293 lines
9.9 KiB
Groff
Executable File
293 lines
9.9 KiB
Groff
Executable File
.TH CJPEG 1 "20 March 1998"
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.SH NAME
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cjpeg \- compress an image file to a JPEG file
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.SH SYNOPSIS
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.B cjpeg
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[
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.I options
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]
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[
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.I filename
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]
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.LP
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.SH DESCRIPTION
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.LP
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.B cjpeg
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compresses the named image file, or the standard input if no file is
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named, and produces a JPEG/JFIF file on the standard output.
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The currently supported input file formats are: PPM (PBMPLUS color
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format), PGM (PBMPLUS gray-scale format), BMP, Targa, and RLE (Utah Raster
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Toolkit format). (RLE is supported only if the URT library is available.)
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.SH OPTIONS
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All switch names may be abbreviated; for example,
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.B \-grayscale
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may be written
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.B \-gray
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or
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.BR \-gr .
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Most of the "basic" switches can be abbreviated to as little as one letter.
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Upper and lower case are equivalent (thus
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.B \-BMP
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is the same as
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.BR \-bmp ).
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British spellings are also accepted (e.g.,
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.BR \-greyscale ),
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though for brevity these are not mentioned below.
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.PP
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The basic switches are:
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.TP
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.BI \-quality " N"
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Scale quantization tables to adjust image quality. Quality is 0 (worst) to
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100 (best); default is 75. (See below for more info.)
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.TP
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.B \-grayscale
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Create monochrome JPEG file from color input. Be sure to use this switch when
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compressing a grayscale BMP file, because
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.B cjpeg
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isn't bright enough to notice whether a BMP file uses only shades of gray.
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By saying
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.BR \-grayscale ,
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you'll get a smaller JPEG file that takes less time to process.
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.TP
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.B \-optimize
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Perform optimization of entropy encoding parameters. Without this, default
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encoding parameters are used.
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.B \-optimize
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usually makes the JPEG file a little smaller, but
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.B cjpeg
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runs somewhat slower and needs much more memory. Image quality and speed of
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decompression are unaffected by
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.BR \-optimize .
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.TP
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.B \-progressive
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Create progressive JPEG file (see below).
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.TP
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.B \-targa
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Input file is Targa format. Targa files that contain an "identification"
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field will not be automatically recognized by
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.BR cjpeg ;
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for such files you must specify
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.B \-targa
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to make
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.B cjpeg
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treat the input as Targa format.
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For most Targa files, you won't need this switch.
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.PP
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The
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.B \-quality
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switch lets you trade off compressed file size against quality of the
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reconstructed image: the higher the quality setting, the larger the JPEG file,
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and the closer the output image will be to the original input. Normally you
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want to use the lowest quality setting (smallest file) that decompresses into
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something visually indistinguishable from the original image. For this
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purpose the quality setting should be between 50 and 95; the default of 75 is
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often about right. If you see defects at
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.B \-quality
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75, then go up 5 or 10 counts at a time until you are happy with the output
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image. (The optimal setting will vary from one image to another.)
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.PP
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.B \-quality
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100 will generate a quantization table of all 1's, minimizing loss in the
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quantization step (but there is still information loss in subsampling, as well
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as roundoff error). This setting is mainly of interest for experimental
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purposes. Quality values above about 95 are
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.B not
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recommended for normal use; the compressed file size goes up dramatically for
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hardly any gain in output image quality.
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.PP
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In the other direction, quality values below 50 will produce very small files
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of low image quality. Settings around 5 to 10 might be useful in preparing an
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index of a large image library, for example. Try
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.B \-quality
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2 (or so) for some amusing Cubist effects. (Note: quality
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values below about 25 generate 2-byte quantization tables, which are
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considered optional in the JPEG standard.
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.B cjpeg
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emits a warning message when you give such a quality value, because some
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other JPEG programs may be unable to decode the resulting file. Use
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.B \-baseline
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if you need to ensure compatibility at low quality values.)
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.PP
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The
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.B \-progressive
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switch creates a "progressive JPEG" file. In this type of JPEG file, the data
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is stored in multiple scans of increasing quality. If the file is being
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transmitted over a slow communications link, the decoder can use the first
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scan to display a low-quality image very quickly, and can then improve the
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display with each subsequent scan. The final image is exactly equivalent to a
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standard JPEG file of the same quality setting, and the total file size is
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about the same --- often a little smaller.
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.B Caution:
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progressive JPEG is not yet widely implemented, so many decoders will be
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unable to view a progressive JPEG file at all.
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.PP
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Switches for advanced users:
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.TP
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.B \-dct int
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Use integer DCT method (default).
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.TP
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.B \-dct fast
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Use fast integer DCT (less accurate).
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.TP
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.B \-dct float
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Use floating-point DCT method.
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The float method is very slightly more accurate than the int method, but is
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much slower unless your machine has very fast floating-point hardware. Also
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note that results of the floating-point method may vary slightly across
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machines, while the integer methods should give the same results everywhere.
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The fast integer method is much less accurate than the other two.
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.TP
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.BI \-restart " N"
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Emit a JPEG restart marker every N MCU rows, or every N MCU blocks if "B" is
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attached to the number.
