Provided by: netpbm_10.97.00-2_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 ⟨#tupletype⟩ .
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