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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 con‐
       version	filters.  Because the format pays no heed to efficiency, it is
       simple and general enough that one can easily develop programs to  con‐
       vert 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 signif‐
	      icant 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 contiguous.

       ·      Before  the  whitespace  character that delimits the raster, any
	      characters from a '#' through the next carriage return  or  new‐
	      line  character, is a comment and is ignored.  Note that this is
	      rather unconventional, 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 simplis‐
       tic, more lavishly wasteful of space than PBM, called Plain PBM.	 Plain
       PBM actually came first, but even its inventor couldn't stand its reck‐
       lessly  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.

       Netpbm programs generate	 Raw  PBM  format  instead  of	Plain  PBM  by
       default,	 but  the  common  option  ⟨index.html#commonoptions⟩	-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 pnmtoplainpnm 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)
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