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.B \-restart 0
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(the default) means no restart markers.
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.TP
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.BI \-smooth " N"
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Smooth the input image to eliminate dithering noise. N, ranging from 1 to
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100, indicates the strength of smoothing. 0 (the default) means no smoothing.
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.TP
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.BI \-maxmemory " N"
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Set limit for amount of memory to use in processing large images. Value is
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in thousands of bytes, or millions of bytes if "M" is attached to the
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number. For example,
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.B \-max 4m
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selects 4000000 bytes. If more space is needed, temporary files will be used.
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.TP
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.BI \-outfile " name"
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Send output image to the named file, not to standard output.
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.TP
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.B \-verbose
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Enable debug printout. More
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.BR \-v 's
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give more output. Also, version information is printed at startup.
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.TP
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.B \-debug
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Same as
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.BR \-verbose .
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.PP
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The
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.B \-restart
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option inserts extra markers that allow a JPEG decoder to resynchronize after
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a transmission error. Without restart markers, any damage to a compressed
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file will usually ruin the image from the point of the error to the end of the
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image; with restart markers, the damage is usually confined to the portion of
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the image up to the next restart marker. Of course, the restart markers
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occupy extra space. We recommend
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.B \-restart 1
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for images that will be transmitted across unreliable networks such as Usenet.
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.PP
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The
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.B \-smooth
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option filters the input to eliminate fine-scale noise. This is often useful
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when converting dithered images to JPEG: a moderate smoothing factor of 10 to
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50 gets rid of dithering patterns in the input file, resulting in a smaller
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JPEG file and a better-looking image. Too large a smoothing factor will
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visibly blur the image, however.
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.PP
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Switches for wizards:
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.TP
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.B \-baseline
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Force baseline-compatible quantization tables to be generated. This clamps
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quantization values to 8 bits even at low quality settings. (This switch is
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poorly named, since it does not ensure that the output is actually baseline
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JPEG. For example, you can use
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.B \-baseline
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and
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.B \-progressive
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together.)
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.TP
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.BI \-qtables " file"
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Use the quantization tables given in the specified text file.
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.TP
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.BI \-qslots " N[,...]"
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Select which quantization table to use for each color component.
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.TP
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.BI \-sample " HxV[,...]"
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Set JPEG sampling factors for each color component.
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.TP
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.BI \-scans " file"
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Use the scan script given in the specified text file.
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.PP
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The "wizard" switches are intended for experimentation with JPEG. If you
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don't know what you are doing, \fBdon't use them\fR. These switches are
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documented further in the file wizard.doc.
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.SH EXAMPLES
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.LP
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This example compresses the PPM file foo.ppm with a quality factor of
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60 and saves the output as foo.jpg:
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.IP
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.B cjpeg \-quality
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.I 60 foo.ppm
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.B >
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.I foo.jpg
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.SH HINTS
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Color GIF files are not the ideal input for JPEG; JPEG is really intended for
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compressing full-color (24-bit) images. In particular, don't try to convert
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cartoons, line drawings, and other images that have only a few distinct
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colors. GIF works great on these, JPEG does not. If you want to convert a
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GIF to JPEG, you should experiment with
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.BR cjpeg 's
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.B \-quality
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and
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.B \-smooth
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options to get a satisfactory conversion.
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.B \-smooth 10
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or so is often helpful.
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.PP
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Avoid running an image through a series of JPEG compression/decompression
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cycles. Image quality loss will accumulate; after ten or so cycles the image
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may be noticeably worse than it was after one cycle. It's best to use a
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lossless format while manipulating an image, then convert to JPEG format when
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you are ready to file the image away.
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.PP
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The
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.B \-optimize
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option to
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.B cjpeg
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is worth using when you are making a "final" version for posting or archiving.
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It's also a win when you are using low quality settings to make very small
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JPEG files; the percentage improvement is often a lot more than it is on
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larger files. (At present,
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.B \-optimize
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mode is always selected when generating progressive JPEG files.)
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.SH ENVIRONMENT
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.TP
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.B JPEGMEM
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If this environment variable is set, its value is the default memory limit.
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The value is specified as described for the
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.B \-maxmemory
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switch.
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.B JPEGMEM
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overrides the default value specified when the program was compiled, and
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itself is overridden by an explicit
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.BR \-maxmemory .
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.SH SEE ALSO
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.BR djpeg (1),
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.BR jpegtran (1),
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.BR rdjpgcom (1),
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.BR wrjpgcom (1)
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.br
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.BR ppm (5),
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.BR pgm (5)
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.br
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Wallace, Gregory K. "The JPEG Still Picture Compression Standard",
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Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
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.SH AUTHOR
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Independent JPEG Group
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.SH BUGS
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Arithmetic coding is not supported for legal reasons.
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.PP
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GIF input files are no longer supported, to avoid the Unisys LZW patent.
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Use a Unisys-licensed program if you need to read a GIF file. (Conversion
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of GIF files to JPEG is usually a bad idea anyway.)
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.PP
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Not all variants of BMP and Targa file formats are supported.
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.PP
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The
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.B \-targa
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switch is not a bug, it's a feature. (It would be a bug if the Targa format
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designers had not been clueless.)
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.PP
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Still not as fast as we'd like.
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