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CJPEG(1)	    UNIX Programmer's Manual		 CJPEG(1)

NAME
     cjpeg - compress an image file to a JPEG file

SYNOPSIS
     cjpeg [ options ] [ filename ]

DESCRIPTION
     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 input file formats
     are: PPM (PBMPLUS color format), PGM (PBMPLUS gray-scale
     format), BMP, Targa, and RLE (Utah Raster Toolkit format).
     (RLE is supported only if the URT library is available.)

OPTIONS
     All switch names may be abbreviated; for example, -grayscale
     may be written -gray or -gr. Most of the "basic" switches
     can be abbreviated to as little as one letter. Upper and
     lower case are equivalent (thus -BMP is the same as -bmp).
     British spellings are also accepted (e.g., -greyscale),
     though for brevity these are not mentioned below.

     The basic switches are:

     -quality N
	  Scale quantization tables to adjust image quality.
	  Quality is 0 (worst) to 100 (best); default is 75.
	  (See below for more info.)

     -grayscale
	  Create monochrome JPEG file from color input.	 Be sure
	  to use this switch when compressing a grayscale BMP
	  file, because cjpeg isn't bright enough to notice
	  whether a BMP file uses only shades of gray. By saying
	  -grayscale, you'll get a smaller JPEG file that takes
	  less time to process.

     -optimize
	  Perform optimization of entropy encoding parameters for
	  Huffman coding. Without this, default encoding parame-
	  ters are used. -optimize usually makes the JPEG file a
	  little smaller, but cjpeg runs somewhat slower and
	  needs much more memory.  Image quality and speed of
	  decompression are unaffected by -optimize.

     -progressive
	  Create progressive JPEG file, arithmetic of Huffman
	  (see below).

     -arithmetic
	  Perform arithmetic instead of Huffman coding on the
	  output.

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     -targa
	  Input file is Targa format.  Targa files that contain
	  an "identification" field will not be automatically
	  recognized by cjpeg; for such files you must specify
	  -targa to make cjpeg treat the input as Targa format.
	  For most Targa files, you won't need this switch.

     The -quality switch lets you trade off compressed file size
     against quality of the reconstructed image: the higher the
     quality 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 quality setting (smallest file)
     that decompresses into something visually indistinguishable
     from the original image.  For this purpose the quality set-
     ting should be between 50 and 95; the default of 75 is often
     about right.  If you see defects at -quality 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.)

     -quality 100 will generate a quantization table of all 1's,
     minimizing 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 pur-
     poses.  Quality values above about 95 are not recommended
     for normal use; the compressed file size goes up dramati-
     cally for hardly any gain in output image quality.

     In the other direction, quality 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 -quality 2 (or so) for some amus-
     ing Cubist effects.  (Note: quality values below about 25
     generate 2-byte quantization tables, which are considered
     optional in the JPEG standard. cjpeg emits a warning message
     when you give such a quality value, because some other JPEG
     programs may be unable to decode the resulting file.  Use
     -baseline if you need to ensure compatibility at low quality
     values.)

     The -progressive switch creates a "progressive JPEG" file.
     In this type of JPEG file, the data is stored in multiple
     scans of increasing quality.  If the file is being transmit-
     ted over a slow communications link, the decoder can use the
     first scan to display a low-quality image very quickly, and
     can then improve the display with each subsequent scan.  The
     final image is exactly equivalent to a standard JPEG file of
     the same quality setting, and the total file size is about
     the same --- often a little smaller. Caution: progressive
     JPEG is not yet widely implemented, so many decoders will be
     unable to view a progressive JPEG file at all.

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CJPEG(1)	    UNIX Programmer's Manual		 CJPEG(1)

     Switches for advanced users:

     -dct int
	  Use integer DCT method (default).

     -dct fast
	  Use fast integer DCT (less accurate).

     -dct float
	  Use floating-point DCT method. The float method is very
	  slightly more accurate than the int method, but is much
	  slower unless your machine has very fast floating-point
	  hardware.  Also note that results of the floating-point
	  method may vary slightly across machines, while the
	  integer methods should give the same results every-
	  where. The fast integer method is much less accurate
	  than the other two.

     -restart N
	  Emit a JPEG restart marker every N MCU rows, or every N
	  MCU blocks if "B" is attached to the number. -restart 0
	  (the default) means no restart markers.

     -smooth N
	  Smooth the input image to eliminate dithering noise.
	  N, ranging from 1 to 100, indicates the strength of
	  smoothing.  0 (the default) means no smoothing.

