pam(5)pam(5)NAMEpam - 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. Actually, 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
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 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 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, delimiters, 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
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
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 chan
nels). 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 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 repre
sent 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 image. The tuple type
for PBM images 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
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 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 lit
tle 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 ALSOpbm(5), pgm(5), ppm(5), pnm(5), libpnm(3)
31 July 2000 pam(5)
