pgm man page on Scientific

Man page or keyword search:  
man Server   26626 pages
apropos Keyword Search (all sections)
Output format
Scientific logo
[printable version]

PGM Format Specification(5)			   PGM Format Specification(5)

       pgm - Netpbm grayscale image format

       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
       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.

       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 definition is as follows.  You can use the libnetpbm(1)Csub‐
       routinelibrarytoconveniently and accurately read and interpret the for‐

       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 signifi‐
	      cant 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	trans‐
	      fer  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.

	      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.   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 trans‐
	      fer 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.

       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‐
       somecommentaryonhowplain 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 num‐
	      ber (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.
       # feep.pgm
       24 7
       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.

       All  characters	referred  to  herein  are encoded in ASCII.  'newline'
       refers the 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).

       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.

       pnm(1),	pbm(1),	  ppm(1),   pam(1),   libnetpbm(1),   programsthatpro‐

       Copyright (C) 1989, 1991 by Jef Poskanzer.

netpbm documentation		03 October 2003	   PGM Format Specification(5)

List of man pages available for Scientific

Copyright (c) for man pages and the logo by the respective OS vendor.

For those who want to learn more, the polarhome community provides shell access and support.

[legal] [privacy] [GNU] [policy] [cookies] [netiquette] [sponsors] [FAQ]
Polarhome, production since 1999.
Member of Polarhome portal.
Based on Fawad Halim's script.
Vote for polarhome
Free Shell Accounts :: the biggest list on the net