pgm

pgm(5)                        File Formats Manual                       pgm(5)

NAME
       pgm - portable graymap file format

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 be-
       cause 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 ex-
       cept 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
         transfer  function  specifies  a gamma number of 2.2 and has a linear
         section for small intensities).  A value of zero is therefore  black.
         A value of Maxval represents CIE D65 white and the most intense value
         in the image and any other image to which the  image  might  be  com-
         pared.

       - Note  that  a  common variation on the PGM format is to have the gray
         value be "linear," i.e. as specified above except without  the  gamma
         adjustment.   pnmgamma takes such a PGM variant as input and produces
         a true PGM as output.

       - In the transparency mask  variation  on  PGM,  the  value  represents
         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 in-
         tensity 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, ac-
       cepting 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-
       ple.  Old programs may depend on this.

       Before July 2000, there could be at most one image in a PGM file.  As a
       result,  most  tools  to  process PGM files ignore (and don't read) any
       data after the first image.

SEE ALSO
       fitstopgm(1), fstopgm(1),  hipstopgm(1),  lispmtopgm(1),  psidtopgm(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.

                               12 November 1991                         pgm(5)