Provided by: libjpeg-turbo-progs_2.1.2-0ubuntu1_amd64 bug


       cjpeg - compress an image file to a JPEG file


       cjpeg [ options ] [ filename ]


       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 grayscale format), BMP, GIF, and


       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.)

              Create monochrome JPEG file from color input.  Be sure  to  use  this  switch  when
              compressing  a  grayscale  BMP  or  GIF  file, because cjpeg isn't bright enough to
              notice whether a BMP or GIF 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.

              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.

              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 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 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
       quantization 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 compressed 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
       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 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 total file size is about the same --- often a little smaller.

       Switches for advanced users:

              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
              algorithm  provided  noticeable  performance  benefits.   On  modern  CPUs  running
              libjpeg-turbo, however, the compression  time  for  a  1-megapixel  JPEG  image  is
              measured  in  milliseconds, and thus the performance benefits of the fast algorithm
              are  much  less  noticeable.   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 generally 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 difference 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 machines.

       -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,
              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  error  will

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

              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 compression progress.

              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

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

       -debug Same as -verbose.

              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:

              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


       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


       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

       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


              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.


       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.


       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


       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 November 2021                                CJPEG(1)