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 (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  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 parame-
              ters.  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  con-
              tain an "identification" field will not be automat-
              ically 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 pur-
       pose 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 pro-
       duce 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 trans-
       mitted 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 subse-
       quent  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 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 num-
              ber.  -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 stan-
              dard 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
              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  qual-
       ity 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  dis-
       tinct  colors.   GIF  works great on these, JPEG does not.
       If you want to convert a GIF to JPEG, you  should  experi-
       ment  with  cjpeg's  -quality and -smooth options to get a
       satisfactory conversion.  -smooth 10 or so is often  help-
       ful.

       Avoid  running  an image through a series of JPEG compres-
       sion/decompression cycles.  Image quality loss will  accu-
       mulate; 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 progres-
       sive 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 pro-
              gram  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 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.)

       Still not as fast as we'd like.

                          20 March 1998                         1