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NAME
     pgm - portable graymap file format

DESCRIPTION
     The portable graymap format is a lowest  common  denominator
     grayscale file format.  The definition is as follows:

     - A "magic number" for identifying the  file  type.   A  pgm
       file's magic number is the two characters "P2".

     - Whitespace (blanks, TABs, CRs, LFs).

     - A width, formatted as ASCII characters in decimal.

     - Whitespace.

     - A height, again in ASCII decimal.

     - Whitespace.

     - The maximum gray value, again in ASCII decimal.

     - Whitespace.

     - Width * height gray values, each in ASCII decimal, between
       0  and  the  specified  maximum  value,  separated by whi-
       tespace, starting at the top-left corner of  the  graymap,
       proceding  in  normal English reading order.  A value of 0
       means black, and the maximum value means white.

     - Characters from a "#" to the next end-of-line are  ignored
       (comments).

     - No line should be longer than 70 characters.

     Here is an example of a small graymap in this format:

     P2
     # feep.pgm
     24 7
     15
     0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
     0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
     0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
     0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
     0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
     0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
     0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0

     Programs that read this format should be as lenient as  pos-
     sible,  accepting  anything that looks remotely like a gray-
     map.
     There is also a variant on the format, available by  setting
     the  RAWBITS  option  at compile time.  This variant is dif-
     ferent in the following ways:

     - The "magic number" is "P5" instead of "P2".

     - The gray values are stored  as  plain  bytes,  instead  of
       ASCII decimal.

     - No whitespace is allowed in the grays section.

     - The files are smaller and many times faster  to  read  and
       write.

     Note that this raw format can only be used for maxvals  less
     than or equal to 255.  If you use the PGM library and try to
     write a file with a larger  maxval,  it  will  automatically
     fall back on the slower but more general ASCII format.

SEE ALSO
     fitstopgm(1),   fstopgm(1),   hipstopgm(1),    psidtopgm(1),
     rawtopgm(1),    tifftopgm(1),   pgmenhance(1),   pgmhist(1),
     pgmnorm(1), pgmtofits(1), pgmtopbm(1),  pgmtops(1),  pnm(5),
     pbm(5), ppm(5)

AUTHOR
     Copyright (C) 1989 by Jef Poskanzer.

     Permission  to  use,  copy,  modify,  and  distribute   this
     software  and  its documentation for any purpose and without
     fee is hereby granted, provided  that  the  above  copyright
     notice  appear  in  all  copies and that both that copyright
     notice and this permission notice appear in supporting docu-
     mentation.   This  software  is  provided  "as  is"  without
     express or implied warranty.

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