jpeg_write_coefficients() is to realize any virtual arrays that have been
|
|
requested from the compression object's memory manager. Thus, when obtaining
|
|
the virtual arrays from the compression object, you should fill the arrays
|
|
after calling jpeg_write_coefficients(). The data is actually written out
|
|
when you call jpeg_finish_compress(); jpeg_write_coefficients() only writes
|
|
the file header.
|
|
|
|
When writing raw DCT coefficients, it is crucial that the JPEG quantization
|
|
tables and sampling factors match the way the data was encoded, or the
|
|
resulting file will be invalid. For transcoding from an existing JPEG file,
|
|
we recommend using jpeg_copy_critical_parameters(). This routine initializes
|
|
all the compression parameters to default values (like jpeg_set_defaults()),
|
|
then copies the critical information from a source decompression object.
|
|
The decompression object should have just been used to read the entire
|
|
JPEG input file --- that is, it should be awaiting jpeg_finish_decompress().
|
|
|
|
jpeg_write_coefficients() marks all tables stored in the compression object
|
|
as needing to be written to the output file (thus, it acts like
|
|
jpeg_start_compress(cinfo, TRUE)). This is for safety's sake, to avoid
|
|
emitting abbreviated JPEG files by accident. If you really want to emit an
|
|
abbreviated JPEG file, call jpeg_suppress_tables(), or set the tables'
|
|
individual sent_table flags, between calling jpeg_write_coefficients() and
|
|
jpeg_finish_compress().
|
|
|
|
|
|
Progress monitoring
|
|
-------------------
|
|
|
|
Some applications may need to regain control from the JPEG library every so
|
|
often. The typical use of this feature is to produce a percent-done bar or
|
|
other progress display. (For a simple example, see cjpeg.c or djpeg.c.)
|
|
Although you do get control back frequently during the data-transferring pass
|
|
(the jpeg_read_scanlines or jpeg_write_scanlines loop), any additional passes
|
|
will occur inside jpeg_finish_compress or jpeg_start_decompress; those
|
|
routines may take a long time to execute, and you don't get control back
|
|
until they are done.
|
|
|
|
You can define a progress-monitor routine which will be called periodically
|
|
by the library. No guarantees are made about how often this call will occur,
|
|
so we don't recommend you use it for mouse tracking or anything like that.
|
|
At present, a call will occur once per MCU row, scanline, or sample row
|
|
group, whichever unit is convenient for the current processing mode; so the
|
|
wider the image, the longer the time between calls. During the data
|
|
transferring pass, only one call occurs per call of jpeg_read_scanlines or
|
|
jpeg_write_scanlines, so don't pass a large number of scanlines at once if
|
|
you want fine resolution in the progress count. (If you really need to use
|
|
the callback mechanism for time-critical tasks like mouse tracking, you could
|
|
insert additional calls inside some of the library's inner loops.)
|
|
|
|
To establish a progress-monitor callback, create a struct jpeg_progress_mgr,
|
|
fill in its progress_monitor field with a pointer to your callback routine,
|
|
and set cinfo->progress to point to the struct. The callback will be called
|
|
whenever cinfo->progress is non-NULL. (This pointer is set to NULL by
|
|
jpeg_create_compress or jpeg_create_decompress; the library will not change
|
|
it thereafter. So if you allocate dynamic storage for the progress struct,
|
|
make sure it will live as long as the JPEG object does. Allocating from the
|
|
JPEG memory manager with lifetime JPOOL_PERMANENT will work nicely.) You
|
|
can use the same callback routine for both compression and decompression.
