Provided by: netpbm_11.05.02-1.1build1_amd64 
NAME
pam - Netpbm common 2-dimensional bitmap format
GENERAL
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.
The name "PAM" is an acronym derived from "Portable Arbitrary Map." This derivation makes more sense if
you consider it in the context of the other Netpbm format names: PBM, PGM, and PPM.
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 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 basic 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 basic 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. Some tuple types are
defined as official subformats of PAM. See Defined Tuple Types .
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 even a new program must work with them. Having
the library handle all the formats makes it convenient to write programs that use the newer PAM format as
well.
THE LAYOUT
A convenient way to read and write the PAM format accurately is via the libnetpbm(1) C subroutine
library.
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
TUPLTYPE RGB
ENDHDR
The header begins with the ASCII characters "P7" followed by newline. This is the magic number.
Note: xv thumbnail images also start with the "P7" magic number. (This and PAM were independent
extensions to the Netpbm formats). The rest of the format makes it easy to distinguish PAM from that
format, though).
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. There must be something other than white space after the TUPLTYPE token.
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 TUPLTYPE 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.
The character referred to as "newline" herein is the character known in ASCII as Line Feed or LF.
LIMITATIONS
Height, width, depth, and maxval are at least 1.
Height, width, and depth have no defined maximum, but processors and generators of images usually have
their own limitations.
The maxval of an image is never greater than 65535. (The reason it is limited is to make it easier to
build an image processor, in which intermediate arithmetic values often have to fit within 31 or 32
bits). There was no specified limitation before October, 2005, but essentially all implementations have
always observed it.
DEFINED TUPLE TYPES
Some tuple types are defined in this specification to specify official subformats of PAM for especially
popular applications of the format. Users of the format may also define their own tuple types, and thus
their own subformats.
Tuple type affects only the meanings of the samples (which are unsigned integers) in the tuples of the
image. It does not affect how the samples or tuples are encoded. Tuple type may affect the meaning of a
tuple's position in the array (e.g. it may indicate in a visual image that a tuple in Row 1 is one at the
top of the image rather than the bottom).
Tuple type never determines how many samples are in a tuple (that is instead determined by the DEPTH
header line). Tuple type could be said to imply a depth (number of samples per tuple) because certain
tuple types are valid only in combination with certain DEPTH values, but it is good programming practice
to use DEPTH for the depth when decoding the raster and separately validate that the depth is consistent
with the tuple type. Also, it is good practice to accept a depth that is too great and just ignore the
higher numbered planes.
PAM Used For Visual Images
A common use of PAM images is to represent visual images such as are typically represented by images in
the older and more concrete PBM, PGM, and PPM formats.
Black And White
A black and white image, such as would alternatively be represented by a PBM image, has a tuple type of
"BLACKANDWHITE". Such a PAM image has a depth of 1 and maxval 1 where the one sample in each tuple is 0
to represent a black pixel and 1 to represent a white one. The maxval, height, width, and order of
tuples in the raster bear the obvious relationship to those of the equivalent PGM image.
Note that in the PBM format, a sample value of zero means white, but in PAM, zero means black.
Grayscale
A grayscale image, such as would alternatively be represented by a PGM image, has a tuple type of
"GRAYSCALE". Such a PAM image has a depth of 1. The maxval, height, width, and raster bear the obvious
relationship to those of the equivalent PGM image.
Color
A color image, such as would alternatively be represented by a PPM image, has a tuple type of "RGB".
Such a PAM image has a depth of 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.
Transparent
Each of the visual image formats mentioned above has a variation that contains transparency information.
In that variation, the tuple type has "_ALPHA" added to it (e.g. "RGB_ALPHA") and one more plane. The
highest numbered plane is the opacity plane (sometimes called an alpha plane or transparency plane).
In this kind of image, the color represented by a pixel is actually a combination of an explicitly
specified foreground color and a background color to be identified later.
The planes other than the opacity plane describe the foreground color. A sample in the opacity plane
tells how opaque the pixel is, by telling what fraction of the pixel's light comes from the foreground
color. The rest of the pixel's light comes from the (unspecified) background color.
For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indicates a gray tone 60% of white and Plane 1
indicates opacity 25%. The foreground color is the 60% gray, and 25% of that contributes to the ultimate
color of the pixel. The other 75% comes from some background color. So let's assume further that the
background color of the pixel is full white. Then the color of the pixel is 90% of white: 25% of the
foreground 60%, plus 75% of the background 100%.
The sample value is the opacity fraction just described, as a fraction of the maxval. Note that it is
not gamma-adjusted like the foreground color samples.
INTERNET MEDIA TYPE
No Internet Media Type (aka MIME type, content type) for PBM has been registered with IANA, but the
unofficial value image/x-portable-arbitrarymap is assigned by this specification, to be consistent with
conventional values for the older Netpbm formats.
FILE NAME
The conventional suffix for the name of a PAM file is ".pam". But this is not required.
SEE ALSO
Netpbm(1), pbm(1), pgm(1), ppm(1), pnm(1), libnetpbm(1)
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation
is at
http://netpbm.sourceforge.net/doc/pam.html
netpbm documentation 27 November 2013 PAM format specification(5)