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 stan‐
       dard output.  The currently supported input file formats are: PPM (PBMPLUS color format), PGM  (PBMPLUS  grayscale  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 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,  be‐
              cause  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.

       -rgb   Create RGB JPEG file.  Using this switch suppresses the conversion from RGB colorspace input  to  the  default  YCbCr
              JPEG colorspace.

       -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 qual‐
              ity 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 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 qual‐
       ity 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 generally be between 50 and 95 (the default is 75) for photographic  im‐
       ages.   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 in‐
       formation  loss in subsampling, as well as roundoff error.)  For most images, specifying a quality value above about 95 will
       increase the size of the compressed file dramatically, and while the quality gain from these higher quality values  is  mea‐
       surable (using metrics such as PSNR or SSIM), it is rarely perceivable by human vision.

       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 luminance 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 levels 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 chrominance subsam‐
       pling.  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).  In libjpeg-turbo, the fast method is generally about 5-15% faster than the int
              method when using the x86/x86-64 SIMD extensions (results may vary with other SIMD  implementations,  or  when  using
              libjpeg-turbo without SIMD extensions.)  For quality levels of 90 and below, there should be little or no perceptible
              difference between the two algorithms.  For quality levels above 90, however, the difference between the fast and the
              int  methods becomes more pronounced.  With quality=97, for instance, the fast method incurs generally about a 1-3 dB
              loss (in PSNR) relative to the int method, but this can be larger for some images.  Do not use the fast  method  with
              quality  levels  above  97.   The algorithm often degenerates at quality=98 and above and can actually produce a more
              lossy image than if lower quality levels had been used.  Also, in libjpeg-turbo, the fast method is not fully  accel‐
              erated for quality levels above 97, so it will be slower than the int method.

       -dct float
              Use  floating-point DCT method.  The float method is mainly a legacy feature.  It does not produce significantly more
              accurate results than the int method, and it is much slower.  The float method may also  give  different  results  on
              different  machines due to varying roundoff behavior, whereas the integer methods should give the same results on all
              machines.

       -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, an er‐
              ror will occur.

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

       -memdst
              Compress to memory instead of a file.  This feature was implemented mainly as a way of testing the in-memory destina‐
              tion manager (jpeg_mem_dest()), but it is also useful for benchmarking, since it reduces the I/O overhead.

       -verbose
              Enable debug printout.  More -v's give more output.  Also, version information is printed at startup.

       -debug Same as -verbose.

       -version
              Print version information and exit.

       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  unreli‐
       able 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.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 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 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

       This file was modified by The libjpeg-turbo Project to include only information relevant to libjpeg-turbo, to wordsmith cer‐
       tain sections, and to describe features not present in libjpeg.

ISSUES
       Support  for  GIF input files was removed in cjpeg v6b due to concerns over the Unisys LZW patent.  Although this patent ex‐
       pired in 2006, cjpeg still lacks GIF support, for these historical reasons.  (Conversion of GIF files to JPEG is  usually  a
       bad idea anyway, since GIF is a 256-color format.)

       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.)

                                                           18 March 2017                                                   CJPEG(1)

usage: cjpeg [switches] [inputfile]
Switches (names may be abbreviated):
  -quality N[,...]   Compression quality (0..100; 5-95 is most useful range,
                     default is 75)
  -grayscale     Create monochrome JPEG file
  -rgb           Create RGB JPEG file
  -optimize      Optimize Huffman table (smaller file, but slow compression)
  -progressive   Create progressive JPEG file
  -targa         Input file is Targa format (usually not needed)
Switches for advanced users:
  -arithmetic    Use arithmetic coding
  -dct int       Use integer DCT method (default)
  -dct fast      Use fast integer DCT (less accurate)
  -dct float     Use floating-point DCT method
  -restart N     Set restart interval in rows, or in blocks with B
  -smooth N      Smooth dithered input (N=1..100 is strength)
  -maxmemory N   Maximum memory to use (in kbytes)
  -outfile name  Specify name for output file
  -memdst        Compress to memory instead of file (useful for benchmarking)
  -verbose  or  -debug   Emit debug output
  -version       Print version information and exit
Switches for wizards:
  -baseline      Force baseline quantization tables
  -qtables FILE  Use quantization tables given in FILE
  -qslots N[,...]    Set component quantization tables
  -sample HxV[,...]  Set component sampling factors
  -scans FILE    Create multi-scan JPEG per script FILE