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pam(5) File Formats Manual pam(5)
NAME
pam - portable arbitrary map file format
DESCRIPTION
The PAM image format is a lowest common denominator 2 dimensional map format.
It is designed to be used for any of myriad kinds of graphics, but can theoretically be
used for any kind of data that is arranged as a two dimensional rectangular array. Actu-
ally, from another perspective it can be seen as a format for data arranged as a three di-
mensional array.
This format does not define the meaning of the data at any particular point in the array.
It could be red, green, and blue light intensities such that the array represents a visual
image, or it could be the same red, green, and blue components plus a transparency compo-
nent, or it could contain annual rainfalls for places on the surface of the Earth. Any
process that uses the PAM format must further define the format to specify the meanings of
the data.
A PAM image describes a two dimensional grid of tuples. The tuples are arranged in rows
and columns. The width of the image is the number of columns. The height of the image is
the number of rows. All rows are the same width and all columns are the same height. The
tuples may have any degree, but all tuples have the same degree. The degree of the tuples
is called the depth of the image. Each member of a tuple is called a sample. A sample is
an unsigned integer which represents a locus along a scale which starts at zero and ends
at a certain maximum value greater than zero called the maxval. The maxval is the same
for every sample in the image. The two dimensional array of all the Nth samples of each
tuple is called the Nth plane or Nth channel of the image.
Though the format does not assign any meaning to the tuple values, it does include an op-
tional string that describes that meaning. The contents of this string, called the tuple
type, are arbitrary from the point of view of the PAM format, but users of the format may
assign meaning to it by convention so they can identify their particular implementations
of the PAM format.
The Layout
A PAM file consists of a sequence of one or more PAM images. There are no data, delim-
iters, or padding before, after, or between images.
Each PAM image consists of a header followed immediately by a raster.
Here is an example header:
P7
WIDTH 227
HEIGHT 149
DEPTH 3
MAXVAL 255
TUPLTYPE RGB
ENDHDR
The header begins with the ASCII characters "P7" followed by newline. This is the magic
number.
The header continues with an arbitrary number of lines of ASCII text. Each line ends with
and is delimited by a newline character.
Each header line consists of zero or more whitespace-delimited tokens or begins with "#".
If it begins with "#" it is a comment and the rest of this specification does not apply to
it.
A header line which has zero tokens is valid but has no meaning.
The type of header line is identified by its first token, which is 8 characters or less:
ENDHDR This is the last line in the header. The header must contain exactly one of these
header lines.
HEIGHT The second token is a decimal number representing the height of the image (number
of rows). The header must contain exactly one of these header lines.
WIDTH The second token is a decimal number representing the width of the image (number of
columns). The header must contain exactly one of these header lines.
DEPTH The second token is a decimal number representing the depth of the image (number of
planes or channels). The header must contain exactly one of these header lines.
MAXVAL The second token is a decimal number representing the maxval of the image. The
header must contain exactly one of these header lines.
TUPLTYPE
The header may contain any number of these header lines, including zero. The rest
of the line is part of the tuple type. The rest of the line is not tokenized, but
the tuple type does not include any white space immediately following TUPLTYPE or
at the very end of the line. It does not include a newline. If there are multiple
TUPLTYPE header lines, the tuple type is the concatenation of the values from each
of them, separated by a single blank, in the order in which they appear in the
header. If there are no TUPLTYPE header lines the tuple type is the null string.
The raster consists of each row of the image, in order from top to bottom, consecutive
with no delimiter of any kind between, before, or after, rows.
Each row consists of every tuple in the row, in order from left to right, consecutive with
no delimiter of any kind between, before, or after, tuples.
Each tuple consists of every sample in the tuple, in order, consecutive with no delimiter
of any kind between, before, or after, samples.
Each sample consists of an unsigned integer in pure binary format, with the most signifi-
cant byte first. The number of bytes is the minimum number of bytes required to represent
the maxval of the image.
PAM Used For PNM (PBM, PGM, or PPM) Images
A common use of PAM images is to represent the older and more concrete PBM, PGM, and PPM
images.
A PBM image is conventionally represented as a PAM image of depth 1 with maxval 1 where
the one sample in each tuple is 0 to represent a black pixel and 1 to represent a white
one. The height, width, and raster bear the obvious relationship to those of the PBM im-
age. The tuple type for PBM images represented as PAM images is conventionally "BLACKAND-
WHITE".
A PGM image is conventionally represented as a PAM image of depth 1. The maxval, height,
width, and raster bear the obvious relationship to those of the PGM image. The tuple type
for PGM images represented as PAM images is conventionally "GRAYSCALE".
A PPM image is conventionally represented as a PAM image of depth 3. The maxval, height,
width, and raster bear the obvious relationship to those of the PPM image. The first
plane represents red, the second green, and the third blue. The tuple type for PPM images
represented as PAM images is conventionally "RGB".
The Confusing Universe of Netpbm Formats
It is easy to get confused about the relationship between the PAM format and PBM, PGM,
PPM, and PNM. Here is a little enlightenment:
"PNM" is not really a format. It is a shorthand for the PBM, PGM, and PPM formats collec-
tively. It is also the name of a group of library functions that can each handle all
three of those formats.
"PAM" is in fact a fourth format. But it is so general that you can represent the same
information in a PAM image as you can in a PBM, PGM, or PPM image. And in fact a program
that is designed to read PBM, PGM, or PPM and does so with a recent version of the Netpbm
library, will read an equivalent PAM image just fine and the program will never know the
difference.
To confuse things more, there is a collection of library routines called the "pam" func-
tions that read and write the PAM format, but also read and write the PBM, PGM, and PPM
formats. They do this because the latter formats are much older and more popular, so this
makes it convenient to write programs that use the newer PAM format.
SEE ALSO
pbm(5), pgm(5), ppm(5), pnm(5), libpnm(3)
31 July 2000 pam(5)
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