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)