# Introduction to GIFLIB

GIFLIB is a package of portable tools and library routines for working with GIF images. You can find the latest version at the GIFLIB home page http://www.ccil.org/~esr/giflib.

The Graphics Interchange Format(c) specification is the copyrighted property of CompuServe Incorporated. GIF(sm) is a service mark property of CompuServe Incorporated. As this package existed before UniSys's lawyeritis attack of New Years' Day 1995, it is grandfathered in under their license terms and you do not have to pay fees for using it.

This package has been released under an X Consortium-life freeware license. Use and copy as you see fit. If you make useful changes, add new tools, or find and fix bugs, please send your mods to the maintainers for general distribution.

The util directory includes programs to clip, rotate, scale, and position GIF images. It includes an X11 viewer, code to dump GIFs to an Epson-compatible printer in graphics mode, and many conversion utilities. These are no replacement for an interactive graphics editor, but they can be very useful for scripted image generation or transformation.

The library includes program-callable entry points for reading and writing GIF files, an 8x8 utility font for embedding text in GIFs, and an error handler. GIF manipulation can be done at a relatively low level by sequential I/O (which automatically does/undoes image compression) or at a higher level by slurping an entire GIF into allocated core.

This library speaks both GIF87a and GIF89. The differences between GIF87 and GIF89 are minor: in the latter, the interpretation of some extension block types is defined. The library never needs to actually interpret these, but giftext notices them.

# Utilities

Here is a summary of the utilities in this package. If you're looking at this page through a web browser, each utility name should be a hotlink to HTML documentation.

## Conversion Utilities

gif2bgi
  display GIFs on IBM PC displays using the BGI (Borland) driver
gif2epsn
  dump images saved as GIF files on Epson type printers
gif2herc
  display GIFs on IBM PC displays using the Hercules graphic card
gif2iris
  display GIFs under SGI NeWs window system
gif2ps
  print GIF file on laser printers supporting PostScript
gif2rgb
  convert images saved as GIF to 24-bit RGB image(s)
raw2gif
  convert raw 8-bit image data into GIF files
rgb2gif
  convert 24 bit images to a GIF image using color quantization

## Test Pattern Generators

gifbg
  generate a single-color test pattern GIF
gifcolor
  generate color test patterns
gifwedge
  create a test GIF image resembling a color monitor test pattern

## Image Manipulation Components

gifasm
  assemble multiple GIFs into one, or burst a multiple-mage GIF
gifclip
  clip or crop a GIF image
gifclrmp
  modify GIF image colormaps
gifcomb
  combine 2 GIF images of exactly the same size into one
giffix
  clumsily attempts to fix truncated GIF images
gifflip
  flip GIF image along X or Y axis or rotate by 90 degrees
gifinter
  convert between interlaced and non interlaced images
gifovly
  generate one composite GIF from a multiple-image GIF
gifpos
  change a GIF's screen size or recondition it.
gifrotat
  rotate a GIF through any desired angle
gifrsize
  resize a GIF by deletion or duplication of bits
gifburst
  burst a GIF image into subrectangles.

## Report Generators

giftext
  print (text only) general information about a GIF
gifhisto
  generate color-frequency histogram from a GIF

## GIF Composition Tools

icon2gif
  converter/deconverter to/from an editable text format
text2gif
  generate GIF images out of regular text in 8x8 font
gifinto
  end-of-pipe fitting for GIF-processing pipelines
gifcompose
  use giflib tools to compose images

## C Code Templates

giffiltr
  template code for filtering a GIF sequentially
gifspnge
  template code for filtering a GIF with in-core operations

Under MS-DOS, most filters will print the current input scan line number (counting up) whenever they read image input, and will print output image line number (counting down) when they dump output. Utilities that only read or write always print in increasing order. Utilities (like GifPos that only change positions) that copy the image as a block of compressed data will print nothing --- they cannot identify a scan line number, and are enough faster that the feedback to the user doesn't seem necessary.

Some of the utilities require memory on the order of the whole screen, while others read one scan line at a time. Each utility HTML file has entry called Memory Usage which will be one of:

Line
  memory required is on the order of one scan line
Image
  proportional to the size of the biggest image in GIF file
Screen
  proportional to GIF screen size

In all cases a byte is allocated per pixel, so an image of 320 by 200 pixels will requires approximately 64k bytes of main memory.

# Library Functions

The library contains two groups of C functions. One group does sequential I/O on the stream-oriented GIF format. The other supports grabbing an entire GIF into allocated core, operating on it in core, and then writing the modified in-core GIF out to disk.

Unless you are on a 286 or some other very memory-limited machine running under DOS, you probably want to use the second group.

Detailed documentation on the library entry points is in gif_lib.html. Library error codes are described in liberror.html

# The GIF Standard

The doc subdurector includes flat-ASCII descriptions of GIF89 format and Lempel-Ziv Compression.

# History

This package was originally written by Gershon Elber <gershon@cs.utah.edu> in 1990 on an IBM PC under MS-DOS using Borland Turbo C. He made it portable to several UNIX environments.

The 2.1 version featured substantial changes and additions by Eric S. Raymond <esr@snark.thyrsus.com>. These included the DGifSlurp/EGifSpew function pair for enabling non-sequential operations on GIF images and the tools icon2gif, gifovly, gifburst, and gifcompose.

The 2.4 version converted all the docs from an idiosyncratic plain-text formal to to HTML.

# Package Status

GIFLIB's current maintainer is Eric S. Raymond. You can find his home page at http://www.ccil.org/esr.

GIFLIB is not under active development, but bug fixes are being accepted.