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'up' may (is) not really specific enough... is there some way to base it on direction to light-source?

Sure but why bother. Just shine the light on it and get rid of AO.

We're dealing with a particular phenomenon here, which you guys are not seeing in this thread. Ice-boy saw a render I posted and noted that for that particular outdoor scenario, I had too much AO on the ears and face. This is because AO is an approximate technique, not accurate. (Given the approximation, I wish it were a lot faster than a proper GI technique.)

Anyway, the fault lies with material based AO and the inaccuracy it produces for directional lights. We should use light-based AO and then directional lights don't matter. Everybody needs to bug Smith Micro about this. Then we can get the problem dealt with and stop hacking.

I don't see a point to dealing with directional lights in AO because AO is supposed to be for non-directional lights. The kind of detailed adjustment needed for directional lights could only work if we could actually tell if the directional light is shining on this currently rendered point or not. We can't tell that in the shader. Also, we can't tell which direction is mostly occluded either. I'm just showing you another hack that - under particular circumstances - will reduce the error you observe.

If you want to modulate it - modulate however you like. I'm thinking this modulation is pretty helpful.

I'll post some images in a second.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)