     -maxmemory N
	  Set limit for amount of memory to use in processing
	  large images.	 Value is in thousands of bytes, or mil-
	  lions of bytes if "M" is attached to the number.  For
	  example, -max 4m selects 4000000 bytes.  If more space
	  is needed, temporary files will be used.

     -outfile name
	  Send output image to the named file, not to standard
	  output.

     -verbose
	  Enable debug printout.  More -v's give more output.
	  Also, version information is printed at startup.

     -debug
	  Same as -verbose.

     The -restart option inserts extra markers that allow a JPEG
     decoder to resynchronize after a transmission error.
     Without restart markers, any damage to a compressed file
     will usually ruin the image from the point of the error to
     the end of the image; with restart markers, the damage is
     usually confined to the portion of the image up to the next

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     restart marker.  Of course, the restart markers occupy extra
     space.  We recommend -restart 1 for images that will be
     transmitted across unreliable networks such as Usenet.

     The -smooth option filters the input to eliminate fine-scale
     noise.  This is often useful when converting dithered images
     to JPEG: a moderate smoothing factor of 10 to 50 gets rid of
     dithering patterns in the input file, resulting in a smaller
     JPEG file and a better-looking image.  Too large a smoothing
     factor will visibly blur the image, however.

     Switches for wizards:

     -baseline
	  Force baseline-compatible quantization tables to be
	  generated.  This clamps quantization values to 8 bits
	  even at low quality settings.	 (This switch is poorly
	  named, since it does not ensure that the output is
	  actually baseline JPEG.  For example, you can use
	  -baseline and -progressive together.)

     -qtables file
	  Use the quantization tables given in the specified text
	  file.

     -qslots N[,...]
	  Select which quantization table to use for each color
	  component.

     -sample HxV[,...]
	  Set JPEG sampling factors for each color component.

     -scans file
	  Use the scan script given in the specified text file.

     The "wizard" switches are intended for experimentation with
     JPEG.  If you don't know what you are doing, don't use them.
     These switches are documented further in the file
     wizard.doc.

EXAMPLES
     This example compresses the PPM file foo.ppm with a quality
     factor of 60 and saves the output as foo.jpg:

	  cjpeg -quality 60 foo.ppm > foo.jpg

HINTS
     Color GIF files are not the ideal input for JPEG; JPEG is
     really intended for compressing full-color (24-bit) images.
     In particular, don't try to convert cartoons, line drawings,
     and other images that have only a few distinct colors.  GIF
     works great on these, JPEG does not.  If you want to convert

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CJPEG(1)	    UNIX Programmer's Manual		 CJPEG(1)

     a GIF to JPEG, you should experiment with cjpeg's -quality
     and -smooth options to get a satisfactory conversion.
     -smooth 10 or so is often helpful.

     Avoid running an image through a series of JPEG
     compression/decompression cycles.	Image quality loss will
     accumulate; after ten or so cycles the image may be notice-
     ably worse than it was after one cycle.  It's best to use a
     lossless format while manipulating an image, then convert to
     JPEG format when you are ready to file the image away.

     The -optimize option to cjpeg is worth using when you are
     making a "final" version for posting or archiving. It's also
     a win when you are using low quality settings to make very
     small JPEG files; the percentage improvement is often a lot
     more than it is on larger files.  (At present, -optimize
     mode is always selected when generating progressive JPEG
     files.)

ENVIRONMENT
     JPEGMEM
	  If this environment variable is set, its value is the
	  default memory limit. The value is specified as
	  described for the -maxmemory switch. JPEGMEM overrides
	  the default value specified when the program was com-
	  piled, and itself is overridden by an explicit -max-
	  memory.

SEE ALSO
     djpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1)
     ppm(5), pgm(5)
     Wallace, Gregory K.  "The JPEG Still Picture Compression
     Standard", Communications of the ACM, April 1991 (vol. 34,
     no. 4), pp. 30-44.

AUTHOR
     Independent JPEG Group

BUGS
     Arithmetic coding is not supported for legal reasons.

     GIF input files are no longer supported, to avoid the Unisys
     LZW patent. Use a Unisys-licensed program if you need to
     read a GIF file.  (Conversion of GIF files to JPEG is usu-
     ally a bad idea anyway.)

     Not all variants of BMP and Targa file formats are sup-
     ported.

     The -targa switch is not a bug, it's a feature.  (It would
     be a bug if the Targa format designers had not been clue-
     less.)

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CJPEG(1)	    UNIX Programmer's Manual		 CJPEG(1)

     Still not as fast as we'd like.

MirOS BSD #10-current	  20 March 1998				6

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