pgm



DESCRIPTION
       The  PGM  format  is a lowest common denominator grayscale file format.
       It is designed to be extremely easy to learn and  write  programs  for.
       (It's  so  simple  that  most  people  will  simply reverse engineer it
       because it's easier than reading this specification).

       A PGM image represents a  grayscale  graphic  image.   There  are  many
       psueudo-PGM  formats  in  use  where  everything is as specified herein
       except for the meaning of individual pixel values.  For most  purposes,
       a  PGM image can just be thought of an array of arbitrary integers, and
       all the programs in the world that think they're processing a grayscale
       image can easily be tricked into processing something else.

       One  official  variant of PGM is the transparency mask.  A transparency
       mask in Netpbm is represented by a PGM image, except that in  place  of
       pixel intensities, there are opaqueness values.  See below.


       The format definition is as follows.

       A  PGM file consists of a sequence of one or more PGM images. There are
       no data, delimiters, or padding before, after, or between images.

       Each PGM image consists of the following:

       - A "magic number" for identifying the file type.  A pgm image's  magic
         number is the two characters "P5".

       - 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  (Maxval), again in ASCII decimal.  Must be
         less than 65536.

       - Newline or other single whitespace character.

       - A raster of Width * Height gray values, proceeding through the  image
         in  normal English reading order.  Each gray value is a number from 0
         through Maxval, with 0 being black and Maxval being white.  Each gray
         value  is  represented in pure binary by either 1 or 2 bytes.  If the
         Maxval is less than 256, it is 1 byte.  Otherwise,  it  is  2  bytes.
         The most significant byte is first.

       - Each  gray  value  is  a  number proportional to the intensity of the
         pixel, adjusted by the CIE Rec. 709 gamma transfer  function.   (That
         opaqueness.   It  is  proportional  to the fraction of intensity of a
         pixel that would show in place of an underlying pixel, with the  same
         gamma  transfer  function  mentioned above applied.  So what normally
         means white represents total opaqueness and what normally means black
         represents  total  transparency.   In  between, you would compute the
         intensity of a composite pixel of an  "under"  and  "over"  pixel  as
         under * (1-(alpha/alpha_maxval)) + over * (alpha/alpha_maxval).<

       - Characters  from  a  "#"  to  the next end-of-line, before the maxval
         line, are comments and are ignored.

       Note that you can use pnmdepth To convert between a the format  with  1
       byte per gray value and the one with 2 bytes per gray value.

       There  is  actually  another  version  of the PGM format that is fairly
       rare: "plain" PGM format.  The format above, which generally considered
       the  normal one, is known as the "raw" PGM format.  See pbm(5) for some
       commentary on how plain and raw formats relate to one another.

       The difference in the plain format is:

       - There is exactly one image in a file.

       - The magic number is P2 instead of P5.

       - Each pixel in the raster is represented as an  ASCII  decimal  number
         (of arbitrary size).

       - Each  pixel in the raster has white space before and after it.  There
         must be at least one character of white space between any two pixels,
         but there is no maximum.

       - 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 possible,
       accepting anything that looks remotely like a graymap.


COMPATIBILITY
       Before April 2000, a raw format  PGM  file  could  not  have  a  maxval
       greater than 255.  Hence, it could not have more than one byte per sam-
       ppm(5)

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



                               12 November 1991                         pgm(5)
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