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
       TUPLETYPE 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 TUPLETYPE 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)