pam

pam(5)                        File Formats Manual                       pam(5)

NAME
       pam - portable arbitrary map file format

DESCRIPTION
       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.

       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  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 im-
       age.

       Though the format does not assign any meaning to the tuple  values,  it
       does  include an optional string that describes that meaning.  The con-
       tents of this string, called the tuple type,  are  arbitrary  from  the
       point  of  view  of  the PAM format, but users of the format may assign
       meaning to it by convention so they can identify their  particular  im-
       plementations of the PAM format.

   The Layout
       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.

       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 con-
              tain 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, includ-
              ing  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.  If  there
              are  multiple  TUPLTYPE header lines, the tuple type is the con-
              catenation of the values from each of them, separated by a  sin-
              gle  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 bot-
       tom, consecutive with no delimiter of any kind between, before, or  af-
       ter, 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.

   PAM Used For PNM (PBM, PGM, or PPM) Images
       A common use of PAM images is to represent the older and more  concrete
       PBM, PGM, and PPM images.

       A  PBM  image  is  conventionally represented as a PAM image of depth 1
       with 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  PBM  image.   The
       tuple  type  for PBM images represented as PAM images is conventionally
       "BLACKANDWHITE".

       A PGM image is conventionally represented as a PAM image  of  depth  1.
       The  maxval, height, width, and raster bear the obvious relationship to
       those of the PGM image.  The tuple type for PGM images  represented  as
       PAM images is conventionally "GRAYSCALE".

       A  PPM  image  is conventionally represented as a PAM image of depth 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.  The tuple type for PPM  images  represented
       as PAM images is conventionally "RGB".

   The Confusing Universe of Netpbm Formats
       It  is easy to get confused about the relationship between the PAM for-
       mat 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 this makes it conve-
       nient to write programs that use the newer PAM format.

SEE ALSO
       pbm(5), pgm(5), ppm(5), pnm(5), libpnm(3)

                                 31 July 2000                           pam(5)