jpegtopnm(1)

NAME

   jpegtopnm - convert JPEG/JFIF file to portable pixmap or graymap

SYNOPSIS

   jpegtopnm    [-dct   {int|fast|float}]   [-nosmooth]   [-maxmemory   N]
   [{-adobe|-notadobe}]    [-comments]    [-dumpexif]     [-exif=filespec]
   [-verbose] [-tracelevel N] [ filename ]

   All options may be abbreviated to their shortest unique prefix.

DESCRIPTION

   jpegtopnm  converts  the  named  JFIF file, or the standard input if no
   file is named to a PPM or PGM image file on the  standard  output.   If
   the  JFIF  file  is of the grayscale variety, jpegtopnm generates a PGM
   (Portable Graymap) file.   Otherwise,  it  generates  a  PPM  (Portable
   Pixmap) file.

   jpegtopnm  uses  the Independent JPEG Group's JPEG library to interpret
   the input file.  See http://www.ijg.org for information on the library.

   "JFIF" is the correct name for  the  image  format  commonly  known  as
   "JPEG."  Strictly speaking, JPEG is a method of compression.  The image
   format using JPEG compression that is by far the most common  is  JFIF.
   There is also a subformat of TIFF that uses JPEG compression.

   EXIF  is  an  image  format that is a subformat of JFIF (to wit, a JFIF
   file that contains an  EXIF  header  as  an  APP1  marker).   jpegtopnm
   handles EXIF.

   JFIF  files  can  have  either 8 bits per sample or 12 bits per sample.
   The 8 bit variety is by far the most common.  There are two versions of
   the  IJG  JPEG library.  One reads only 8 bit files and the other reads
   only 12 bit  files.   You  must  link  the  appropriate  one  of  these
   libraries  with  jpegtopnm.   Ordinarily,  this means the library is in
   your shared library search path when you run jpegtopnm.

   jpegtopnm generates output with either one byte or two bytes per sample
   depending  on  whether  the JFIF input has either 8 bits or 12 bits per
   sample.  You can use pnmdepth to reduce a two-byte-per-sample file to a
   one-byte-per-sample file if you need to.

   If  the JFIF file uses the CMYK or YCCK color space, the input does not
   actually contain enough information to know what color each  pixel  is.
   To know what color a pixel is, one would have to know the properties of
   the inks to which the color space  refers.   jpegtopnm  interprets  the
   colors  using  the common transformation which assumes all the inks are
   simply subtractive and linear.

OPTIONS

   The options are only 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
          accurate than the other two.

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

   -maxmemory N
          Set limit on the amount of memory jpegtopnm uses  in  processing
          large  images.   Value  is in thousands of bytes, or millions of
          bytes if "M" is suffixed to the number.  For example, -maxmemory
          4m  selects  4000000  bytes.   If jpegtopnm needs more space, it
          uses temporary files.

   -adobe

   -notadobe
          There are two variations on the CMYK (and likewise  YCCK)  color
          space  that may be used in the JFIF input.  In the normal one, a
          zero value for a color components indicates absence of ink.   In
          the  other,  a  zero  value means the maximum ink coverage.  The
          latter is used by Adobe Photoshop when it creates  a  bare  JFIF
          output  file  (but  not  when  it creates JFIF output as part of
          Encapsulated Postscript output).

          These options tell jpegtopnm which version of the CMYK  or  YCCK
          color  space  the image uses.  If you specify neither, jpegtopnm
          tries to figure it out on its own.  In the present  version,  it
          doesn't  try  very  hard  at  all: It just assumes the Photoshop
          version, since Photoshop and its emulators seem to be  the  main
          source  of  CMYK  and  YCCK images.  But with experience of use,
          future versions might be more sophisticated.

          If the JFIF image does not indicate that it  is  CMYK  or  YCCK,
          these options have no effect.

          If  you don't use the right one of these options, the symptom is
          output that looks like a negative.

