LZW

File Format
Name	LZW
Ontology	Electronic File Formats Compression LZW
LoCFDD	fdd000135
PRONOM	x-cmp/12
Released	1984

LZW (Lempel-Ziv-Welch) is a lossless compression algorithm based on LZ78. It was developed by Terry Welch in 1984. It is infamous for being widely used in various standard formats (especially GIF) by developers unaware that it was encumbered by patents. The patents expired around 2003.

There are many variations of LZW in use. In general, LZW compresses a stream of symbols into a stream of bits. For general-purpose file compression, the symbols are usually taken to be bytes, but for GIF format the symbols are pixels.

Related formats

• GIF
• TIFF
• compress (Unix)
• LZMW

Resources

• A Technique for High-Performance Data Compression Welch, Terry A. Computer, Vol. 17, No. 6 (June 1984): pp. 8-19
• Lempel-Ziv-Welch : Wikipedia
• LZW Revisited a 2011 update to an article by Mark Nelson written 20 years earlier.
• Lempel-Ziv-Welch (LZW) Compression, from the Encyclopedia of Graphics File Formats
• How LZW (GIF) Compression Works
• US Patent 4,558,302