pam
NAME DESCRIPTION SEE ALSO
pam - portable arbitrary map file format
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
pbm(5), pgm(5), ppm(5), pnm(5), libpnm(3)?
4 pages link to pam(5):
