The TIFF PackBits Algorithm
--- TIFF PackBits algorithm
This document describes a simple compression scheme for bilevel
scanned and paint type files.
The TIFF specification defines a number of compression schemes.
Compression type 1 is really no compression, other than basic
pixel packing. Compression type 2, based on CCITT 1D
compression, is powerful, but not trivial to implement.
Compression type 5 is typically very effective for most bilevel
images, as well as many deeper images such as palette color and
grayscale images, but is also not trivial to implement. PackBits
is a simple but often effective alternative.
Several good schemes were already in use in various settings. We
somewhat arbitrarily picked the Macintosh PackBits scheme. It is
byte oriented, so there is no problem with word alignment. And
it has a good worst case behavior (at most 1 extra byte for every
128 input bytes). For Macintosh users, there are toolbox
utilities PackBits and UnPackBits that will do the work for you,
but it is easy to implement your own routines.
A pseudo code fragment to unpack might look like this:
Loop until you get the number of unpacked bytes you are
Read the next source byte into n.
If n is between 0 and 127 inclusive, copy the next n+1 bytes
Else if n is between -127 and -1 inclusive, copy the next
byte -n+1 times.
Else if n is 128, noop.
In the inverse routine, it's best to encode a 2-byte repeat run
as a replicate run except when preceded and followed by a literal
run, in which case it's best to merge the three into one literal
run. Always encode 3-byte repeats as replicate runs.
So that's the algorithm. Here are some other rules:
o Each row must be packed separately. Do not compress across
o The number of uncompressed bytes per row is defined to be
(ImageWidth + 7) / 8. If the uncompressed bitmap is required to
have an even number of bytes per row, decompress into word-
o If a run is larger than 128 bytes, simply encode the
remainder of the run as one or more additional replicate runs.
When PackBits data is uncompressed, the result should be
interpreted as per compression type 1 (no compression).
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