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NAME

       pam - Netpbm common 2-dimensional bitmap format

GENERAL

       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.

       The name "PAM" is an acronym derived from "Portable Arbitrary Map." This derivation  makes
       more  sense  if you consider it in the context of the other Netpbm format names: PBM, PGM,
       and PPM.

       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 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  basic 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 basic 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.  Some tuple types are defined as official subformats of
       PAM.  See Defined Tuple Types ⟨#tupletype⟩ .

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
       even a new program must work with them.  Having the library handle all the  formats  makes
       it convenient to write programs that use the newer PAM format as well.

THE LAYOUT

       A  convenient  way  to  read and write the PAM format accurately is via the libnetpbm(1) C
       subroutine library.

       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.

       Note: xv thumbnail images also start with the "P7"  magic  number.   (This  and  PAM  were
       independent  extensions  to  the Netpbm formats).  The rest of the format makes it easy to
       distinguish PAM from that format, though).

       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.  There must be
              something other than white space after the TUPLTYPE token.

              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.

       The character referred to as "newline" herein is the character known in ASCII as Line Feed
       or LF.

LIMITATIONS

       Height, width, depth, and maxval are at least 1.

       Height,  width, and depth have no defined maximum, but processors and generators of images
       usually have their own limitations.

       The maxval of an image is never greater than 65535.  (The reason it is limited is to  make
       it  easier to build an image processor, in which intermediate arithmetic values often have
       to fit within 31 or 32 bits).  There was no specified limitation before October, 2005, but
       essentially all implementations have always observed it.

DEFINED TUPLE TYPES

       Some  tuple  types are defined in this specification to specify official subformats of PAM
       for especially popular applications of the format.  Users of the format  may  also  define
       their own tuple types, and thus their own subformats.

       Tuple  type  affects only the meanings of the samples (which are unsigned integers) in the
       tuples of the image.  It does not affect how the samples or  tuples  are  encoded.   Tuple
       type  may affect the meaning of a tuple's position in the array (e.g. it may indicate in a
       visual image that a tuple in Row 1 is one at the top of the image rather than the bottom).

       Tuple type never determines how many samples are in a tuple (that is instead determined by
       the  DEPTH header line).  Tuple type could be said to imply a depth (number of samples per
       tuple) because certain tuple types are  valid  only  in  combination  with  certain  DEPTH
       values,  but  it is good programming practice to use DEPTH for the depth when decoding the
       raster and separately validate that the depth is consistent with the tuple type.  Also, it
       is  good  practice to accept a depth that is too great and just ignore the higher numbered
       planes.

   PAM Used For Visual Images
       A common use of PAM images is to represent visual images such as are typically represented
       by images in the older and more concrete PBM, PGM, and PPM formats.

       Black And White

       A  black and white image, such as would alternatively be represented by a PBM image, has a
       tuple type of "BLACKANDWHITE".  Such a PAM image has a depth of 1 and maxval 1  where  the
       one  sample  in each tuple is 0 to represent a black pixel and 1 to represent a white one.
       The maxval, height, width, and order of tuples in the raster bear the obvious relationship
       to those of the equivalent PGM image.

       Note  that  in  the PBM format, a sample value of zero means white, but in PAM, zero means
       black.

       Grayscale

       A grayscale image, such as would alternatively be represented by a PGM image, has a  tuple
       type  of  "GRAYSCALE".  Such a PAM image has a depth of 1.  The maxval, height, width, and
       raster bear the obvious relationship to those of the equivalent PGM image.

       Color

       A color image, such as would alternatively be represented by a PPM image, has a tuple type
       of  "RGB".  Such a PAM image has a depth of 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.

       Transparent

       Each  of  the  visual  image  formats  mentioned  above  has  a  variation  that  contains
       transparency information.  In that variation, the tuple type  has  "_ALPHA"  added  to  it
       (e.g.  "RGB_ALPHA")  and  one more plane.  The highest numbered plane is the opacity plane
       (sometimes called an alpha plane or transparency plane).

       In this kind of image, the color represented by a pixel is actually a  combination  of  an
       explicitly specified foreground color and a background color to be identified later.

       The  planes  other  than the opacity plane describe the foreground color.  A sample in the
       opacity plane tells how opaque the pixel is, by telling what fraction of the pixel's light
       comes  from  the  foreground  color.   The  rest  of  the  pixel's  light  comes  from the
       (unspecified) background color.

       For example, in a GRAYSCALE_ALPHA image, assume Plane 0 indicates a gray tone 60% of white
       and  Plane 1 indicates opacity 25%.  The foreground color is the 60% gray, and 25% of that
       contributes to the ultimate color of the pixel.  The other 75% comes from some  background
       color.   So  let's  assume  further  that the background color of the pixel is full white.
       Then the color of the pixel is 90% of white:  25% of the foreground 60%, plus 75%  of  the
       background 100%.

       The  sample  value  is  the  opacity fraction just described, as a fraction of the maxval.
       Note that it is not gamma-adjusted like the foreground color samples.

INTERNET MEDIA TYPE

       No Internet Media Type (aka MIME type, content type) for  PBM  has  been  registered  with
       IANA,   but  the  unofficial  value  image/x-portable-arbitrarymap  is  assigned  by  this
       specification, to be consistent with conventional values for the older Netpbm formats.

FILE NAME

       The conventional suffix for the name of a PAM file is ".pam".  But this is not required.

SEE ALSO

       Netpbm(1), pbm(1), pgm(1), ppm(1), pnm(1), libnetpbm(1)

DOCUMENT SOURCE

       This manual page was generated by the Netpbm tool 'makeman' from HTML source.  The  master
       documentation is at

              http://netpbm.sourceforge.net/doc/pam.html