ppm - 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 pgm
file'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
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 proportional to the
intensity of the CIE Rec. 709 red, green, and blue in
the pixel. A value of Maxval for all three samples rep
resents 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).
- 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
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 ALSO
giftopnm(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), ppm
toacad(1), ppmtogif(1), ppmtoicr(1), ppmtoilbm(1), ppm
topcx(1), ppmtopgm(1), ppmtopi1(1), ppmtopict(1), ppm
topj(1), ppmtopuzz(1), ppmtorgb3(1), ppmtosixel(1), ppm
totga(1), ppmtouil(1), ppmtoxpm(1), ppmtoyuv(1),
ppmdither(1), ppmforge(1), ppmhist(1), ppmmake(1), ppm
pat(1), ppmquant(1), ppmquantall(1), ppmrelief(1), pnm(5),
pgm(5), pbm(5)
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
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