The TIFF PackBits Algorithm

The TIFF PackBits Algorithm


Original Documentation

--- TIFF PackBits algorithm

		  Abstract

		  This document  describes a  simple compression scheme for bilevel
		  scanned and paint type files.


		  Motivation

		  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.


		  Description

		  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
		  expecting:
			   Read the next source byte into n.
			   If n is between 0 and 127 inclusive, copy the next n+1 bytes
		  literally.
			   Else if  n is  between -127  and -1 inclusive, copy the next
		  byte -n+1 times.
			   Else if n is 128, noop.
		  Endloop

		  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
		  row boundaries.

		  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-
		  aligned buffers.
		  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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