The Independent JPEG Group's JPEG software v2

cjpeg.1

@@ -0,0 +1,130 @@
+.TH CJPEG 1 "11 December 1991"
+.SH NAME
+cjpeg \- compress an image file to a JPEG file
+.SH SYNOPSIS
+.B cjpeg
+[
+.BI \-Q " quality"
+]
+[
+.B \-oTIad
+]
+[
+.I filename
+]
+.LP
+.SH DESCRIPTION
+.LP
+.B cjpeg
+compresses the named image file, or the standard input if no file is
+named, and produces a JPEG/JFIF file on the standard output.
+The currently supported image file formats are: PPM (PBMPLUS color
+format), PGM (PBMPLUS gray-scale format), GIF, Targa, and RLE (Utah Raster
+Toolkit format).  (RLE is supported only if the URT library is available.)
+.SH OPTIONS
+.TP
+.BI \-Q " quality"
+Scale quantization tables to adjust image quality.  Quality is 0 (worst) to
+100 (best); default is 75.  (See below for more info.)
+.TP
+.B \-o
+Perform optimization of entropy encoding parameters.  Without this, default
+encoding parameters are used.
+.B \-o
+usually makes the JPEG file a little smaller, but
+.B cjpeg
+runs much slower.  Image quality and speed of decompression are unaffected by
+.BR \-o .
+.TP
+.B \-T
+Input file is Targa format.  Targa files that contain an "identification"
+field will not be automatically recognized by
+.BR cjpeg ;
+for such files you must specify
+.B \-T
+to force
+.B cjpeg
+to treat the input as Targa format.
+.TP
+.B \-I
+Generate noninterleaved JPEG file (not yet supported).
+.TP
+.B \-a
+Use arithmetic coding rather than Huffman coding (not currently
+supported for legal reasons).
+.TP
+.B \-d
+Enable debug printout.  More
+.BR \-d 's
+give more output.  Also, version information is printed at startup.
+.PP
+The
+.B \-Q
+switch lets you trade off compressed file size against quality of the
+reconstructed image: the higher the
+.B \-Q
+setting, the larger the JPEG file, and the closer the output image will be to
+the original input.  Normally you want to use the lowest
+.B \-Q
+setting (smallest file) that decompresses into something visually
+indistinguishable from the original image.  For this purpose the
+.B \-Q
+setting should be between 50 and 95; the default of 75 is often about right.
+If you see defects at 
+.B \-Q
+75, then go up 5 or 10 counts at a time until you are happy with the output
+image.  (The optimal setting will vary from one image to another.)
+.PP
+.B \-Q
+100 will generate a quantization table of all 1's, eliminating loss in the
+quantization step (but there is still information loss in subsampling, as well
+as roundoff error).  This setting is mainly of interest for experimental
+purposes.  
+.B \-Q
+values above about 95 are
+.B not
+recommended for normal use; the compressed file size goes up dramatically for
+hardly any gain in output image quality.
+.PP
+In the other direction, 
+.B \-Q
+values below 50 will produce very small files of low image quality.  Settings
+around 5 to 10 might be useful in preparing an index of a large image library,
+for example.  Try
+.B \-Q
+2 (or so) for some amusing Cubist effects.  (Note: 
+.B \-Q
+values below about 25 generate 2-byte quantization tables, which are
+considered optional in the JPEG standard.  
+.B cjpeg
+emits a warning message when you give such a
+.B \-Q
+value, because some commercial JPEG programs may be unable to decode the
+resulting file.)
+.SH EXAMPLES
+.LP
+This example compresses the PPM file foo.ppm with a quality factor of
+60 and saves the output as foo.jpg:
+.IP
+.B cjpeg \-Q
+.I 60 foo.ppm
+.B >
+.I foo.jpg
+.SH SEE ALSO
+.BR djpeg (1)
+.br
+Wallace, Gregory K.  "The JPEG Still Picture Compression Standard",
+Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.
+.SH AUTHOR
+Independent JPEG Group
+.SH BUGS
+Arithmetic coding and interleaved output not yet supported.
+.PP
+Not all variants of Targa file format are supported.
+.PP
+The
+.B -T
+switch is not a bug, it's a feature.  (It would be a bug if the Targa format
+designers had not been clueless.)
+.PP
+Not as fast as we'd like.