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PPM Format Specification
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ppm
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Updated: 03 October 2003
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Table Of Contents
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NAME
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PPM - Netpbm color image format
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DESCRIPTION
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This program is part of Netpbm.
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The PPM format is a lowest common denominator color image file
format.
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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.
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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 format.  They are informally called PPM files, but to be
absolutely precise, you should indicate the variation from true PPM.
For example, "PPM using the red, green, and blue colors that the
scanner in question uses."
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The name "PPM" is an acronym derived from "Portable Pixel Map."
Images in this format (or a precursor of it) were once also called
"portable pixmaps."
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The format definition is as follows.  You can use the libnetpbm C subroutine library to read and
interpret the format conveniently and accurately.
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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.
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Each PPM image consists of the following:
.IP 1
A "magic number" for identifying the file type.
A ppm image's magic number is the two characters "P6".
.IP 2
Whitespace (blanks, TABs, CRs, LFs).
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A width, formatted as ASCII characters in decimal.
.IP 4
Whitespace.
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A height, again in ASCII decimal.
.IP 6
Whitespace.
.IP 7
The maximum color value (Maxval), again in ASCII decimal.  Must be less
than 65536 and more than zero.
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Newline or other single whitespace character.
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A raster of Height rows, in order from top to bottom.  Each row
consists of Width pixels, in order from left to right.  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 first.
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A row of an image is horizontal.  A column is vertical.  The pixels
in the image are square and contiguous.
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In the raster, the sample values are "nonlinear." They are
proportional to the intensity of the ITU-R Recommendation BT.709 red,
green, and blue in the pixel, adjusted by the BT.709 gamma transfer
function.  (That transfer function specifies a gamma number of 2.2 and
has a linear section for small intensities).  A value of Maxval for
all three samples represents CIE D65 white and the most intense color
in the color universe of which the image is part (the color universe
is all the colors in all images to which this image might be
compared).
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ITU-R Recommendation BT.709 is a renaming of the former CCIR
Recommendation 709.  When CCIR was absorbed into its parent
organization, the ITU, ca. 2000, the standard was renamed.  This
document once referred to the standard as CIE Rec. 709, but it isn't
clear now that CIE ever sponsored such a standard.
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Note that another popular color space is the newer sRGB.  A common
variation on PPM is to subsitute this color space for the one specified.
.IP 11
Note that a common variation on the PPM format is to have the sample
values be "linear," i.e. as specified above except without
the gamma adjustment.  \fBpnmgamma\fR takes such a PPM variant as
input and produces a true PPM as output.
.IP 12
Characters from a "#" to the next end-of-line, before
the maxval line, are comments and are ignored.
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Note that you can use \fBpamdepth\fR to convert between a the
format with 1 byte per sample and the one with 2 bytes per sample.
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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 \fBpbm\fR for some commentary on how plain
and raw formats relate to one another and how to use them.
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The difference in the plain format is:
.RS
.IP "-"
There is exactly one image in a file.
.IP "-"
The magic number is P3 instead of P6.
.IP "-"
Each sample in the raster is represented as an ASCII decimal number
(of arbitrary size).
.IP "-"
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 samples, 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.
.IP "-"
No line should be longer than 70 characters.
.RE
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Here is an example of a small pixmap in this format.
.DS L
P3
# feep.ppm
4 4
15
 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
.DE
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There is a newline character at the end of each of these lines.
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Programs that read this format should be as lenient as possible,
accepting anything that looks remotely like a pixmap.
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COMPATIBILITY
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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 sample.  Old
programs may depend on this.
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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.
.SH 2
SEE ALSO
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pnm,
pgm,
pbm,
pam,
programs that process PPM
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\l'5i'
.SH 2
Table Of Contents
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.IP \(bu
DESCRIPTION
.IP \(bu
COMPATIBILITY
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SEE ALSO
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