[fitz-dev] On LZW filters

[Paul Sladen]

On Sat, 20 Dec 2003, tor andersson wrote:
> Uhm. We will need to add a LZW filter to fitz eventually.
> What's the status on patents and such?

According to:

  http://www.unisys.com/about__unisys/lzw/

  ``The U.S. LZW patent expires June 20, 2003, the counterpart Canadian
  patent expires July 7, 2004, the counterpart patents in the United
  Kingdom, France, Germany and Italy expire June 18, 2004, and the
  Japanese counterpart patents expire June 20, 2004.''

My memory tells me that it is LZW compression code that is the problem--and
decompression-only code should be fine, *however*, I can't find anything
emphatic to back that up and Unisys are unlikely to say so themselves...

> Should I go ahead with an implementation? If so, is there any reference
> code I should be using?

BSD*'s `compress' is probably pretty canonical, there's also stuff like
GNU's `libungif' which is a cut-down version of `libgif' and claims to be
free of any questionable algorithms (for decompression only):

  http://www.gnu.org/directory/libs/image/libungif.html

For `libtiff' there's a drop-over tarball that contains a single 1000-line
file (`tif_lzw.c') which does the business along with a README filing
reminding that what you're about to do is (was) probably illegal in the
States:

  ftp://ftp.remotesensing.org/libtiff/libtiff-lzw-compression-kit-1.3.tar.gz

Hope that helps,

	-Paul
-- 
War is inconsistent with Truth.  London, GB