It's mostly straightforward except for a few odd quirks, so I'm writing them down.
spec of gif89a
A breakdown of a GIF decoder
The GIFLIB Library
Frame Delay Times for Animated GIFs by humpy77 on deviantART
ImageMagick - Animation Basics -- IM v6 Examples
(Optional) Building zlib, libjpeg, libpng, libtiff and giflib � Leptonica & Visual Studio 2008
theimage.com gif Disposal Methods 2
theimage.com GIF Table of Contents
My notes :
A GIF is a "canvas" and the size of the canvas is in the file header as the "screen width/height". There are then multiple "images" in the file drawn into the canvas.
In theory multiple images could be used even for non-animated gifs. Each image can have its own palette, which lets you do true color gifs by assigning different palettes to different parts of the image. So you should not assume that a GIF decodes into an 8-bit palettized image. I have yet to see any GIF in the wild that does this. (and if you had one, most viewers would interpret it as an animated gif, since delay=0 is not respected literally)
(one hacky way around that which I have seen suggested elsewhere : a gif with multiple images but no GCE blocks should be treated as compositing to form a single still image, whereas GCE blocks even with delay of 0 must be interpreted as animation frames)
Note that animation frames which only update part of the image *is* common. Also the transparency in a GIF must be used when drawing frames onto the canvas - it does not indicate that the final pixel should be transparent. That is, an animation frame may mark some pixels as transparent, and that means don't update those pixels.
There is an (optional) global palette and an (optional) per-image palette. In the global header there is a
"background color". That is an index to the global palette, if it exists. The background color will be visible
in parts of the canvas where there is no image rectangle, and also where images are transparent all the way down to the canvas. However, the ImageMagick link above has this
There is some thinking that rather than clearing the overlaid area to
the transparent color, this disposal should clear it to the 'background'
color meta-data setting stored in the GIF animation. In fact the old
"Netscape" browser (version 2 and 3), did exactly that. But then it also
failed to implement the 'Previous' dispose method correctly.
On the other hand the initial canvas should also be set from the formats
'background' color too, and that is also not done. However all modern
web browsers clear just the area that was last overlaid to transparency,
as such this is now accepted practice, and what IM now follows.
which makes me think that many decoders (eg. web browsers) ignore background and just make those pixels transparent.
(ADD : I've seen quite a few cases where the "background" value is larger than the global palette. eg. global palette has 64 colors, and "background" is 80 or 152.)
In the optional GCE block, each image can have a transparent color set. This is a palette index which acts as a color-key transparency. Tranparent pixels should show whatever was beneath them in the canvas. That is, they do not necessarily result in transparent pixels in the output if there was a solid pixel beneath them in the canvas from a previous image.
Each image has a "delay" time and "dispose" setting in the optional GCE block (which occurs before the image data). These apply *after* that frame.
Delay is the frame time; it can vary per frame, there is no global constant fps. Delay is in centiseconds, and the support for delay < 10 is nasty. In practice you must interpret a delay of 0 or 1 centiseconds to mean "not set" rather than to mean they actually wanted a delay of 0 or 1. (people who take delay too literally are why some gifs play way too fast in some viewers).
Dispose is an action to take on the image after it has been displayed and held for delay time. Note that it applies to the *image* not the canvas (the image may be just a rectangular portion of the canvas). It essentially tells how that image's data will be committed to the canvas for future frames. Unfortunately the dispose value of 0 for "not specified" is extremely common. It appears to be correct to treat that the same as a value of 1 (leave in place).
(ADD : I've seen several cases of a dispose value of "5". Dispososal is supposed to be a 3 bit value, of which only the values 0-3 are defined (and fucking 0 means "undefined"). Values 4-7 are supposed to be reserved.)
The ImageMagick and "theimage.com" links above are very handy for testing disposal and other funny animation details.
It's a shame that zero-delay is so widely mis-used and not supported, because it is the most interesting feature in GIF for encoder flexibility.