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CJPEG(1)                    General Commands 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 Tool-
       kit 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.  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 "iden-
              tification" 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 indistin-
       guishable 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 purposes.  Quality values above about 95 are
       not recommended for normal use; the compressed file size goes  up  dra-
       matically 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  -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.  Caution: progressive JPEG is  not  yet  widely
       implemented, so many decoders will be unable to view a progressive JPEG
       file at all.

       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 accu-
              rate 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,  ver-
              sion 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  recom-
       mend  -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 moder-
       ate 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,  how-
       ever.

       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 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/decompres-
       sion  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  percent-
       age  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 speci-
              fied 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
       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 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.)

       Still not as fast as we'd like.



                                 20 March 1998                        CJPEG(1)