Provided by: libjpeg-turbo-progs_1.4.2-0ubuntu3.4_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 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 dramatically 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 -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, temporary files will be used.

       -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.

BUGS

       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.)

       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.)

                                                21 November 2014                                        CJPEG(1)