...just rambling here (brain-fart)...

Let's assume the "up" you mention above is really just another way of describing where the light is coming from (which, it is :) ).

Suppose you found the location / direction of each light-source (unsure atm how to treat IBL, without knowing the pixel brightnesses).

Suppose you also assigned a weight to each light-source, based on it's intensity.

Then you compute 'up' (direction of light) as the average of the positions (or at least directions) of the sources in the scene, weighted by thier respective intensities.

...would that do us any good as 'up', relative to your AO calcs, above?

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