[prospero]

I don't have time for a full conversion just now, but an eight-way diffusion on the GPU in Penumbra looks like this:

  (let [sum 0.0
        count 0.0]
    (convolution 1
      (+= sum %1)
      (+= count 1))
    (/ sum count))

"convolution" is a keyword that iterates over the neighbors (with a radius of 1, in this case), and does not overrun the boundaries of the source textures, hence the need to keep a running count.

For a more extensive example, see this implementation of Sobel edge detection on the GPU: http://github.com/ztellman/penumbra/blob/master/src/example/...