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       cjpeg - compress an image file to a JPEG file


       cjpeg [ options ] [ filename ]


       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


       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

              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.

              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.

              Create progressive JPEG file (see below).

       -scale M/N
              Scale the output image by a  factor  M/N.   Currently  supported
              scale factors are 8/N with all N from 1 to 16.

       -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 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 IJG version 7 for support of
       separate quality settings for luminance and chrominance (or in general,
       for  every  provided  quantization table slot).  This feature is useful
       for high-quality applications which cannot accept the damage  of  color
       data  by  coarse  subsampling  settings.  You can now easily reduce the
       color data amount more smoothly with  finer  control  without  separate
       subsampling.   The resulting file is fully compliant with standard JPEG
       decoders.  Note that the -quality ratings  refer  to  the  quantization
       table slots, and that the last value is replicated if there are more q-
       table slots than parameters.  The  default  q-table  slots  are  0  for
       luminance  and  1  for  chrominance with default tables as given in the
       JPEG standard.  This is compatible with the old behaviour in case  that
       only  one parameter is given, which is then used for both luminance and
       chrominance (slots 0 and 1).  More or custom quantization tables can be
       set  with  -qtables  and  assigned to components with -qslots parameter
       (see the "wizard" switches below).  Caution: You  must  explicitly  add
       -sample  1x1  for efficient separate color quality selection, since the
       default value used by library is 2x2!

       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:

       -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.

              Don’t use high-quality downsampling.

       -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

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

              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,

       Switches for wizards:

              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.

              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, dont use them.  These switches  are
       documented further in the file wizard.txt.


       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


       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 improvement is often a lot more than it is on larger  files.
       (At   present,  -optimize  mode  is  always  selected  when  generating
       progressive JPEG files.)


              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.


       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.


       Independent JPEG Group


       GIF  input  files  are  no  longer  supported,  to avoid the Unisys LZW
       patent.  (Conversion of GIF  files  to  JPEG  is  usually  a  bad  idea

       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.)

                               30 December 2009