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libjpeg - A library for manipulating JPEG image format files.…  more info»

cjpeg.1.gz

CJPEG(1)                                                    CJPEG(1)



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 gray-scale for‐
       mat), 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  say‐
              ing  -grayscale,  you'll  get a smaller JPEG file that
              takes less time to process.

       -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  amus‐
       ing  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  -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 transmit‐
       ted 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.  Caution: progressive
       JPEG is not yet widely implemented, so many decoders will  be
       unable to view a progressive JPEG file at all.

       Switches for advanced users:

       -dct int
              Use integer DCT method (default).

       -dct fast
              Use fast integer DCT (less accurate).

       -dct float
              Use  floating-point  DCT  method.  The float method is
              very slightly more accurate than the int  method,  but
              is  much  slower  unless  your  machine  has very fast
              floating-point hardware.  Also note  that  results  of
              the  floating-point  method  may  vary slightly across
              machines, while the integer methods  should  give  the
              same  results  everywhere.  The fast integer method is
              much less accurate than the other two.

       -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 mil‐
              lions 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.

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

       -debug Same as -verbose.

       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 recom‐
       mend  -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.doc.

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  compres‐
       sion/decompression  cycles.   Image quality loss will accumu‐
       late; after ten or so cycles  the  image  may  be  noticeably
       worse  than it was after one cycle.  It's best to use a loss‐
       less 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  over‐
              rides 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

BUGS
       Arithmetic coding is not supported for legal reasons.

       GIF  input files are no longer supported, to avoid the Unisys
       LZW patent.  Use a Unisys-licensed program  if  you  need  to
       read a GIF file.  (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.)

       Still not as fast as we'd like.



                            20 March 1998                   CJPEG(1)
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