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

netpbm documentation                             09 October 2016                     PGM Format Specification(5)