The portable pixmap format is a lowest common denominator color image
It should be noted that this format is egregiously inefficient. It is
highly redundant, while containing a lot of information that the human
eye can't even discern. Furthermore, the format allows very little
information about the image besides basic color, which means you may
have to couple a file in this format with other independent information
to get any decent use out of it. However, it is very easy to write and
analyze programs to process this format, and that is the point.
It should also be noted that files often conform to this format in
every respect except the precise semantics of the sample values. These
files are useful because of the way PPM is used as an intermediary for-
mat. They are informally called PPM files, but to be absolutely pre-
cise, you should indicate the variation from true PPM. For example,
"PPM using the red, green, and blue colors that the scanner in question
The format definition is as follows.
A PPM file consists of a sequence of one or more PPM images. There are
no data, delimiters, or padding before, after, or between images.
Each PPM image consists of the following:
- A "magic number" for identifying the file type. A ppm image's magic
number is the two characters "P6".
- Whitespace (blanks, TABs, CRs, LFs).
- A width, formatted as ASCII characters in decimal.
- A height, again in ASCII decimal.
- The maximum color value (Maxval), again in ASCII decimal. Must be
less than 65536.
- Newline or other single whitespace character.
- A raster of Width * Height pixels, proceeding through the image in
normal English reading order. Each pixel is a triplet of red, green,
and blue samples, in that order. Each sample 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
- In the raster, the sample values are "nonlinear." They are propor-
- 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 sample and the one with 2 bytes per sample.
There is actually another version of the PPM format that is fairly
rare: "plain" PPM format. The format above, which generally considered
the normal one, is known as the "raw" PPM 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 P3 instead of P6.
- Each sample in the raster is represented as an ASCII decimal number
(of arbitrary size).
- Each sample in the raster has white space before and after it. There
must be at least one character of white space between any two sam-
ples, but there is no maximum. There is no particular separation of
one pixel from another -- just the required separation between the
blue sample of one pixel from the red sample of the next pixel.
- No line should be longer than 70 characters.
Here is an example of a small pixmap in this format:
0 0 0 0 0 0 0 0 0 15 0 15
0 0 0 0 15 7 0 0 0 0 0 0
0 0 0 0 0 0 0 15 7 0 0 0
15 0 15 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 pixmap.
Before April 2000, a raw format PPM 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 PPM file. As a
result, most tools to process PPM files ignore (and don't read) any
data after the first image.
Copyright (C) 1989, 1991 by Jef Poskanzer.
08 April 2000 ppm(5)
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