Fast averaging function for images

Not yet. I need to do this as a filter that samples and generates progressively smaller images from the source. Using hardware texture filtering, each step would reduce the image by one fourth in each direction until you arrive at a 1x1 image containing the average image color (potentially adjusted based on mismatches between width and height dimensions).

Apple uses this process in their mean color Core Image filter, and describes it here: http://http.developer.nvidia.com/GPUGems3/gpugems3_ch26.html

I just need to implement something that handles this operation.

Unfortunately, you can't write to floating point textures on iOS, so the only way to extract a value other than 0-255 is to encode it as a color. Your best bet probably would be to extract that from a 1x1 RGBA image.