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
   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)


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