Provided by: netpbm_10.0-12.1ubuntu1_i386 bug

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)