NAME

       pgm - Netpbm grayscale image format

DESCRIPTION

       This program is part of Netpbm(1).

       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 pseudo-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.

       The name "PGM" is an acronym derived from "Portable Gray Map."

       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

       The format definition is as follows.  You can use the libnetpbm(1) C subroutine library to
       conveniently and accurately read and interpret the format.

       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,
              and more than zero.

       •      A single whitespace character (usually a newline).

       •      A  raster  of Height rows, in order from top to bottom.  Each row consists of Width
              gray values, in order from left to right.  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.

              A  row  of  an image is horizontal.  A column is vertical.  The pixels in the image
              are square and contiguous.

              Each gray value is a number proportional to the intensity of the pixel, adjusted by
              the  ITU-R  Recommendation BT.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
              compared.

              BT.709's range of channel values (16-240) is irrelevant to PGM.

              Note  that  a  common  variation  from  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.

              Another  popular  variation  from  PGM  is  to  substitute  the newer sRGB transfer
              function for the BT.709  one.   You  can  use  pnmgamma  to  convert  between  this
              variation and true PGM.

              In  the  transparency mask variation from 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.  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).  Note that  there  is  no
              gamma transfer function in the transparency mask.

       Strings starting with "#" may be comments, the same as with PBM(1).

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

       All characters referred to herein are encoded in ASCII.  "newline" refers to the character
       known  in  ASCII as Line Feed or LF.  A "white space" character is space, CR, LF, TAB, VT,
       or FF (I.e. what the ANSI standard C isspace() function calls white space).

   Plain PGM
       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(1) for some commentary on how plain and raw formats  relate  to
       one another and how to use them.

       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 image in the plain PGM 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

       There is a newline character at the end of each of these lines.

       Programs  that  read this format should be as lenient as possible, accepting anything that
       looks remotely like a PGM.

INTERNET MEDIA TYPE

       No Internet Media Type (aka MIME type, content type) for  PGM  has  been  registered  with
       IANA, but the value image/x-portable-graymap is conventional.

       Note that the PNM Internet Media Type image/x-portable-anymap also applies.

FILE NAME

       There  are  no  requirements  on  the name of a PGM file, but the convention is to use the
       suffix ".pgm".  "pnm" is also conventional, for cases  where  distinguishing  between  the
       particular subformats of PNM is not convenient.

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 sample.  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

       pnm(1), pbm(1), ppm(1), pam(1), libnetpbm(1), programs that process PGM(1),

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.

DOCUMENT SOURCE

       This  manual page was generated by the Netpbm tool 'makeman' from HTML source.  The master
       documentation is at

              http://netpbm.sourceforge.net/doc/pgm.html