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The PBM Format(5) File Formats Manual 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 intermedi-
ary format. In it's purest use, it lives only in a pipe between two other programs.
THE LAYOUT
The format definition is as follows.
A PBM file consists of a sequence of one or more PBM images. There are no data, delim-
iters, or padding before, after, or between images.
Each PBM image consists of the following:
o A "magic number" for identifying the file type. A pbm image's magic number is the
two characters "P4".
o Whitespace (blanks, TABs, CRs, LFs).
o The width in pixels of the image, formatted as ASCII characters in decimal.
o Whitespace.
o The height in pixels of the image, again in ASCII decimal.
o A single whitespace character (usually a newline).
o 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 signifi-
cant bit to least significant bit. The order of the file bytes is from the begin-
ning 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.
o 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 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 suf-
ficient to delimit the raster.
All characters referred to herein are encoded in ASCII. "newline" refers to 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).
Plain PBM
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 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:
o
There is exactly one image in a file.
o
The "magic number" is "P1" instead of "P4".
o
Each pixel in the raster is represented by a byte containing ASCII '1' or '0', rep-
resenting black and white respectively. There are no fill bits at the end of a
row.
o
White space in the raster section is ignored.
o
You can put any junk you want after the raster, if it starts with a white space
character.
o
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.
INTERNET MEDIA TYPE
No Internet Media Type (aka MIME type, content type) for PBM has been registered with
IANA, but the value image/x-portable-bitmap is conventional.
Note that the PNM Internet Media Type image/x-portable-anymap also applies.
FILE NAME
There are no requirements on the name of a PBM file, but the convention is to use the suf-
fix ".pbm". "pnm" is also conventional, for cases where distinguishing between the par-
ticular subformats of PNM is not convenient.
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), programs that process PBM(1)
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master
documentation is at
http://netpbm.sourceforge.net/doc/pbm.html
netpbm documentation 27 November 2013 The PBM Format(5)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
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