Provided by: libjpeg-turbo-progs_2.0.1-0ubuntu2_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.

       -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
              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)