1PPM Format Specification(5) File Formats Manual PPM Format Specification(5)
2
6 PPM - Netpbm color image format
7
10 This program is part of Netpbm(1).
11 The PPM format is a lowest common denominator color image file format.
13 It should be noted that this format is egregiously inefficient. It is
15 highly redundant, while containing a lot of information that the human
16 eye can't even discern. Furthermore, the format allows very little in‐
17 formation about the image besides basic color, which means you may have
18 to couple a file in this format with other independent information to
19 get any decent use out of it. However, it is very easy to write and
20 analyze programs to process this format, and that is the point.
21 It should also be noted that files often conform to this format in ev‐
23 ery respect except the precise semantics of the sample values. These
24 files are useful because of the way PPM is used as an intermediary for‐
25 mat. They are informally called PPM files, but to be absolutely pre‐
26 cise, you should indicate the variation from true PPM. For example,
27 "PPM using the red, green, and blue colors that the scanner in question
28 uses."
29 The name "PPM" is an acronym derived from "Portable Pixel Map." Images
31 in this format (or a precursor of it) were once also called "portable
32 pixmaps."
33
36 The format definition is as follows. You can use the libnetpbm(1) C
37 subroutine library to read and interpret the format conveniently and
38 accurately.
39 A PPM file consists of a sequence of one or more PPM images. There are
41 no data, delimiters, or padding before, after, or between images.
42 Each PPM image consists of the following:
44 • A "magic number" for identifying the file type. A ppm image's
48 magic number is the two characters "P6".
49 •
51 Whitespace (blanks, TABs, CRs, LFs).
53 •
55 A width, formatted as ASCII characters in decimal.
57 •
59 Whitespace.
61 •
63 A height, again in ASCII decimal.
65 •
67 Whitespace.
69 •
71 The maximum color value (Maxval), again in ASCII decimal. Must
73 be less than 65536 and more than zero.
74 • A single whitespace character (usually a newline).
77 • A raster of Height rows, in order from top to bottom. Each row
80 consists of Width pixels, in order from left to right. Each
81 pixel is a triplet of red, green, and blue samples, in that or‐
82 der. Each sample is represented in pure binary by either 1 or 2
83 bytes. If the Maxval is less than 256, it is 1 byte. Other‐
84 wise, it is 2 bytes. The most significant byte is first.
85 A row of an image is horizontal. A column is vertical. The
87 pixels in the image are square and contiguous.
88 In the raster, the sample values are "nonlinear." They are pro‐
90 portional to the intensity of the ITU-R Recommendation BT.709
91 red, green, and blue in the pixel, adjusted by the BT.709 gamma
92 transfer function. (That transfer function specifies a gamma
93 number of 2.2 and has a linear section for small intensities).
94 A value of Maxval for all three samples represents CIE D65 white
95 and the most intense color in the color universe of which the
96 image is part (the color universe is all the colors in all im‐
97 ages to which this image might be compared).
98 BT.709's range of channel values (16-240) is irrelevant to PPM.
100 ITU-R Recommendation BT.709 is a renaming of the former CCIR
102 Recommendation 709. When CCIR was absorbed into its parent or‐
103 ganization, the ITU, ca. 2000, the standard was renamed. This
104 document once referred to the standard as CIE Rec. 709, but it
105 isn't clear now that CIE ever sponsored such a standard.
106 Note that another popular color space is the newer sRGB. A com‐
108 mon variation from PPM is to substitute this color space for the
109 one specified. You can use pnmgamma to convert between this
110 variation and true PPM.
111 Note that a common variation from the PPM format is to have the
113 sample values be "linear," i.e. as specified above except with‐
114 out the gamma adjustment. pnmgamma takes such a PPM variant as
115 input and produces a true PPM as output.
116 Strings starting with "#" may be comments, the same as with PBM(1).
120 Note that you can use pamdepth to convert between a the format with 1
122 byte per sample and the one with 2 bytes per sample.
123 All characters referred to herein are encoded in ASCII. "newline"
125 refers to the character known in ASCII as Line Feed or LF. A "white
126 space" character is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI
127 standard C isspace() function calls white space).
128 Plain PPM
131 There is actually another version of the PPM format that is fairly
132 rare: "plain" PPM format. The format above, which generally considered
133 the normal one, is known as the "raw" PPM format. See pbm(1) for some
134 commentary on how plain and raw formats relate to one another and how
135 to use them.
136 The difference in the plain format is:
138 •
142 There is exactly one image in a file.
144 •
146 The magic number is P3 instead of P6.
148 •
150 Each sample in the raster is represented as an ASCII decimal
152 number (of arbitrary size).
153 •
155 Each sample in the raster has white space before and after it.
157 There must be at least one character of white space between any
158 two samples, but there is no maximum. There is no particular
159 separation of one pixel from another -- just the required sepa‐
160 ration between the blue sample of one pixel from the red sample
161 of the next pixel.
162 •
164 No line should be longer than 70 characters.
166 Here is an example of a small image in this format.
169 P3
170 # feep.ppm
171 4 4
172 15
173 0 0 0 0 0 0 0 0 0 15 0 15
174 0 0 0 0 15 7 0 0 0 0 0 0
175 0 0 0 0 0 0 0 15 7 0 0 0
176 15 0 15 0 0 0 0 0 0 0 0 0
177 There is a newline character at the end of each of these lines.
180 Programs that read this format should be as lenient as possible, ac‐
182 cepting anything that looks remotely like a PPM image.
183
187 No Internet Media Type (aka MIME type, content type) for PPM has been
188 registered with IANA, but the value image/x-portable-pixmap is conven‐
189 tional.
190 Note that the PNM Internet Media Type image/x-portable-anymap also ap‐
192 plies.
193
197 There are no requirements on the name of a PPM file, but the convention
198 is to use the suffix ".ppm". "pnm" is also conventional, for cases
199 where distinguishing between the particular subformats of PNM is not
200 convenient.
201
205 Before April 2000, a raw format PPM file could not have a maxval
206 greater than 255. Hence, it could not have more than one byte per sam‐
207 ple. Old programs may depend on this.
208 Before July 2000, there could be at most one image in a PPM file. As a
210 result, most tools to process PPM files ignore (and don't read) any
211 data after the first image.
212
215 pnm(1), pgm(1), pbm(1), pam(1), programs that process PPM(1)
216
218 This manual page was generated by the Netpbm tool 'makeman' from HTML
219 source. The master documentation is at
220 http://netpbm.sourceforge.net/doc/ppm.html
222netpbm documentation 09 October 2016 PPM Format Specification(5)