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CentOS 7.0 - man page for cjpeg (centos section 1)

CJPEG(1)										 CJPEG(1)

       cjpeg - compress an image file to a JPEG file

       cjpeg [ options ] [ filename ]

       cjpeg compresses the named image file, or the standard input if no file is named, and pro-
       duces 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.)

       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.)

	      Create monochrome JPEG file from color input.  Be sure to use this switch when com-
	      pressing	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 col-
	      orspace input to the default YCbCr JPEG colorspace.

	      Perform optimization of entropy encoding parameters.  Without this, default  encod-
	      ing  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.

	      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  recon-
       structed  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 qual-
       ity  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 quanti-
       zation 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 this version of cjpeg to support separate quality
       settings for luminance and chrominance (or, in general, separate settings for every  quan-
       tization  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  fea-
       ture  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-qual-
       ity  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:

	      Use  arithmetic  coding.	 Caution:  arithmetic coded JPEG is not yet widely imple-
	      mented, so many decoders will be unable to view an arithmetic coded  JPEG  file  at

       -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 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.

	      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 use-
	      ful for benchmarking, since it reduces the I/O overhead.

	      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  usu-
       ally  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:

	      Force baseline-compatible quantization tables to be generated.  This clamps quanti-
	      zation 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 wiz-

       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

       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 -qual-
       ity and -smooth options to get a satisfactory conversion.  -smooth 10 or so is often help-

       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

	      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.

       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.

       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

       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.)

					 18 January 2013				 CJPEG(1)

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