ppm(5)ppm(5)NAMEppm - portable pixmap file format
DESCRIPTION
The portable pixmap format is a lowest common denominator
color image file format.
It should be noted that this format is egregiously ineffi
cient. It is highly redundant, while containing a lot of
information that the human eye can't even discern. Fur
thermore, 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 infor
mation 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 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."
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.
- Whitespace.
- A height, again in ASCII decimal.
- Whitespace.
- The maximum color value (Maxval), again in ASCII deci
mal. 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 signifi
cant byte is first.
- In the raster, the sample values are "nonlinear." They
are proportional to the intensity of the CIE Rec. 709
red, green, and blue in the pixel, adjusted by the CIE
Rec. 709 gamma transfer function. (That transfer func
tion 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).
- 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. pnmgamma
takes such a PPM variant as input and produces a true
PPM as output.
- 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 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.
- No line should be longer than 70 characters.
Here is an example of a small pixmap 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
Programs that read this format should be as lenient as
possible, accepting anything that looks remotely like a
pixmap.
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 sample. 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 ALSOgiftopnm(1), gouldtoppm(1), ilbmtoppm(1), imgtoppm(1),
mtvtoppm(1), pcxtoppm(1), pgmtoppm(1), pi1toppm(1), pict
toppm(1), pjtoppm(1), qrttoppm(1), rawtoppm(1),
rgb3toppm(1), sldtoppm(1), spctoppm(1), sputoppm(1),
tgatoppm(1), ximtoppm(1), xpmtoppm(1), yuvtoppm(1), ppmtoacad(1), ppmtogif(1), ppmtoicr(1), ppmtoilbm(1), ppmtopcx(1), ppmtopgm(1), ppmtopi1(1), ppmtopict(1), ppmtopj(1), ppmtopuzz(1), ppmtorgb3(1), ppmtosixel(1), ppmtotga(1), ppmtouil(1), ppmtoxpm(1), ppmtoyuv(1),
ppmdither(1), ppmforge(1), ppmhist(1), ppmmake(1), ppmpat(1), ppmquant(1), ppmquantall(1), ppmrelief(1), pnm(5),
pgm(5), pbm(5)AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
08 April 2000 ppm(5)
