78 lines
2.6 KiB
Groff
78 lines
2.6 KiB
Groff
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.TH pbm 5 "27 September 1991"
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.SH NAME
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pbm - portable bitmap file format
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.SH DESCRIPTION
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The portable bitmap format is a lowest common denominator monochrome
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file format.
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.IX "PBM file format"
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It was originally designed to make it reasonable to mail bitmaps
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between different types of machines using the typical stupid network
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mailers we have today.
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Now it serves as the common language of a large family of bitmap
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conversion filters.
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The definition is as follows:
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.IP - 2
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A "magic number" for identifying the file type.
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A pbm file's magic number is the two characters "P1".
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.IX "magic numbers"
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.IP - 2
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Whitespace (blanks, TABs, CRs, LFs).
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.IP - 2
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A width, formatted as ASCII characters in decimal.
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.IP - 2
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Whitespace.
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.IP - 2
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A height, again in ASCII decimal.
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.IP - 2
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Whitespace.
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.IP - 2
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Width * height bits, each either '1' or '0', starting at the top-left
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corner of the bitmap, proceeding in normal English reading order.
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.IP - 2
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The character '1' means black, '0' means white.
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.IP - 2
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Whitespace in the bits section is ignored.
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.IP - 2
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Characters from a "#" to the next end-of-line are ignored (comments).
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.IP - 2
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No line should be longer than 70 characters.
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.PP
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Here is an example of a small bitmap in this format:
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.nf
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P1
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# feep.pbm
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24 7
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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 1 1 1 0
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0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 1 0
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0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 0 0 0 1 1 1 1 0
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0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0
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0 1 0 0 0 0 0 1 1 1 1 0 0 1 1 1 1 0 0 1 0 0 0 0
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0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
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.fi
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.PP
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Programs that read this format should be as lenient as possible,
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accepting anything that looks remotely like a bitmap.
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.PP
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There is also a variant on the format, available
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by setting the RAWBITS option at compile time. This variant is
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.IX RAWBITS
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different in the following ways:
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.IP - 2
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The "magic number" is "P4" instead of "P1".
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.IP - 2
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The bits are stored eight per byte, high bit first low bit last.
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.IP - 2
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No whitespace is allowed in the bits section, and only a single character
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of whitespace (typically a newline) is allowed after the height.
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.IP - 2
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The files are eight times smaller and many times faster to read and write.
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.SH AUTHOR
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Copyright (C) 1989, 1991 by Jef Poskanzer.
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.\" Permission to use, copy, modify, and distribute this software and its
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.\" documentation for any purpose and without fee is hereby granted, provided
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.\" that the above copyright notice appear in all copies and that both that
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.\" copyright notice and this permission notice appear in supporting
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.\" documentation. This software is provided "as is" without express or
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.\" implied warranty.
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