
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,
       proceeding  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, and only	a
       single  character  of  whitespace (typically a newline) is
       allowed after the maxval.

     - 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 plain format.

SEE ALSO
     fitstopgm(1), fstopgm(1), hipstopgm(1), lispmtopgm(1), psid-
     topgm(1),	  rawtopgm(1),	  pgmbentley(1),    pgmcrater(1),
     pgmedge(1),    pgmenhance(1),    pgmhist(1),     pgmnorm(1),
     pgmoil(1),	   pgmramp(1),	  pgmtexture(1),    pgmtofits(1),
     pgmtofs(1),  pgmtolispm(1),  pgmtopbm(1),	pnm(5),	  pbm(5),
     ppm(5)

AUTHOR
     Copyright (C) 1989, 1991 by Jef Poskanzer.