   -dumpexif
          Print the interpreted contents of any Exif header in  the  input
          file  to  the Standard Error file.  Similar to the program jhead
          (not part of the Netpbm package).

   -exif=filespec
          Extract the contents of the EXIF header from the input image and
          write  it  to the file filespec.  filespec = - means write it to
          Standard Output.  In this case, jpegtopnm does  not  output  the
          converted image at all.

          jpegtopnm  writes the contents of the EXIF header byte-for-byte,
          starting with the two byte length field (which  length  includes
          those two bytes).

          You  can  use  this  file  as  input  to  ppmtojpeg to insert an
          identical EXIF header into a new JFIF image.

          If there is no EXIF header, jpegtopnm writes two bytes of binary
          zero and nothing else.

          An  EXIF  header takes the form of a JFIF APP1 marker.  Only the
          first such marker within the JFIF header counts.

   -comments
          Print any comments in the input file to the Standard Error file.

   -verbose
          Print details about the conversion to the Standard Error file.

   -tracelevel n
          Turn on the JPEG library's trace messages to the Standard  Error
          file.   A  higher  value  of  n  gets  more  trace  information.
          -verbose implies a trace level of at least 1.

EXAMPLES

   This example converts the color JFIF file foo.jpg to a PPM  file  named
   foo.ppm:

          jpegtopnm foo.jpg >foo.ppm

HINTS

   You  can  use ppmquant to color quantize the result, i.e. to reduce the
   number of distinct colors in the image.  In fact, you may  have  to  if
   you  want  to convert the PPM file to certain other formats.  ppmdither
   Does a more sophisticated quantization.

   Use pnmscale to change the dimensions of the resulting image.

   Use ppmtopgm to convert a color JFIF file to a grayscale PGM file.

   You can easily use these converters together.  E.g.:

          jpegtopnm foo.jpg | ppmtopgm | pnmscale .25
          >foo.pgm

   -dct fast and/or -nosmooth gain speed at a small sacrifice in quality.

   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.

   Another program,  djpeg,  is  similar.   djpeg  is  maintained  by  the
   Independent  JPEG  Group  and  packaged  with  the  JPEG  library which
   jpegtopnm uses for all its JPEG work.  Because of that, you may  expect
   it to exploit more current JPEG features.  Also, since you have to have
   the library to run jpegtopnm, but not vice versa,  cjpeg  may  be  more
   commonly available.

   On  the other hand, djpeg does not use the NetPBM libraries to generate
   its output, as all the NetPBM tools such as jpegtopnm do.   This  means
   it  is  less  likely  to be consistent with all the other programs that
   deal with the NetPBM formats.  Also, the command syntax of jpegtopnm is
   consistent with that of the other Netpbm tools, unlike djpeg.

ENVIRONMENT

   JPEGMEM
          If  this  environment  variable is set, its value is the default
          memory limit.  The value  is  specified  as  described  for  the
          -maxmemory  option.  An explicit -maxmemory option overrides any
          JPEGMEM.

SEE ALSO

   ppm(5), pgm(5), ppmtojpeg(1),  ppmquant(1),  pnmscale(1),  ppmtopgm(1),
   ppmdither(1), pnmdepth(1),
   djpeg(1), cjpeg(1), jpegtran(1), rdjpgcom(1), wrjpgcom(1), jhead(1)
   Wallace,  Gregory  K.   "The  JPEG Still Picture Compression Standard",
   Communications of the ACM, April 1991 (vol. 34, no. 4), pp. 30-44.

LIMITATIONS

   Arithmetic coding is not supported for legal reasons.

   The program could be much faster.

AUTHOR

   jpegtopnm and this man page were derived in large part from  djpeg,  by
   the  Independent  JPEG  Group.   The  program  is  otherwise  by  Bryan
   Henderson on March 19, 2000.

                             19 March 2000                    JPEGTOPNM(1)

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