|
|
|
|
The jpeg_progress_mgr struct contains four fields which are set by the library:
|
|
long pass_counter; /* work units completed in this pass */
|
|
long pass_limit; /* total number of work units in this pass */
|
|
int completed_passes; /* passes completed so far */
|
|
int total_passes; /* total number of passes expected */
|
|
During any one pass, pass_counter increases from 0 up to (not including)
|
|
pass_limit; the step size is usually but not necessarily 1. The pass_limit
|
|
value may change from one pass to another. The expected total number of
|
|
passes is in total_passes, and the number of passes already completed is in
|
|
completed_passes. Thus the fraction of work completed may be estimated as
|
|
completed_passes + (pass_counter/pass_limit)
|
|
--------------------------------------------
|
|
total_passes
|
|
ignoring the fact that the passes may not be equal amounts of work.
|
|
|
|
When decompressing, pass_limit can even change within a pass, because it
|
|
depends on the number of scans in the JPEG file, which isn't always known in
|
|
advance. The computed fraction-of-work-done may jump suddenly (if the library
|
|
discovers it has overestimated the number of scans) or even decrease (in the
|
|
opposite case). It is not wise to put great faith in the work estimate.
|
|
|
|
When using the decompressor's buffered-image mode, the progress monitor work
|
|
estimate is likely to be completely unhelpful, because the library has no way
|
|
to know how many output passes will be demanded of it. Currently, the library
|
|
sets total_passes based on the assumption that there will be one more output
|
|
pass if the input file end hasn't yet been read (jpeg_input_complete() isn't
|
|
TRUE), but no more output passes if the file end has been reached when the
|
|
output pass is started. This means that total_passes will rise as additional
|
|
output passes are requested. If you have a way of determining the input file
|
|
size, estimating progress based on the fraction of the file that's been read
|
|
will probably be more useful than using the library's value.
|
|
|
|
|
|
Memory management
|
|
-----------------
|
|
|
|
This section covers some key facts about the JPEG library's built-in memory
|
|
manager. For more info, please read structure.doc's section about the memory
|
|
manager, and consult the source code if necessary.
|
|
|
|
All memory and temporary file allocation within the library is done via the
|
|
memory manager. If necessary, you can replace the "back end" of the memory
|
|
manager to control allocation yourself (for example, if you don't want the
|
|
library to use malloc() and free() for some reason).
|
|
|
|
Some data is allocated "permanently" and will not be freed until the JPEG
|
|
object is destroyed. Most data is allocated "per image" and is freed by
|
|
jpeg_finish_compress, jpeg_finish_decompress, or jpeg_abort. You can call the
|
|
memory manager yourself to allocate structures that will automatically be
|
|
freed at these times. Typical code for this is
|
|
ptr = (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, size);
|
|
Use JPOOL_PERMANENT to get storage that lasts as long as the JPEG object.
|
|
Use alloc_large instead of alloc_small for anything bigger than a few Kbytes.
|
|
There are also alloc_sarray and alloc_barray routines that automatically
|
|
build 2-D sample or block arrays.
|
|
|
|
The library's minimum space requirements to process an image depend on the
|
|
image's width, but not on its height, because the library ordinarily works
|
|
with "strip" buffers that are as wide as the image but just a few rows high.
|
|
Some operating modes (eg, two-pass color quantization) require full-image
|
|
buffers. Such buffers are treated as "virtual arrays": only the current strip
|
|
need be in memory, and the rest can be swapped out to a temporary file.
|
|
|
|
If you use the simplest memory manager back end (jmemnobs.c), then no
|
|
temporary files are used; virtual arrays are simply malloc()'d. Images bigger
|
|
than memory can be processed only if your system supports virtual memory.
|
|
The other memory manager back ends support temporary files of various flavors
|
|
and thus work in machines without virtual memory. They may also be useful on
|
|
Unix machines if you need to process images that exceed available swap space.
|
|
|
|
When using temporary files, the library will make the in-memory buffers for
|
|
its virtual arrays just big enough to stay within a "maximum memory" setting.
