SunOS man pages : pam (3)
Standards, Environments, and Macros pam(5)
NAME
pam - portable arbitrary map file format
DESCRIPTION
The PAM image format is a lowest common denominator 2 dimen-
sional map format.
It is designed to be used for any of myriad kinds of graph-
ics, 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 dimensional 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 component, or it could con-
tain 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 optional string that describes
that meaning. The contents of this string, called the tuple
type, are arbitrary from the point of view of the PAM for-
mat, but users of the format may assign meaning to it by
convention so they can identify their particular implementa-
tions of the PAM format.
The Layout
A PAM file consists of a sequence of one or more PAM images.
There are no data, delimiters, or padding before, after, or
between images.
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Standards, Environments, and Macros pam(5)
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
TUPLETYPE 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 new-
line 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
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Standards, Environments, and Macros pam(5)
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 TUPLETYPE 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 significant 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,
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 image. The tuple type for PBM images
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Standards, Environments, and Macros pam(5)
represented as PAM images is conventionally "BLACKANDWHITE".
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 convention-
ally "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 blue, and the third green.
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 collectively. 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" functions 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),
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