Provided by: libjpeg-turbo-progs_1.5.2-0ubuntu5.18.04.6_amd64 bug

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 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,  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.

       -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 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 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:

       -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 accelerated 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 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.

       -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 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 certain 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 expired 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)