TIFF 6.0 Specification
Final—June 3, 1992
Section 13: LZW Compression
This section describes TIFF compression scheme 5, an adaptive compression
scheme for raster images.
Restrictions
When LZW compression was added to the TIFF specification, in Revision 5.0, it
was thought to be public domain. This is, apparently, not the case.
The following paragraph has been approved by the Unisys Corporation:
“The LZW compression method is said to be the subject of United States patent
number 4,558,302 and corresponding foreign patents owned by the Unisys Cor-
poration. Software and hardware developers may be required to license this patent
in order to develop and market products using the TIFF LZW compression option.
Unisys has agreed that developers may obtain such a license on reasonable, non-
discriminatory terms and conditions. Further information can be obtained from:
Welch Licensing Department, Office of the General Counsel, M/S C1SW19,
Unisys Corporation, Blue Bell, Pennsylvania, 19424.”
Reportedly, there are also other companies with patents that may affect LZW
implementors.
Reference
Terry A. Welch, “A Technique for High Performance Data Compression”, IEEE
Computer, vol. 17 no. 6 (June 1984). Describes the basic Lempel-Ziv & Welch
(LZW) algorithm in very general terms. The author’s goal is to describe a hard-
ware-based compressor that could be built into a disk controller or database en-
gine and used on all types of data. There is no specific discussion of raster images.
This section gives sufficient information so that the article is not required reading.
Characteristics
LZW compression has the following characteristics:
• LZW works for images of various bit depths.
• LZW has a reasonable worst-case behavior.
• LZW handles a wide variety of repetitive patterns well.
• LZW is reasonably fast for both compression and decompression.
• LZW does not require floating point software or hardware.
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