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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 format), BMP, Targa, and RLE (Utah Raster Tool‐
       kit 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).

       -scale M/N
	      Scale  the  output  image	 by a factor M/N.  Currently supported
	      scale factors are 8/N with all N from 1 to 16.

       -targa Input file is Targa format.  Targa files that contain an	"iden‐
	      tification" 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  indistin‐
       guishable  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 purposes.  Quality values above about 95  are
       not  recommended	 for normal use; the compressed file size goes up dra‐
       matically 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 IJG version 7 for  support  of
       separate quality settings for luminance and chrominance (or in general,
       for every provided quantization table slot).  This  feature  is	useful
       for  high-quality  applications which cannot accept the damage of color
       data by coarse subsampling settings.  You can  now  easily  reduce  the
       color  data  amount  more  smoothly with finer control without separate
       subsampling.  The resulting file is fully compliant with standard  JPEG
       decoders.  Note that the -quality ratings refer to the quantization ta‐
       ble slots, and that the last value is replicated if there are  more  q-
       table slots than parameters.  The default q-table slots are 0 for lumi‐
       nance and 1 for chrominance with default tables as given	 in  the  JPEG
       standard.   This is compatible with the old behaviour in case that only
       one parameter is given, which is	 then  used  for  both	luminance  and
       chrominance (slots 0 and 1).  More or custom quantization tables can be
       set with -qtables and assigned to  components  with  -qslots  parameter
       (see  the  "wizard"  switches below).  Caution: You must explicitly add
       -sample 1x1 for efficient separate color quality selection,  since  the
       default value used by library is 2x2!

       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:

       -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 accu‐
	      rate than the other two.

       -nosmooth
	      Don't use high-quality downsampling.

       -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.

       -verbose
	      Enable debug printout.  More -v's give more output.  Also,  ver‐
	      sion 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 moder‐
       ate 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,  how‐
       ever.

       Switches for wizards:

       -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.

       -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/decompres‐
       sion  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  percent‐
       age  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 speci‐
	      fied 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
       GIF input files are no  longer  supported,  to  avoid  the  Unisys  LZW
       patent.	 (Conversion  of  GIF files to JPEG is usually a bad idea any‐
       way.)

       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.)

			       30 December 2009			      CJPEG(1)
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