Ambient Occlusion - How to comp


I ran out an AO pass in Maya and thought I would use an over node to comp it in but found out that an over doesnt work. So I used an iMult instead, is this the correct node to use here?


Yes, you have to multiply your occlusion pass over your image.


Some people think doing a strait multiply is wrong, giving you a “greasy” occlusion. If you are comping together your indirect and direct lighting from separate passes (aovs in prman) then you can multiply the the occlusion to only the indirect lighting. It stands to reason that ambient occlusion would only occlude ambiance.

In the end, do what looks right, this science for arts sake, not the other way around. Often I end up doing a tight occlusion and a simple multiply. If you are doing arch/vis I doubt that would be a good idea.


we have an occlusion script node that we use. its a mix based on iMult where you have something like fade included in there amongst others (for extra control without having too much of a mess). like others said. use what makes it look better. also depends on how you rendered out ur occ pass. (is it darker or lighter, high contrast or low contrast with lots of greys)

try stuff out.


The way i have it set up is Im multiplying the occlusion over my diffuse pass. Both the occlussion and diffuse have a premultiplied halo around the object so Ive used a Matt Div to help with this. Although it has worked for the diffuse pass, it doesnt work properly with the occlusion and starts to effect other parts of the image that is unrelated to the matt.

Both these passes are then over’d by a volumetric pass, it is this pass that is getting the strange artifacts on when I MDiv the occlusion.

Any ideas here before I re-render my occlusion without premult on?


I normally multiply the occlusion over the indirect lighting, leaving the direct lighting untouched. Sometimes I invert the occlusion and use it as a mask for a colour adjustment instead of multiplying. By using a colour adjustment node, it makes it easy to add a colour cast (usually blue, I guess) to your occlusiony bits, which I find rather useful in a lot of circumstances.


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