A simple GIF image decoder in LispWorks.
A GIF object is created using either the read-gif or load-gif functions:
(read-gif input-stream)
(load-gif pathname)
Once you have a GIF object, you can create a graphics port image using the make-gif-image function:
(make-gif-image port gif &optional data-block overlay merge-p)
data-block is one of the image data blocks within the GIF object. Each data-block represents a portion of the overall image. If data-block is nil, then the image will be built by writing all the data blocks in the GIF to a single image.
overlay is the previous image that should be used as the start for the data-block to be written to. Usually this is only valuable when creating animation frames, because each frame builds on the previous one.
merge-p should be T if a new image should not be returned, but the overlay image should be written to directly.
If you'd like to create an animated GIF object, knowing that the GIF is animated, call make-gif-animation.
(make-gif-animation port gif)
If you'd like to know if a GIF is animated, call animated-gif-p on the GIF object. It's possible this returns a false-positive. It simply checks for the "NETSCAPE" application extension in the GIF.