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RedHat 9 (Linux i386) - man page for pgm (redhat section 5)

pgm(5)				       File Formats Manual				   pgm(5)

NAME
       pgm - portable graymap file format

DESCRIPTION
       The portable graymap format is a lowest common denominator grayscale file format.

       The format definition is as follows.

       A  PGM  file  consists  of a sequence of one or more PGM images. There are no data, delim-
       iters, 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 compared.

       - 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.

       - 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 for-
       mat.  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 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
       fitstopgm(1), fstopgm(1), hipstopgm(1), lispmtopgm(1), psidtopgm(1), rawtopgm(1), pgmbent-
       ley(1),	pgmcrater(1),  pgmedge(1), pgmenhance(1), pgmhist(1), pgmnorm(1), pgmoil(1), pgm-
       ramp(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)


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