djpeg man page on MirBSD

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DJPEG(1)	    UNIX Programmer's Manual		 DJPEG(1)

NAME
     djpeg - decompress a JPEG file to an image file

SYNOPSIS
     djpeg [ options ] [ filename ]

DESCRIPTION
     djpeg decompresses the named JPEG file, or the standard
     input if no file is named, and produces an image file on the
     standard output.  PBMPLUS (PPM/PGM), BMP, GIF, Targa, or RLE
     (Utah Raster Toolkit) output format can be selected. (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:

     -colors N
	  Reduce image to at most N colors.  This reduces the
	  number of colors used in the output image, so that it
	  can be displayed on a colormapped display or stored in
	  a colormapped file format.  For example, if you have an
	  8-bit display, you'd need to reduce to 256 or fewer
	  colors.

     -quantize N
	  Same as -colors. -colors is the recommended name,
	  -quantize is provided only for backwards compatibility.

     -fast
	  Select recommended processing options for fast, low
	  quality output.  (The default options are chosen for
	  highest quality output.)  Currently, this is equivalent
	  to -dct fast -nosmooth -onepass -dither ordered.

     -grayscale
	  Force gray-scale output even if JPEG file is color.
	  Useful for viewing on monochrome displays; also, djpeg
	  runs noticeably faster in this mode.

     -scale M/N
	  Scale the output image by a factor M/N.  Currently the
	  scale factor must be 1/1, 1/2, 1/4, or 1/8.  Scaling is
	  handy if the image is larger than your screen; also,
	  djpeg runs much faster when scaling down the output.

MirOS BSD #10-current	 22 August 1997				1

DJPEG(1)	    UNIX Programmer's Manual		 DJPEG(1)

     -bmp Select BMP output format (Windows flavor).  8-bit
	  colormapped format is emitted if -colors or -grayscale
	  is specified, or if the JPEG file is gray-scale; other-
	  wise, 24-bit full-color format is emitted.

     -gif Select GIF output format.  Since GIF does not support
	  more than 256 colors, -colors 256 is assumed (unless
	  you specify a smaller number of colors).

     -os2 Select BMP output format (OS/2 1.x flavor).  8-bit
	  colormapped format is emitted if -colors or -grayscale
	  is specified, or if the JPEG file is gray-scale; other-
	  wise, 24-bit full-color format is emitted.

     -pnm Select PBMPLUS (PPM/PGM) output format (this is the
	  default format). PGM is emitted if the JPEG file is
	  gray-scale or if -grayscale is specified; otherwise PPM
	  is emitted.

     -rle Select RLE output format.  (Requires URT library.)

     -targa
	  Select Targa output format.  Gray-scale format is emit-
	  ted if the JPEG file is gray-scale or if -grayscale is
	  specified; otherwise, colormapped format is emitted if
	  -colors is specified; otherwise, 24-bit full-color for-
	  mat is emitted.

     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 every-
	  where. The fast integer method is much less accurate
	  than the other two.

     -dither fs
	  Use Floyd-Steinberg dithering in color quantization.

     -dither ordered
	  Use ordered dithering in color quantization.

MirOS BSD #10-current	 22 August 1997				2

DJPEG(1)	    UNIX Programmer's Manual		 DJPEG(1)

     -dither none
	  Do not use dithering in color quantization. By default,
	  Floyd-Steinberg dithering is applied when quantizing
	  colors; this is slow but usually produces the best
	  results.  Ordered dither is a compromise between speed
	  and quality; no dithering is fast but usually looks
	  awful.  Note that these switches have no effect unless
	  color quantization is being done. Ordered dither is
	  only available in -onepass mode.

     -map file
	  Quantize to the colors used in the specified image
	  file.	 This is useful for producing multiple files with
	  identical color maps, or for forcing a predefined set
	  of colors to be used.	 The file must be a GIF or PPM
	  file. This option overrides -colors and -onepass.

     -nosmooth
	  Use a faster, lower-quality upsampling routine.

     -onepass
	  Use one-pass instead of two-pass color quantization.
	  The one-pass method is faster and needs less memory,
	  but it produces a lower-quality image. -onepass is
	  ignored unless you also say -colors N. Also, the one-
	  pass method is always used for gray-scale output (the
	  two-pass method is no improvement then).

     -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.

EXAMPLES
     This example decompresses the JPEG file foo.jpg, quantizes
     it to 256 colors, and saves the output in 8-bit BMP format
     in foo.bmp:

	  djpeg -colors 256 -bmp foo.jpg > foo.bmp

MirOS BSD #10-current	 22 August 1997				3

DJPEG(1)	    UNIX Programmer's Manual		 DJPEG(1)

HINTS
     To get a quick preview of an image, use the -grayscale
     and/or -scale switches. -grayscale -scale 1/8 is the fastest
     case.

     Several options are available that trade off image quality
     to gain speed. -fast turns on the recommended settings.

     -dct fast and/or -nosmooth gain speed at a small sacrifice
     in quality. When producing a color-quantized image, -onepass
     -dither ordered is fast but much lower quality than the
     default behavior. -dither none may give acceptable results
     in two-pass mode, but is seldom tolerable in one-pass mode.

     If you are fortunate enough to have very fast floating point
     hardware, -dct float may be even faster than -dct fast.  But
     on most machines -dct float is slower than -dct int; in this
     case it is not worth using, because its theoretical accuracy
     advantage is too small to be significant in practice.

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
     cjpeg(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.

     To avoid the Unisys LZW patent, djpeg produces uncompressed
     GIF files.	 These are larger than they should be, but are
     readable by standard GIF decoders.

     Still not as fast as we'd like.

MirOS BSD #10-current	 22 August 1997				4

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