pam

PAM format specification(5)    File Formats Manual   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 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(3)
        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.


   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 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(5) , pgm(5) , ppm(5) , pnm(5) , libnetpbm(3)



netpbm documentation            27 November 2013     PAM format specification(5)