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 image.

       Though the 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 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.

   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 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.  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.

   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 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 this makes it convenient 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)