Sunday, June 21, 2009

Fixed cost gaussian blur (or: what no-one else will tell you)

Well the title's a bit of an exaggeration. But this piece of code is used in ColorLab for calculating the gaussian blur kernel. You get an almost arbitrary blur strength (sigma) for an almost fixed cost (in fact it's faster the stronger the blur). It does this by approximating a little:

- Assumes you first scale down the image using a box filter N times (the algorithm gives you the N).
- You then run the shader pass, using M bilinear texture samples. Most of those samples actually pick up two pixels, for a near 2x speedup.

I haven't actually seen correct calculations of the bilinear sample points online so I worked it out myself. Turned out to be trivial, I wonder why I couldn't find it.

Since this kind of set-up code is hard to come by (for some reason) I'm posting it here:

Click to get the code

I rather like how it turned out. With contrast rich images you can sometimes get "popping" due to the decimations, but by setting MAX_WEIGHTS arbitrarily high you can pretty much bypass the decimation step and still use the generator (and in particular OptimizeBilinear).

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