1PGM Format Specification(5) File Formats Manual PGM Format Specification(5)
2
6 pgm - Netpbm grayscale image format
7
10 This program is part of Netpbm(1).
11 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 be‐
15 cause it's easier than reading this specification).
16 A PGM image represents a grayscale graphic image. There are many
18 pseudo-PGM formats in use where everything is as specified herein ex‐
19 cept for the meaning of individual pixel values. For most purposes, a
20 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.
23 The name "PGM" is an acronym derived from "Portable Gray Map."
25 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.
29
32 The format definition is as follows. You can use the libnetpbm(1) C
33 subroutine library to conveniently and accurately read and interpret
34 the format.
35 A PGM file consists of a sequence of one or more PGM images. There are
37 no data, delimiters, or padding before, after, or between images.
38 Each PGM image consists of the following:
40 • A "magic number" for identifying the file type. A pgm image's
45 magic number is the two characters "P5".
46 • Whitespace (blanks, TABs, CRs, LFs).
49 • A width, formatted as ASCII characters in decimal.
52 • Whitespace.
55 • A height, again in ASCII decimal.
58 • Whitespace.
61 • The maximum gray value (Maxval), again in ASCII decimal. Must
64 be less than 65536, and more than zero.
65 • A single whitespace character (usually a newline).
68 • A raster of Height rows, in order from top to bottom. Each row
71 consists of Width gray values, in order from left to right.
72 Each gray value is a number from 0 through Maxval, with 0 being
73 black and Maxval being white. Each gray value is represented in
74 pure binary by either 1 or 2 bytes. If the Maxval is less than
75 256, it is 1 byte. Otherwise, it is 2 bytes. The most signifi‐
76 cant byte is first.
77 A row of an image is horizontal. A column is vertical. The
79 pixels in the image are square and contiguous.
80 Each gray value is a number proportional to the intensity of the
82 pixel, adjusted by the ITU-R Recommendation BT.709 gamma trans‐
83 fer function. (That transfer function specifies a gamma number
84 of 2.2 and has a linear section for small intensities). A value
85 of zero is therefore black. A value of Maxval represents CIE
86 D65 white and the most intense value in the image and any other
87 image to which the image might be compared.
88 BT.709's range of channel values (16-240) is irrelevant to PGM.
90 Note that a common variation from the PGM format is to have the
92 gray value be "linear," i.e. as specified above except without
93 the gamma adjustment. pnmgamma takes such a PGM variant as in‐
94 put and produces a true PGM as output.
95 Another popular variation from PGM is to substitute the newer
97 sRGB transfer function for the BT.709 one. You can use pnmgamma
98 to convert between this variation and true PGM.
99 In the transparency mask variation from PGM, the value repre‐
101 sents opaqueness. It is proportional to the fraction of inten‐
102 sity of a pixel that would show in place of an underlying pixel.
103 So what normally means white represents total opaqueness and
104 what normally means black represents total transparency. In be‐
105 tween, you would compute the intensity of a composite pixel of
106 an "under" and "over" pixel as under * (1-(alpha/alpha_maxval))
107 + over * (alpha/alpha_maxval). Note that there is no gamma
108 transfer function in the transparency mask.
109 Strings starting with "#" may be comments, the same as with PBM(1).
113 Note that you can use pamdepth to convert between a the format with 1
115 byte per gray value and the one with 2 bytes per gray value.
116 All characters referred to herein are encoded in ASCII. "newline"
118 refers to the character known in ASCII as Line Feed or LF. A "white
119 space" character is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI
120 standard C isspace() function calls white space).
121 Plain PGM
124 There is actually another version of the PGM format that is fairly
125 rare: "plain" PGM format. The format above, which generally considered
126 the normal one, is known as the "raw" PGM format. See pbm(1) for some
127 commentary on how plain and raw formats relate to one another and how
128 to use them.
129 The difference in the plain format is:
131 •
135 There is exactly one image in a file.
137 •
139 The magic number is P2 instead of P5.
141 •
143 Each pixel in the raster is represented as an ASCII decimal num‐
145 ber (of arbitrary size).
146 •
148 Each pixel in the raster has white space before and after it.
150 There must be at least one character of white space between any
151 two pixels, but there is no maximum.
152 •
154 No line should be longer than 70 characters.
156 Here is an example of a small image in the plain PGM format.
159 P2
161 # feep.pgm
162 24 7
163 15
164 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
165 0 3 3 3 3 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 15 15 15 0
166 0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 15 0
167 0 3 3 3 0 0 0 7 7 7 0 0 0 11 11 11 0 0 0 15 15 15 15 0
168 0 3 0 0 0 0 0 7 0 0 0 0 0 11 0 0 0 0 0 15 0 0 0 0
169 0 3 0 0 0 0 0 7 7 7 7 0 0 11 11 11 11 0 0 15 0 0 0 0
170 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
171 There is a newline character at the end of each of these lines.
174 Programs that read this format should be as lenient as possible, ac‐
176 cepting anything that looks remotely like a PGM.
177
181 No Internet Media Type (aka MIME type, content type) for PGM has been
182 registered with IANA, but the value image/x-portable-graymap is conven‐
183 tional.
184 Note that the PNM Internet Media Type image/x-portable-anymap also ap‐
186 plies.
187
191 There are no requirements on the name of a PGM file, but the convention
192 is to use the suffix ".pgm". "pnm" is also conventional, for cases
193 where distinguishing between the particular subformats of PNM is not
194 convenient.
195
199 Before April 2000, a raw format PGM file could not have a maxval
200 greater than 255. Hence, it could not have more than one byte per sam‐
201 ple. Old programs may depend on this.
202 Before July 2000, there could be at most one image in a PGM file. As a
204 result, most tools to process PGM files ignore (and don't read) any
205 data after the first image.
206
209 pnm(1), pbm(1), ppm(1), pam(1), libnetpbm(1), programs that process
210 PGM(1),
211
214 Copyright (C) 1989, 1991 by Jef Poskanzer.
215
217 This manual page was generated by the Netpbm tool 'makeman' from HTML
218 source. The master documentation is at
219 http://netpbm.sourceforge.net/doc/pgm.html
221netpbm documentation 09 October 2016 PGM Format Specification(5)