.TL
PAM format specification
Table Of Contents
.SH 1
pam
.LP
Updated: 09 October 2005
.br
<?makeman .SH NAME ?>
<?makeman pam - Netpbm common 2-dimensional bitmap format ?>
.SH 2
GENERAL
.LP
.LP
The PAM image format is a lowest common denominator 2 dimensional map
format.
.LP
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.
.LP
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.
.LP
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.
.LP
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.
.LP
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.
.SH 2
The Confusing Universe of Netpbm Formats
.LP
.LP
It is easy to get confused about the relationship between the PAM
format and PBM, PGM, PPM, and PNM.  Here is a little enlightenment:
.LP
"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.
.LP
"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.
.LP
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.
.SH 2
THE LAYOUT
.LP
.LP
A convenient way to read and write the PAM format accurately is via the
libnetpbm C subroutine library.
.LP
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.
.LP
Each PAM image consists of a header followed immediately by a raster.
.LP
Here is an example header:
.LP
\fBP7\fR
.br
\fBWIDTH 227\fR
.br
\fBHEIGHT 149\fR
.br
\fBDEPTH 3\fR
.br
\fBMAXVAL 255\fR
.br
\fBTUPLTYPE RGB\fR
.br
\fBENDHDR\fR
.LP
The header begins with the ASCII characters "P7" followed
by newline.  This is the magic number.
.LP
Note: \fBxv\fR 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).
.LP
The header continues with an arbitrary number of lines of ASCII
text.  Each line ends with and is delimited by a newline character.
.LP
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.
.LP
A header line which has zero tokens is valid but has no meaning.
.LP
The type of header line is identified by its first token, which is
8 characters or less:
.RS
.IP "\fBENDHDR  \fR"
This is the last line in the header.  The header must contain
exactly one of these header lines.
.IP "\fBHEIGHT  \fR"
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.
.IP "\fBWIDTH\fR"
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.
.IP "\fBDEPTH\fR"
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.
.IP "\fBMAXVAL\fR"
The second token is a decimal number representing the maxval of the image.
The header must contain exactly one of these header lines.
.IP "\fBTUPLTYPE\fR"
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 \fBTUPLTYPE \fR or at the very end
of the line.  It does not include a newline.  If there are multiple
\fBTUPLTYPE\fR 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
\fBTUPLTYPE\fR header lines the tuple type is the null string.  
.RE
.LP
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.
.LP
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.
.LP
Each tuple consists of every sample in the tuple, in order,
consecutive with no delimiter of any kind between, before, or after,
samples.
.LP
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.
.SH 2
LIMITATIONS
.LP
.LP
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.
.LP
Height and width are at least 1.
.LP
Height and width have no defined maximum, but processors and generators
of images usually have their own limitations.
.SH 2
DEFINED TUPLE TYPES
.LP
.LP
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.
.SH 3
PAM Used For Visual Images
.LP
.LP
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.
.SH 4
Black And White (PBM)
.LP
.LP
A black and white image, such as would 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
height, width, and raster bear the obvious relationship to those of
the equivalent PBM image.
.LP
Note that in the PBM format, a zero value means white, but in PAM,
zero means black.
.SH 4
Grayscale (PGM)
.LP
.LP
A grayscale image, such as would 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.
.SH 4
Color (PPM)
.LP
.LP
A color image, such as would be represented by a PPM image, has a
typle 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 blue, and
the third green.
.SH 4
Transparent
.LP
.LP
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).
.LP
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.
.LP
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.
.LP
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%.
.LP
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.
.SH 2
SEE ALSO
.LP
\fBNetpbm\fR,
\fBpbm\fR,
\fBpgm\fR,
\fBppm\fR,
\fBpnm\fR,
\fBlibnetpbm\fR
.br
\l'5i'
.SH 2
Table Of Contents
.LP
.IP \(bu
GENERAL
.IP \(bu
THE LAYOUT
.IP \(bu
LIMITATIONS
.IP \(bu
The Confusing Universe of Netpbm Formats
.IP \(bu
DEFINED TUPLE TYPES
.IP \(bu
PAM Used For Visual Images
.IP \(bu
Black And White
.IP \(bu
Grayscale
.IP \(bu
Color
.LP
.LP
.IP \(bu
SEE ALSO
.LP