Table Of Contents
pam
Updated: 09 October 2005
GENERAL
The PAM image format is a lowest common denominator 2 dimen‐
sional 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 perspec‐
tive it can be
seen as a format for data arranged as a three dimensional ar‐
ray.
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 intensi‐
ties 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 mean‐
ings 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 im‐
age. 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 tu‐
ple 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 mean‐
ing to it by
convention so they can identify their particular implementa‐
tions 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 li‐
brary 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 rou‐
tines 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 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 num‐
ber. (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 mean‐
ing.
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 ex‐
actly 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 in‐
clude 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 val‐
ues 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 mini‐
mum number of
bytes required to represent the maxval of the image.
LIMITATIONS
The maxval of an image is never greater than 65535. (The rea‐
son 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.
Height and width are at least 1.
Height and width have no defined maximum, but processors and
generators of
images usually have their own limitations.
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.
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 (PBM)
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.
Note that in the PBM format, a zero value means white, but in
PAM, zero
means black.
Grayscale (PGM)
A grayscale image, such as would be represented by a PGM im‐
age, 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 (PPM)
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 im‐
age. The first
plane represents red, the second blue, and the third green.
Transparent
Each of the visual image formats mentioned above has a varia‐
tion that
contains transparency information. In that variation, the tu‐
ple 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 explcitly specified foreground color and a
background
color to be identified later.
The planes other than the opacity plane describe the fore‐
ground 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 col‐
or.
For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indi‐
cates a gray
tone 60% of white and Plane 1 indicates opacity 25%. The fore‐
ground color is
the 60% gray, and 25% of that contributes to the ultimate col‐
or 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 fore‐
ground color
samples.
SEE ALSO
Netpbm, pbm, pgm, ppm, pnm, libnetpbm
_________________________________________________________________
Table Of Contents
* GENERAL
* THE LAYOUT
* LIMITATIONS
* The Confusing Universe of Netpbm Formats
* DEFINED TUPLE TYPES
+ PAM Used For Visual Images
o Black And White
o Grayscale
o Color
* SEE ALSO