|
|
Your application can set this limit by setting cinfo->mem->max_memory_to_use
|
|
after creating the JPEG object. (Of course, there is still a minimum size for
|
|
the buffers, so the max-memory setting is effective only if it is bigger than
|
|
the minimum space needed.) If you allocate any large structures yourself, you
|
|
must allocate them before jpeg_start_compress() or jpeg_start_decompress() in
|
|
order to have them counted against the max memory limit. Also keep in mind
|
|
that space allocated with alloc_small() is ignored, on the assumption that
|
|
it's too small to be worth worrying about; so a reasonable safety margin
|
|
should be left when setting max_memory_to_use.
|
|
|
|
If you use the jmemname.c or jmemdos.c memory manager back end, it is
|
|
important to clean up the JPEG object properly to ensure that the temporary
|
|
files get deleted. (This is especially crucial with jmemdos.c, where the
|
|
"temporary files" may be extended-memory segments; if they are not freed,
|
|
DOS will require a reboot to recover the memory.) Thus, with these memory
|
|
managers, it's a good idea to provide a signal handler that will trap any
|
|
early exit from your program. The handler should call either jpeg_abort()
|
|
or jpeg_destroy() for any active JPEG objects. A handler is not needed with
|
|
jmemnobs.c, and shouldn't be necessary with jmemansi.c or jmemmac.c either,
|
|
since the C library is supposed to take care of deleting files made with
|
|
tmpfile().
|
|
|
|
|
|
Memory usage
|
|
------------
|
|
|
|
Working memory requirements while performing compression or decompression
|
|
depend on image dimensions, image characteristics (such as colorspace and
|
|
JPEG process), and operating mode (application-selected options).
|
|
|
|
As of v6b, the decompressor requires:
|
|
1. About 24K in more-or-less-fixed-size data. This varies a bit depending
|
|
on operating mode and image characteristics (particularly color vs.
|
|
grayscale), but it doesn't depend on image dimensions.
|
|
2. Strip buffers (of size proportional to the image width) for IDCT and
|
|
upsampling results. The worst case for commonly used sampling factors
|
|
is about 34 bytes * width in pixels for a color image. A grayscale image
|
|
only needs about 8 bytes per pixel column.
|
|
3. A full-image DCT coefficient buffer is needed to decode a multi-scan JPEG
|
|
file (including progressive JPEGs), or whenever you select buffered-image
|
|
mode. This takes 2 bytes/coefficient. At typical 2x2 sampling, that's
|
|
3 bytes per pixel for a color image. Worst case (1x1 sampling) requires
|
|
6 bytes/pixel. For grayscale, figure 2 bytes/pixel.
|
|
4. To perform 2-pass color quantization, the decompressor also needs a
|
|
128K color lookup table and a full-image pixel buffer (3 bytes/pixel).
|
|
This does not count any memory allocated by the application, such as a
|
|
buffer to hold the final output image.
|
|
|
|
The above figures are valid for 8-bit JPEG data precision and a machine with
|
|
32-bit ints. For 12-bit JPEG data, double the size of the strip buffers and
|
|
quantization pixel buffer. The "fixed-size" data will be somewhat smaller
|
|
with 16-bit ints, larger with 64-bit ints. Also, CMYK or other unusual
|
|
color spaces will require different amounts of space.
|
|
|
|
The full-image coefficient and pixel buffers, if needed at all, do not
|
|
have to be fully RAM resident; you can have the library use temporary
|
|
files instead when the total memory usage would exceed a limit you set.
|
|
(But if your OS supports virtual memory, it's probably better to just use
|
|
jmemnobs and let the OS do the swapping.)
|
|
|
|
The compressor's memory requirements are similar, except that it has no need
|
|
for color quantization. Also, it needs a full-image DCT coefficient buffer
|
|
if Huffman-table optimization is asked for, even if progressive mode is not
|
|
requested.
|
|
|
|
If you need more detailed information about memory usage in a particular
|
|
situation, you can enable the MEM_STATS code in jmemmgr.c.
|
|
|
|
|
|
Library compile-time options
|
|
----------------------------
|
|
|
|
A number of compile-time options are available by modifying jmorecfg.h.
