PPM Format Specification(5) PPM Format Specification(5)NAME
PPM - Netpbm color image format
DESCRIPTION
This program is part of Netpbm(1)
The PPM format is a lowest common denominator color image file format.
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
uses.'
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."
The format definition is as follows. You can use the libnetpbm(1)
C subroutine library to read and interpret the format conveniently and
accurately.
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.
·
Whitespace.
·
A height, again in ASCII decimal.
·
Whitespace.
·
The maximum color value (Maxval), again in ASCII decimal. Must
be less than 65536 and more than zero.
· A single whitespace character (usually a newline).
· 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. Other‐
wise, it is 2 bytes. The most significant byte is first.
A row of an image is horizontal. A column is vertical. The
pixels in the image are square and contiguous.
In the raster, the sample values are 'nonlinear.' They are pro‐
portional 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).
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.
Note that another popular color space is the newer sRGB. A com‐
mon variation on PPM is to substitute this color space for the
one specified.
Note that a common variation on the PPM format is to have the
sample values be 'linear,' i.e. as specified above except with‐
out the gamma adjustment. pnmgamma takes such a PPM variant as
input and produces a true PPM as output.
Strings starting with '#' may be comments, the same as with PBM(1)
Note that you can use pamdepth 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(1)
for some commentary on how plain and raw formats relate to one another
and how to use them.
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 samples, but there is no maximum. There is no particular
separation of one pixel from another -- just the required sepa‐
ration 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 image in this format.
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
There is a newline character at the end of each of these lines.
Programs that read this format should be as lenient as possible,
accepting anything that looks remotely like a PPM image.
All characters referred to herein are encoded in ASCII. 'newline'
refers the character known in ASCII as Line Feed or LF. A 'white
space' character is space, CR, LF, TAB, VT, or FF (I.e. what the ANSI
standard C isspace() function calls white space).
COMPATIBILITY
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.
SEE ALSOpnm(1) , pgm(1) , pbm(1) , pam(1) , programsthatprocessPPM(1)netpbm documentation 03 October 2003 PPM Format Specification(5)
