pbm(5)pbm(5)NAMEpbm - portable bitmap file format
DESCRIPTION
The portable bitmap format is a lowest common denominator
monochrome file format. It serves as the common language
of a large family of bitmap 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.
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 interme
diary 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 deci
mal.
- Newline or other single whitespace character.
- 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.
- 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 squander
ous 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 impossi
ble 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 orig
inal image. And we hardly need to define the format here,
because you can decode it by inspection.
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 bitmap in the plain PBM for
mat:
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
You can generate the Plain PBM format from the regular PBM
format (first image in the file only) with the pnmto
plainpnm program.
Programs that read this format should be as lenient as
possible, 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 ALSOlibpbm(3),pnm(5),pgm(5),ppm(5)AUTHOR
Copyright (C) 1989, 1991 by Jef Poskanzer.
05 March 2000 pbm(5)
