cjpeg



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 avail-
       able.)


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, dont 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)

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