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CJPEG(1)                                                              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  (PBM-
       PLUS  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  men-
       tioned 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.  Without  this,  default  encoding  parameters  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 (see below).

       -targa Input file is Targa format.  Targa files that contain an "identification" field will  not  be  automati-
              cally 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 setting 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 purposes.  Quality values above about 95 are not recommended for normal use; the  compressed  file
       size goes up dramatically 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 amusing 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 -quality option has been extended in this version of cjpeg to support separate quality settings  for  lumi-
       nance  and chrominance (or, in general, separate settings for every quantization table slot.)  The principle is
       the same as chrominance subsampling:  since the human eye is more sensitive to spatial  changes  in  brightness
       than  spatial  changes in color, the chrominance components can be quantized more than the luminance components
       without incurring any visible image quality loss.  However, unlike subsampling, this feature  reduces  data  in
       the frequency domain instead of the spatial domain, which allows for more fine-grained control.  This option is
       useful in quality-sensitive applications, for which the artifacts generated by subsampling may be unacceptable.

       The  -quality option accepts a comma-separated list of parameters, which respectively refer to the quality lev-
       els that should be assigned to the quantization table slots.  If there are more q-table slots than  parameters,
       then  the  last  parameter  is replicated.  Thus, if only one quality parameter is given, this is used for both
       luminance and chrominance (slots 0 and 1, respectively), preserving the legacy behavior of cjpeg v6b and prior.
       More  (or  customized)  quantization tables can be set with the -qtables option and assigned to components with
       the -qslots option (see the "wizard" switches below.)

       JPEG files generated with separate luminance and chrominance quality are fully  compliant  with  standard  JPEG
       decoders.

       CAUTION:  For this setting to be useful, be sure to pass an argument of -sample 1x1 to cjpeg to disable chromi-
       nance subsampling.  Otherwise, the default subsampling level (2x2, AKA "4:2:0") will be used.

       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 transmitted 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.

       Switches for advanced users:

       -arithmetic
              Use arithmetic coding.  Caution: arithmetic coded JPEG is not yet widely implemented, so  many  decoders
              will be unable to view an arithmetic coded JPEG file at all.

       -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 everywhere.  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
              millions  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 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 out-
              put 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.txt.

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 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 noticeably 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  improve-
       ment  is often a lot more than it is on larger files.  (At present, -optimize mode is always selected when gen-
       erating 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
              compiled, and itself is overridden by an explicit -maxmemory.

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
       Support for GIF input files was removed in cjpeg v6b due to concerns over the Unisys LZW patent.  Although this
       patent expired in 2006, cjpeg still lacks GIF support, for these historical reasons.  (Conversion of GIF  files
       to JPEG is usually a bad idea anyway.)

       Not all variants of BMP and Targa file formats are supported.

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



                                31 January 2012                       CJPEG(1)