|
|
|
|
The JPEG standard provides for both the baseline 8-bit DCT process and
|
|
a 12-bit DCT process. The IJG code supports 12-bit lossy JPEG if you define
|
|
BITS_IN_JSAMPLE as 12 rather than 8. Note that this causes JSAMPLE to be
|
|
larger than a char, so it affects the surrounding application's image data.
|
|
The sample applications cjpeg and djpeg can support 12-bit mode only for PPM
|
|
and GIF file formats; you must disable the other file formats to compile a
|
|
12-bit cjpeg or djpeg. (install.doc has more information about that.)
|
|
At present, a 12-bit library can handle *only* 12-bit images, not both
|
|
precisions. (If you need to include both 8- and 12-bit libraries in a single
|
|
application, you could probably do it by defining NEED_SHORT_EXTERNAL_NAMES
|
|
for just one of the copies. You'd have to access the 8-bit and 12-bit copies
|
|
from separate application source files. This is untested ... if you try it,
|
|
we'd like to hear whether it works!)
|
|
|
|
Note that a 12-bit library always compresses in Huffman optimization mode,
|
|
in order to generate valid Huffman tables. This is necessary because our
|
|
default Huffman tables only cover 8-bit data. If you need to output 12-bit
|
|
files in one pass, you'll have to supply suitable default Huffman tables.
|
|
You may also want to supply your own DCT quantization tables; the existing
|
|
quality-scaling code has been developed for 8-bit use, and probably doesn't
|
|
generate especially good tables for 12-bit.
|
|
|
|
The maximum number of components (color channels) in the image is determined
|
|
by MAX_COMPONENTS. The JPEG standard allows up to 255 components, but we
|
|
expect that few applications will need more than four or so.
|
|
|
|
On machines with unusual data type sizes, you may be able to improve
|
|
performance or reduce memory space by tweaking the various typedefs in
|
|
jmorecfg.h. In particular, on some RISC CPUs, access to arrays of "short"s
|
|
is quite slow; consider trading memory for speed by making JCOEF, INT16, and
|
|
UINT16 be "int" or "unsigned int". UINT8 is also a candidate to become int.
|
|
You probably don't want to make JSAMPLE be int unless you have lots of memory
|
|
to burn.
|
|
|
|
You can reduce the size of the library by compiling out various optional
|
|
functions. To do this, undefine xxx_SUPPORTED symbols as necessary.
|
|
|
|
You can also save a few K by not having text error messages in the library;
|
|
the standard error message table occupies about 5Kb. This is particularly
|
|
reasonable for embedded applications where there's no good way to display
|
|
a message anyway. To do this, remove the creation of the message table
|
|
(jpeg_std_message_table[]) from jerror.c, and alter format_message to do
|
|
something reasonable without it. You could output the numeric value of the
|
|
message code number, for example. If you do this, you can also save a couple
|
|
more K by modifying the TRACEMSn() macros in jerror.h to expand to nothing;
|
|
you don't need trace capability anyway, right?
|
|
|
|
|
|
Portability considerations
|
|
--------------------------
|
|
|
|
The JPEG library has been written to be extremely portable; the sample
|
|
applications cjpeg and djpeg are slightly less so. This section summarizes
|
|
the design goals in this area. (If you encounter any bugs that cause the
|
|
library to be less portable than is claimed here, we'd appreciate hearing
|
|
about them.)
|
|
|
|
The code works fine on ANSI C, C++, and pre-ANSI C compilers, using any of
|
|
the popular system include file setups, and some not-so-popular ones too.
|
|
See install.doc for configuration procedures.
|
|
|
|
The code is not dependent on the exact sizes of the C data types. As
|
|
distributed, we make the assumptions that
|
|
char is at least 8 bits wide
|
|
short is at least 16 bits wide
|
|
int is at least 16 bits wide
|
|
long is at least 32 bits wide
|
|
(These are the minimum requirements of the ANSI C standard.) Wider types will
|
|
work fine, although memory may be used inefficiently if char is much larger
|
|
than 8 bits or short is much bigger than 16 bits. The code should work
|
|
equally well with 16- or 32-bit ints.
|
|
|
|
In a system where these assumptions are not met, you may be able to make the
|
|
code work by modifying the typedefs in jmorecfg.h. However, you will probably
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have difficulty if int is less than 16 bits wide, since references to plain
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int abound in the code.
