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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 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:
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 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.
o 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 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).
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(3) , pnm(5) , pgm(5) , ppm(5) , pam(5) , programsthatpro-
cessPBM(1)
netpbm documentation 22 September 2006 The PBM Format(5)
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