just thinking out loud, Ambient produces an even surface lighting, and Diffuse produces brigtness in the middle and darness at the sides, so Ambient minus Diffuse might be something in the right direction.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though