pbm(5)                                                          File Formats Manual                                                         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 pro-
       grams 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)