giflib

a library for reading and writing GIF files.

Overview

giflib provides code for reading GIF files and transforming them into RGB bitmaps, and for writing RGB bitmaps as GIF files. GIF is a legacy format and the giflib maintainers recommend against generating new images in the format. PNG has a more extensible design with better color support and better compression.

Animation and on-line identification

The primary reason to use GIF is support for terminal graphics and color animations. GIF specifies a terminal protocol in Appendix G for on-line identification of the GIF Data Stream for display of images in terminals.

Animated PNG (APNG) offers an alternative for animations but has not been ratified as of 2022 and this is not included in the libpng implementation. Here are details of the libpng fork supporting Mozilla's APNG variant:

Project information

This repository if a fork of giflib adding CMake support and minor portability changes to support building with Microsoft Visual Studio.

Latest versions of giflib are currently hosted at:

http://sourceforge.net/projects/giflib/

Please report bugs to the bug tracker on sourceforge:

https://sourceforge.net/p/giflib/bugs/

The project has a long and confusing history, described in the history section of this document which was sourced from history.adoc.

Authors

Gershon Elber, <gershon(at-symbol)cs.technion.sc.il>, original giflib code.
Toshio Kuratomi, <toshio(at-symbol)tiki-lounge.com>, uncompressed gif writing code, former maintainer.
Eric Raymond, <esr(at-symbol)snark.thyrsus.com>, current and long time former maintainer of giflib code.

There have been many other contributors; see the attributions in the version-control history to learn more.

Building

A CMake project has been added to this project to aid in the use of giflib in cross platform projects. The code has been tested to build on Linux, macOS and Windows with GCC, Clang and the Microsoft compiler.

cmake -B build
cmake --build build
cmake --install build

Portability

This codebase is C99-conformant.

The Makefile is required to build the docs and a distribution tarball. You will need xmlto to build the derived forms of the documentation from the DocBook-XML sources. If you are going no modify the website, you will also need to have asciidoc and the ImageMagick convert utility installed.

You can run "make check" after the library and utilities have been built to see a regression test of the codebase. No output (other than the test header lines) is good news.

Release Instructions

Check the SourceForge tracker for bugs and patches.
Bump the version number in gif_lib.h. Do "make version" to confirm that it looks sane when extracted to the Makefile.
Version-stamp the top entry in the NEWS file.
If you are changing major versions, sync the XBS-SourceForge-Folder attribute in the control file.
'make dist' to make a tarball.
Tag the release in the repo.
Ship the release tarball.

The last three steps can be done with "make release" if you have shipper installed.

History

GIF (Graphics Interchange Format) was originally developed on the CompuServe timesharing service in the late 1980s. It was described by a GIF standard issued in 1987 and revised in 1989. A copy of the GIF89 standard is included in the doc/ directory.

This code originated as a linkable library for DOS programs, together with command-line tools for generating and viewing and analyzing GIF images. The DOS code was written by Gershon Elber using Borland C under MS-DOS sometime between the issue of GIF87 and mid-1989 (1.0 was dated 14 June 1989; one portion, getarg.c, was dated 11 Mar 88).

At some time no later than the end of 1989 Eric S. Raymond (aka "ESR") ported this DOS version to System V Unix. Between 1989 and 1992 ESR reworked various portions of the API, improving and simplifying the code's interface.

ESR's 2.1 version was the first to include the DGifSlurp()/EGifSpew() function pair for enabling non-sequential operations on GIF images (also the tools icon2gif, gifovly, and gifcompose; the last was removed in 5.0).

ESR's Unix port was incorporated into the NCSA Mosaic browser in 1994, which is how GIF became (with JPEG) one of the two most popular image formats on the early Web.

Beginning around 1993, patent claims by Unisys over the LZW compression method used in GIF theatened adverse legal consequences for users and developers of programs incorporating the format. The threats became serious in 1999, with Unisys demanding license fees for any software using the format.

One response to this was the development of PNG in 1995. Another was that ESR sought a lead developer outside the U.S. to hand the project off to, and passed it to Toshio Kuratomi. ESR remembers this as happening in 1994, but that date could be wrong as some headers imply 3.0 was issued under ESR's name in 1996. But other files do date Toshio's first release to 1994. Toshio shipped 4.0 in December 1998.

Subsequently, the project shipped for some time as "libungif" with support for compressed GIFs removed to avoid the LZW patent issues. Compression support was merged back in after the last blocking patent expired in 2004; this became release 4.0.0. After that merge the code was again known as giflib.

By 2006, support for PNGs was sufficiently universal that GIF could be described as a legacy format. Anything you can do with it GIF could probably be better done with PNG. Nevertheless (and despite efforts like "Burn All GIFs Day" in November 1999) the GIF format has remained widely popular.

In April 2012 ESR rejoined the project to do some code cleanups and auditing, and Toshio Kuratomi asked him to take back the lead. ESR released version 4.2 in May 2012.

Version 5.0, released in June 2012, fulfilled almost all the to-do items from 18 years of backlog. It made the library thread-safe, added direct support for GIF89 graphics control blocks, and tossed out large amounts of obsolete utility code.

More recent version of the code (5.1.0 and onwards) have been hardened by both static analysis and fuzz testing. While these failed to turn up bugs in normal rendering cases, they did uncover some crash and corruption bugs that could be tickled by carefully crafted malformed GIFs.

This code is very old, very stable, and everywhere - browsers game consoles, smartphones, pretty much everything that opens an HTTP port and does graphics uses it.

The utilities in this source tree were important as GIF production tools early in the format's history, but have been superseded by multi-format viewers and editors. Most installable binary packages shipped as 'giflib' include the library and header file only.

michaeljclark / giflib