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PAM format specification(5)                        PAM format specification(5)



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  cer-
       tain  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  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  subfor-
       mats of PAM.  See Defined Tuple Types .


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)Csubroutinelibrary.

       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
       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.

       Height and width are at least 1.

       Height and width have no defined maximum, but processors and generators of images usually have their own  limi-
       tations.


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 sub-
       formats.


   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 (PBM)

       A black and white image, such as would 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 height, width, and raster bear the obvious relationship to those  of
       the equivalent PBM image.

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

       Grayscale (PGM)

       A  grayscale  image,  such as would 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 (PPM)

       A  color image, such as would be represented by a PPM image, has a typle 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 blue, and the third green.

       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  indi-
       cates  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.



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



netpbm documentation            09 October 2005    PAM format specification(5)