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The PBM Format(5)                                            The PBM Format(5)



NAME
       pbm - Netpbm bi-level image format


DESCRIPTION
       This program is part of Netpbm(1).

       The  PBM  format  is a lowest common denominator monochrome file format.  It serves as the common language of a
       large family of bitmap image 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.

       The name "PBM" is an acronym derived from "Portable Bit Map."

       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.


       ?      A single whitespace character (usually a newline).


       ?      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.

              A row of an image is horizontal.  A column is vertical.  The pixels in the image are square and contigu-
              ous.


       ?      Before  the  whitespace  character  that delimits the raster, any characters from a '#' through the next
              carriage return or newline character, is a comment and is ignored.  Note that this is  rather  unconven-
              tional,  because  a comment can actually be in the middle of what you might consider a token.  Note also
              that this means if you have a comment right before the raster, the newline at the end of the comment  is
              not sufficient to delimit the raster.



       There is actually another version of the PBM format, even 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 read-
       able.  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.

       Netpbm  programs generate Raw PBM format instead of Plain PBM by default, but the common option  -plain chooses
       Plain PBM.

       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 image 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

       There is a newline character at the end of each of these lines.

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

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

       All  characters  referred to herein are encoded in ASCII.  'newline' refers the the character known in ASCII as
       Line Feed or LF.  A 'white space' character is space, CR, LF, TAB, VT, or FF (I.e. what  the  ANSI  standard  C
       isspace() function calls white space).


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
       libnetpbm(1), pnm(1), pgm(1), ppm(1), pam(1), programsthatprocessPBM(1)



netpbm documentation           22 September 2006             The PBM Format(5)