pbm
Updated: 03 October 2003
Table Of Contents
NAMEpbm ‐ Netpbm bi‐level image format
DESCRIPTION
This program is part of Netpbm.
The PBM format is a lowest common denominator monochrome file
format. It
serves as the common language of a large family of bitmap im‐
age 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 im‐
age.
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 expres‐
sive 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:
1. A "magic number" for identifying the file type. A pbm im‐
age’s magic
number is the two characters "P4".
2. Whitespace (blanks, TABs, CRs, LFs).
3. The width in pixels of the image, formatted as ASCII char‐
acters in
decimal.
4. Whitespace.
5. The height in pixels of the image, again in ASCII decimal.
6. Newline or other single whitespace character.
7. 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 signifi‐
cant 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.
8. 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 re‐
dundant ‐‐ 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 in‐
spection.
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 con‐
taining 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 10 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 00 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 10 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 00 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 possi‐
ble, 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
libnetpbm, pnm, pgm, ppm, pam, programs that process PBM
AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
_________________________________________________________________
Table Of Contents
* NAME
* DESCRIPTION
* COMPATIBILITY
* SEE ALSO
* AUTHOR
