man > CJPEG(1)

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 pro‐
       duces a JPEG/JFIF file on the standard output.  The currently supported  input  file  formats
       are: PPM (PBMPLUS color format), PGM (PBMPLUS grayscale format), BMP, GIF, and Targa.

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.  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 quality and speed of de‐
              compression 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  recon‐
       structed  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 set‐
       ting (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 images.  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 quantiza‐
       tion step (but there is still information loss in subsampling, as well  as  roundoff  error.)
       For most images, specifying a quality value above about 95 will increase the size of the com‐
       pressed  file  dramatically,  and  while the quality gain from these higher quality values is
       measurable (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 li‐
       brary, 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 quantiza‐
       tion 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 chromi‐
       nance 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 subsampling.  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 to‐
       tal 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 accurate integer DCT method (default).

       -dct fast
              Use  less  accurate  integer  DCT  method [legacy feature].  When the Independent JPEG
              Group's software was first released in 1991, the compression time  for  a  1-megapixel
              JPEG image on a mainstream PC was measured in minutes.  Thus, the fast integer DCT al‐
              gorithm  provided  noticeable  performance  benefits.  On modern CPUs running libjpeg-
              turbo, however, the compression time for a 1-megapixel JPEG image is measured in  mil‐
              liseconds,  and  thus the performance benefits of the fast algorithm are much less no‐
              ticeable.  On modern x86/x86-64 CPUs that support AVX2 instructions, the fast and  int
              methods  have  similar performance.  On other types of CPUs, the fast method is gener‐
              ally about 5-15% faster than the int method.

              For quality levels of 90 and below, there should be little or no  perceptible  quality
              difference between the two algorithms.  For quality levels above 90, however, the dif‐
              ference  between  the  fast and 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  accelerated  for  quality
              levels above 97, so it will be slower than the int method.

       -dct float
              Use  floating-point  DCT  method  [legacy feature].  The float method 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 ma‐
              chines.

       -icc file
              Embed ICC color management profile contained in the specified file.

       -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, indi‐
              cates 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  thou‐
              sands  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 error 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 destination manager (jpeg_mem_dest()), but it is also useful for
              benchmarking, since it reduces the I/O overhead.

       -report
              Report compression progress.

       -strict
              Treat all warnings as fatal.  Enabling this option will cause the compressor to  abort
              if an LZW-compressed GIF input image contains incomplete or corrupt image data.

       -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 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  quantiza‐
              tion  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 post‐
       ing 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 certain sections, and to describe features not  present  in  lib‐
       jpeg.

ISSUES
       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 de‐
       signers had not been clueless.)

                                          30 November 2021                                  CJPEG(1)