>When the output pixel depends on neighboring pixels (e.g. I strongly
>suspect that bumpmap uses a 3 x 3 neighborhood) the region iterator
>does not work very well.

Not on its own.  However, if you iterate the output region with the
pixel region iterator, and use a pixel fetcher like the one in
whirlpinch for the input sourcing, and there is at least some weak
relationship between input and output locality, the standard tile
cache should be enough to obtain decent efficiency.

Obviously, the more you know about your data access patterns, the more
efficient an access strategy you can develop.  

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