Recently i incorporated support for creating thumbnails in glgif, the backend gif animation library used in Anim8gif.

Rather than taking the easy route out and attempting to load the gif using UIImage, i decided to take advantage of glgif’s decode routines to create an accurate representation of the current state of the gif playback.

Put simply, i took the palette texture which is generated for display, and decoded it into a new UIImage.

The actual process is triggered by assigning a delegate (thumbDelegate), which will be invoked when the next frame in the gif is decoded. The delegate is then cleared.

Simple, right?

More Technical Details

The format of a palette texture in GLES is as follows:

The palette can either be 16 or 256 entries long, depending on the format you use. Likewise the pixel data will either use 4 bits or 8 bits. To keep things simple, glgif uses an 8bit palette with 3 8bit entries for the color (GL_PALETTE8_RGB8_OES).

(Note that an 8bit palette can be insufficient as a gif can actually display more than 256 colours across multiple frames, but this is a necessary trade-off for speed)

So in order to decode it:

1. Read and decode the palette
2. Decode each pixel as an index into the palette
3. Using the decoded pixel data, create a new CoreGraphics bitmap context (using CGBitmapContextCreate)
4. Convert the bitmap context into a UIImage (using CGBitmapContextCreateImage and [UIImage imageWithCGImage:])

The resultant UIImage can then easily be used as a base to generate a sized down thumbnail of the gif, as illustrated below.

As always, the code for glgif is hosted on github, so feel free to check it out!