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pam
!!!pam
NAME
DESCRIPTION
SEE ALSO
----
!!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.
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


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


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 __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, 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


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


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


__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:






To confuse things more, there is a collection of library
routines called the
!!SEE ALSO


pbm(5), pgm(5), ppm(5), pnm(5),
libpnm(3)
----
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Last edited on Tuesday, June 4, 2002 12:30:40 am by "perry"
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