NAME

       pbm - portable bitmap file format

DESCRIPTION

       The  portable  bitmap  format  is  a  lowest common denominator monochrome file format.  It serves as the
       common language of a large family of bitmap conversion filters.  Because  the  format  pays  no  heed  to
       efficiency,  it  is simple and general enough that one can easily develop programs to convert to and from
       just about any other graphics format, or to manipulate the image.

       This is not a format that one would normally use to store a file or to transmit it to someone -- it's too
       expensive  and not expressive enough for that.  It's just an intermediary format.  In it's purest use, it
       lives only in a pipe between two other programs.

       The format definition is as follows.

       A PBM file consists of a sequence of one or more PBM images. There are no data,  delimiters,  or  padding
       before, after, or between images.

       Each PBM image consists of the following:

       - A "magic number" for identifying the file type.  A pbm image's magic number is the two characters "P4".

       - Whitespace (blanks, TABs, CRs, LFs).

       - The width in pixels of the image, formatted as ASCII characters in decimal.

       - Whitespace.

       - The height in pixels of the image, again in ASCII decimal.

       - Newline or other single whitespace character.

       - A raster of Height rows, in order from top to bottom.  Each row is Width bits, packed 8 to a byte, with
         don't care bits to fill out the last byte in the row.  Each bit represents a pixel: 1 is  black,  0  is
         white.   The order of the pixels is left to right.  The order of their storage within each file byte is
         most significant bit to least significant bit.  The order of the file bytes is from  the  beginning  of
         the file toward the end of the file.

       - Characters  from  a  "#"  to  the  next end-of-line, before the width/height line, are comments and are
         ignored.

       There is actually another version of the PBM format, even more more simplistic, more lavishly wasteful of
       space  than  PBM,  called Plain PBM.  Plain PBM actually came first, but even its inventor couldn't stand
       its recklessly squanderous use of resources after a while and switched to what we now know as the regular
       PBM  format.   But Plain PBM is so redundant -- so overstated -- that it's virtually impossible to break.
       You can send it through the most liberal mail system (which was the original purpose of the  PBM  format)
       and  it  will arrive still readable.  You can flip a dozen random bits and easily piece back together the
       original image.  And we hardly need to define the format here, because you can decode it by inspection.

       The difference is:

       - There is exactly one image in a file.

       - The "magic number" is "P1" instead of "P4".

       - Each pixel in the raster is represented by a byte containing ASCII '1' or '0', representing  black  and
         white respectively.  There are no fill bits at the end of a row.

       - White space in the raster section is ignored.

       - You can put any junk you want after the raster, if it starts with a white space character.

       - No line should be longer than 70 characters.

       Here is an example of a small bitmap in the plain PBM format:
       P1
       # feep.pbm
       24 7
       0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
       0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0
       0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0
       0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 1 0
       0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0
       0 1 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 0 0 0 0
       0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

       You can generate the Plain PBM format from the regular PBM format (first image in the file only) with the
       pnmtoplainpnm program.

       Programs that read this format should be as lenient as possible, accepting anything that  looks  remotely
       like a bitmap.

COMPATIBILITY

       Before July 2000, there could be at most one image in a PBM file.  As a result, most tools to process PBM
       files ignore (and don't read) any data after the first image.

SEE ALSO

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

AUTHOR

       Copyright (C) 1989, 1991 by Jef Poskanzer.

                                                  05 March 2000                                           pbm(5)