PAM format specification(5) File Formats Manual PAM format specification(5)
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 PBM 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 PAM 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)
libnetpbm 11.10.05 - Generated Fri Aug 15 07:18:13 CDT 2025