Man page 5 : pam

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 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.

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 green, and the third blue. The tuple type for PPM images represented as PAM images is conventionally "RGB".

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.