LZW hint

Michael A. Peters mpeters at mac.com
Thu May 8 06:56:24 PDT 2003


-------------- next part --------------
Title:		LZW Compression for Graphics Libraries in BLFS
BLFS VERSION:	1.0
Author:		Michael A. Peters <mpeters at mac.com>

SYNOPSIS:
	Adding LZW compression to graphics applications that can utilize it

HINT:
ver 1.0
08/05/2003

Contents
--------

	0. Preface
	1. Why care about LZW
	2. Legal Issues
	3. Giflib as replacement for libungif
		3a. Makefile Issues
	4. libtiff
	5. gd library

0. Preface
----------
	In the early 1980's a compression algorithm known as LZW emerged that
	was very good at lossless compression of data. This algorithm was used
	in a variety of software products, such as the UNIX compress command.

	LZW was chosen as the compression algorithm for the CompuServe GIF image
	format, as well as the TIFF image format. A free library emerged called
	Giflib that allowed freeware and shareware authors to write programs for
	the GIF image format, and as a result, the GIF image format became very
	popular.

	A company called Unisys existed that owned a patent on this algorithm,
	but they did not complain until the GIF image format was already in very
	wide use. At that point in time, they decided they wanted to charge a
	very expensive licensing fee to use the LZW compression algorithm.

	Since free software is free, this became a problem for the free software
	industry. The result was that LZW was ripped out of several products.

	This document tells you how to put it back in since many countries do
	not recognize the Unisys software patent, and the patent is very close
	to expiring in the U.S. if it has not expired already.

1. Why Care About LZW
---------------------
	The PNG image format has largely replaced the GIF image in the free
	software world. However, the evil was not with LZW - but rather, with
	the software patent that restricted its use without licensing.
	
	Since the compression algorithm itself is a very good one, there is no
	reason not to use it where we can. Also, while PNG can be used as a
	replacement for GIF, there is not really a suitable replacement for the
	TIFF image format. Patching LZW support back into libtiff will allow the
	creation of compressed TIFF images, and the compression makes a big
	difference in the final file size.
	
2. Legal Issues
---------------
	In some countries it may not be legal to use the LZW algorithm without
	paying a license fee. To the best of my knowledge the patent expires in
	June 2003 in the United States. However, I believe the patent does not
	expire in Japan until June 2004. You are advised to follow your local
	law with respect to using the LZW compression algorithm and any license
	fees that you are required to pay to do so. You are also advised to
	look up the patent expiration date yourself, rather than rely on the
	information I provide. I am not a patent lawyer.
	
3. Giflib as replacement for libungif
-------------------------------------
	libungif was written as a replacement for Giflib. libungif does not use
	LZW but instead produces uncompressed GIF images. If you would rather
	produce compressed GIF images, then build Giflib instead of libungif.
	
	Giflib 4.1.0 can be downloaded from:
	http://ftp.rge.com/pub/multimedia/libungif/giflib-4.1.0.tar.gz
	
	Follow the same build instructions for libungif in the BLFS book.
	
3a. Makefile Issues
-------------------
	Most configure scripts will find libgif in your library path and use
	that if you don't have libungif install. This is not universally true.
	Some packages, such as emacs, will specifically look for libungif.
	
	There are two ways to solve this issue. The first to make the following
	symlinks in your /usr/lib directory:
		ln -s libgif.a libungif.a
		ln -s libgif.la libungif.la
		ln -s libgif.so libungif.so
		ln -s libgif.so.4 libungif.so.4
		ln -s libgif.so.4.1.0 libungif.so.4.1.0

	The second method, which is a little cleaner IMHO, is to modify the
	configure scripts and Makefiles of the source to the software before
	building it. For example, with emacs, there are two files that need
	to be edited: configure and src/Makefile.in
	
	In both files you just need to change every reference of lungif to lgif:
	
	cp configure configure.orig &&
	sed -e s?"ungif"?"gif"? < configure.orig > configure &&
	cd src &&
	cp Makefile.in Makefile.in.orig &&
	sed -e s?"ungif"?"gif"? < Makefile.in.orig > Makefile.in &&
	cd ..
	
	Then you can proceed to build as normal and emacs will use libgif.
	
4. libtiff
----------
	To put LZW compression back into libtiff, all you need to do is apply
	the LZW Compression Kit to the source before building it.
	You can download the kit from:
	ftp://ftp.remotesensing.org/libtiff/libtiff-lzw-compression-kit-1.3.tar.gz
	
	The official instructions in the kit say:
	"Just copy tif_lzw.c over the copy in libtiff and rebuild libtiff."
	
	In other words, unpack the libtiff source as you would while following
	the BLFS instructions. Before you do anything else, also unpack the
	libtiff-lzw-compression-kit and replace the tif_lzw.c file in the
	libtiff source directory with the one in the compression kit.
	
	Then continue to build libtiff as described in the BLFS book.
	
5. gd library
-------------
	Most applications that offer gif support will use libgif or libungif.
	However, some applications will look for gif support in the gd library
	and use gd for gif support if it finds it.
	
	The author of the gd library no longer includes any gif support in his
	library. However, we can patch gif support (with LZW compression) back
	into gif so that software that wants to use gd for gif support can find
	it.
	
	The gd library can be downloaded from:
	http://www.boutell.com/gd/http/gd-2.0.12.tar.gz
	
	The patch to the gd library can be downloaded from:
	http://downloads.rhyme.com.au/gd/patch_gd2.0.12_gif_20030401.gz
	
	to build:
	patch -p1 < ../patch_gd2.0.12_gif_20030401 &&
	./configure --prefix=/usr &&
	make &&
	make install &&
	/sbin/ldconfig
	
	It is best to build gd after building zlib, libpng, freetype2, libjpeg,
	and XFree86 - as gd will use those libraries if configure finds them.

	
	
	


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