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char can be either signed or unsigned, although the code runs faster if an
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unsigned char type is available. If char is wider than 8 bits, you will need
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to redefine JOCTET and/or provide custom data source/destination managers so
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that JOCTET represents exactly 8 bits of data on external storage.
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The JPEG library proper does not assume ASCII representation of characters.
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But some of the image file I/O modules in cjpeg/djpeg do have ASCII
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dependencies in file-header manipulation; so does cjpeg's select_file_type()
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routine.
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The JPEG library does not rely heavily on the C library. In particular, C
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stdio is used only by the data source/destination modules and the error
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|
handler, all of which are application-replaceable. (cjpeg/djpeg are more
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|
heavily dependent on stdio.) malloc and free are called only from the memory
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|
manager "back end" module, so you can use a different memory allocator by
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|
replacing that one file.
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|
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The code generally assumes that C names must be unique in the first 15
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|
characters. However, global function names can be made unique in the
|
|
first 6 characters by defining NEED_SHORT_EXTERNAL_NAMES.
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|
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More info about porting the code may be gleaned by reading jconfig.doc,
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|
jmorecfg.h, and jinclude.h.
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|
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Notes for MS-DOS implementors
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-----------------------------
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The IJG code is designed to work efficiently in 80x86 "small" or "medium"
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|
memory models (i.e., data pointers are 16 bits unless explicitly declared
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|
"far"; code pointers can be either size). You may be able to use small
|
|
model to compile cjpeg or djpeg by itself, but you will probably have to use
|
|
medium model for any larger application. This won't make much difference in
|
|
performance. You *will* take a noticeable performance hit if you use a
|
|
large-data memory model (perhaps 10%-25%), and you should avoid "huge" model
|
|
if at all possible.
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|
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|
The JPEG library typically needs 2Kb-3Kb of stack space. It will also
|
|
malloc about 20K-30K of near heap space while executing (and lots of far
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|
heap, but that doesn't count in this calculation). This figure will vary
|
|
depending on selected operating mode, and to a lesser extent on image size.
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|
There is also about 5Kb-6Kb of constant data which will be allocated in the
|
|
near data segment (about 4Kb of this is the error message table).
|
|
Thus you have perhaps 20K available for other modules' static data and near
|
|
heap space before you need to go to a larger memory model. The C library's
|
|
static data will account for several K of this, but that still leaves a good
|
|
deal for your needs. (If you are tight on space, you could reduce the sizes
|
|
of the I/O buffers allocated by jdatasrc.c and jdatadst.c, say from 4K to
|
|
1K. Another possibility is to move the error message table to far memory;
|
|
this should be doable with only localized hacking on jerror.c.)
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|
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|
About 2K of the near heap space is "permanent" memory that will not be
|
|
released until you destroy the JPEG object. This is only an issue if you
|
|
save a JPEG object between compression or decompression operations.
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|
|
|
Far data space may also be a tight resource when you are dealing with large
|
|
images. The most memory-intensive case is decompression with two-pass color
|
|
quantization, or single-pass quantization to an externally supplied color
|
|
map. This requires a 128Kb color lookup table plus strip buffers amounting
|
|
to about 40 bytes per column for typical sampling ratios (eg, about 25600
|
|
bytes for a 640-pixel-wide image). You may not be able to process wide
|
|
images if you have large data structures of your own.
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|
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|
Of course, all of these concerns vanish if you use a 32-bit flat-memory-model
|
|
compiler, such as DJGPP or Watcom C. We highly recommend flat model if you
|
|
can use it; the JPEG library is significantly faster in flat model.
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