man > pam(5)

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  ar‐
       ray.

       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 im‐
       age, 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 .


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  collec‐
       tively.   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  in‐
       formation  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 pro‐
       gram 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 sub‐
       routine 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 num‐
       ber.

       Note: xv thumbnail images also start with the "P7" magic number.  (This and PAM were indepen‐
       dent  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  essen‐
       tially 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  tu‐
       ples 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 tu‐
       ple) 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 sepa‐
       rately 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 tu‐
       ple  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 al‐
       pha plane or transparency plane).

       In  this  kind of image, the color represented by a pixel is actually a combination of an ex‐
       plicitly 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 opac‐
       ity 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) back‐
       ground 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  back‐
       ground 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

netpbm documentation                      27 November 2013               PAM format specification(5)