pgm(5) - f7

1PGM Format Specification(5)   File Formats Manual  PGM Format Specification(5)
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NAME

6       pgm - Netpbm grayscale image format
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DESCRIPTION

10       This program is part of Netpbm(1).
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12       The  PGM  format  is a lowest common denominator grayscale file format.
13       It is designed to be extremely easy to learn and  write  programs  for.
14       (It's  so  simple  that  most  people  will  simply reverse engineer it
15       because it's easier than reading this specification).
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17       A PGM image represents a  grayscale  graphic  image.   There  are  many
18       psueudo-PGM  formats  in  use  where  everything is as specified herein
19       except for the meaning of individual pixel values.  For most  purposes,
20       a  PGM image can just be thought of an array of arbitrary integers, and
21       all the programs in the world that think they're processing a grayscale
22       image can easily be tricked into processing something else.
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24       The name "PGM" is an acronym derived from "Portable Gray Map."
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26       One  official  variant of PGM is the transparency mask.  A transparency
27       mask in Netpbm is represented by a PGM image, except that in  place  of
28       pixel intensities, there are opaqueness values.  See below.
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30       The format definition is as follows.  You can use the libnetpbm(1)Csub‐
31       routinelibrarytoconveniently and accurately read and interpret the for‐
32       mat.
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34       A  PGM file consists of a sequence of one or more PGM images. There are
35       no data, delimiters, or padding before, after, or between images.
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37       Each PGM image consists of the following:
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42       ·      A 'magic number' for identifying the file type.  A  pgm  image's
43              magic number is the two characters 'P5'.
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46       ·      Whitespace (blanks, TABs, CRs, LFs).
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49       ·      A width, formatted as ASCII characters in decimal.
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52       ·      Whitespace.
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55       ·      A height, again in ASCII decimal.
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58       ·      Whitespace.
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61       ·      The  maximum  gray value (Maxval), again in ASCII decimal.  Must
62              be less than 65536, and more than zero.
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65       ·      A single whitespace character (usually a newline).
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68       ·      A raster of Height rows, in order from top to bottom.  Each  row
69              consists  of  Width  gray  values,  in order from left to right.
70              Each gray value is a number from 0 through Maxval, with 0  being
71              black and Maxval being white.  Each gray value is represented in
72              pure binary by either 1 or 2 bytes.  If the Maxval is less  than
73              256, it is 1 byte.  Otherwise, it is 2 bytes.  The most signifi‐
74              cant byte is first.
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76              A row of an image is horizontal.  A  column  is  vertical.   The
77              pixels in the image are square and contiguous.
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80       ·      Each gray value is a number proportional to the intensity of the
81              pixel, adjusted by the ITU-R Recommendation BT.709 gamma  trans‐
82              fer  function.  (That transfer function specifies a gamma number
83              of 2.2 and has a linear section for small intensities).  A value
84              of  zero  is  therefore black.  A value of Maxval represents CIE
85              D65 white and the most intense value in the image and any  other
86              image to which the image might be compared.
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88
89       ·      Note  that  a  common variation on the PGM format is to have the
90              gray value be 'linear,' i.e. as specified above  except  without
91              the  gamma  adjustment.   pnmgamma  takes  such a PGM variant as
92              input and produces a true PGM as output.
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95       ·      In the transparency mask variation on PGM, the value  represents
96              opaqueness.   It is proportional to the fraction of intensity of
97              a pixel that would show in place of  an  underlying  pixel.   So
98              what  normally  means white represents total opaqueness and what
99              normally means black represents total transparency.  In between,
100              you  would  compute  the  intensity  of  a composite pixel of an
101              'under' and 'over' pixel as under *  (1-(alpha/alpha_maxval))  +
102              over * (alpha/alpha_maxval).  Note that there is no gamma trans‐
103              fer function in the transparency mask.
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105
106       ·      Strings starting with '#' may be  comments,  the  same  as  with
107              PBM(1).
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111       Note  that  you can use pamdepth to convert between a the format with 1
112       byte per gray value and the one with 2 bytes per gray value.
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114       There is actually another version of the  PGM  format  that  is  fairly
115       rare: 'plain' PGM format.  The format above, which generally considered
116       the normal one, is known as  the  'raw'  PGM  format.   See  pbm(1)for‐
117       somecommentaryonhowplain  and raw formats relate to one another and how
118       to use them.
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120       The difference in the plain format is:
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124       -      There is exactly one image in a file.
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126       -      The magic number is P2 instead of P5.
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128       -      Each pixel in the raster is represented as an ASCII decimal num‐
129              ber (of arbitrary size).
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131       -      Each  pixel  in  the raster has white space before and after it.
132              There must be at least one character of white space between  any
133              two pixels, but there is no maximum.
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135       -      No line should be longer than 70 characters.
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138       Here is an example of a small image in the plain PGM format.
139       P2
140       # feep.pgm
141       24 7
142       15
143       0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
144       0  3  3  3  3  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15 15 15 15  0
145       0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0 15  0
146       0  3  3  3  0  0  0  7  7  7  0  0  0 11 11 11  0  0  0 15 15 15 15  0
147       0  3  0  0  0  0  0  7  0  0  0  0  0 11  0  0  0  0  0 15  0  0  0  0
148       0  3  0  0  0  0  0  7  7  7  7  0  0 11 11 11 11  0  0 15  0  0  0  0
149       0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0  0
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151       There is a newline character at the end of each of these lines.
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153       Programs  that  read  this  format  should  be  as lenient as possible,
154       accepting anything that looks remotely like a PGM.
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156       All characters referred to herein  are  encoded  in  ASCII.   'newline'
157       refers  the  the character known in ASCII as Line Feed or LF.  A 'white
158       space' character is space, CR, LF, TAB, VT, or FF (I.e. what  the  ANSI
159       standard C isspace() function calls white space).
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COMPATIBILITY

164       Before  April  2000,  a  raw  format  PGM  file could not have a maxval
165       greater than 255.  Hence, it could not have more than one byte per sam‐
166       ple.  Old programs may depend on this.
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168       Before July 2000, there could be at most one image in a PGM file.  As a
169       result, most tools to process PGM files ignore  (and  don't  read)  any
170       data after the first image.
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AUTHOR

179       Copyright (C) 1989, 1991 by Jef Poskanzer.
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183netpbm documentation            03 October 2003    PGM Format Specification(5)

NAME

DESCRIPTION

COMPATIBILITY

SEE ALSO

AUTHOR