Provided by: libjpeg-turbo-progs_1.4.2-0ubuntu3_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)