ppm - portable pixmap file format
The portable pixmap format is a lowest common denominator color image file
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 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.
- 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 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 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).
- 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
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:
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 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
giftopnm(1), gouldtoppm(1), ilbmtoppm(1), imgtoppm(1), mtvtoppm(1), pcxtoppm(1),
pgmtoppm(1), pi1toppm(1), picttoppm(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)
Copyright (C) 1989, 1991 by Jef Poskanzer.