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cjpeg (1)
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    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
         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.
    
         -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  set-
         ting 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 pur-
         poses.  Quality values above about 95  are  not  recommended
         for  normal  use;  the compressed file size goes up dramati-
         cally 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  mes-
         sage  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  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.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
         compression/decompression  cycles.   Image quality loss will
         accumulate; after ten or so cycles the image may be  notice-
         ably  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 com-
              piled, and itself is overridden by  an  explicit  -max-
              memory.
    
    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  usu-
         ally a bad idea anyway.)
    
         Not all variants of BMP and  Targa  file  formats  are  sup-
         ported.
    
         The -targa switch is not a bug, it's a feature.   (It  would
         be  a  bug  if the Targa format designers had not been clue-
         less.)
    
         Still not as fast as we'd like.
    
    
    


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