pbm(5)                   (05 March 2000)                   pbm(5)

     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

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            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

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          possible, accepting anything that looks remotely like a bit-
          map.

     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),

     AUTHOR
          Copyright (C) 1989, 1991 by Jef Poskanzer.

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