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ppm
!!!ppm
NAME
DESCRIPTION
COMPATIBILITY
SEE ALSO
AUTHOR
----
!!NAME


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
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,


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


-


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.


-


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


-


Note that a common variation on the PPM format is to have
the sample values be
pnmgamma__
takes such a PPM variant as input and produces a true PPM as
output.


-


Characters from a


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:
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 ALSO


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), 411toppm(1)
!!AUTHOR


Copyright (C) 1989, 1991 by Jef Poskanzer.
----
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Last edited on Wednesday, September 10, 2003 8:33:28 pm by GreigMcGill
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