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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.

       -rgb   Create  RGB JPEG file.  Using this switch suppresses the conver‐
	      sion from RGB colorspace input to the default  YCbCr  JPEG  col‐
	      orspace.	 Use  this  switch  in	combination  with the -block N
	      switch (see below) for lossless JPEG coding.

       -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  M/N with all N from 1 to 16, where M is the
	      destination DCT size, which is 8 by default (see -block N switch
	      below).

       -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:

       -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.

       -block N
	      Set  DCT	block size.  All N from 1 to 16 are possible.  Default
	      is 8 (baseline format).  Larger values produce  higher  compres‐
	      sion,  smaller  values  produce  higher quality (exact DCT stage
	      possible with 1 or 2; with the default quality of 75 and default
	      Luminance	 qtable	 the  DCT+Quantization	stage  is lossless for
	      N=1).  Caution: An implementation of the JPEG SmartScale	exten‐
	      sion  is	required for this feature.  SmartScale enabled JPEG is
	      not yet widely implemented, so many decoders will be  unable  to
	      view a SmartScale extended JPEG file at all.

       -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:

       -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
       (now expired).  (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.)

				28 August 2011			      CJPEG(1)
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