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. PBM-
    PLUS (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 out-
    put.  (The default options are chosen for highest quality out-
    put.) 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.

    -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; otherwise, 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; otherwise, 24-bit full-color format
    is emitted.

    -pnm  Select PBMPLUS (PPM/PGM) output format (this is the default for-
    mat).  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 emitted if the
    JPEG file is gray-scale or if -grayscale is specified; other-
    wise, colormapped format is emitted if -colors is specified;
    otherwise, 24-bit full-color format 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 everywhere. The fast integer method is much less  accu-
    rate than the other two.

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

    -dither ordered
    Use ordered dithering in color quantization.

    -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 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.

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

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 signifi-
    cant 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 speci-
    fied when the program was compiled, and itself is overridden by
    an explicit -maxmemory.

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.