Can I use a Matte object for Ambient Occlusion?

[davision]

I know how to use a Matte object to render moving objects separate from the background. I know the matte object will catch direct sun shadows and lighting/shadows from an irradiance map.

The question is: can I do fake GI using a VrayAmbientLight with VrayDirt and use a Matte to catch sun shadows AND catch the AO related shading in the alpha channel so it can be composited over a pre-rendered background?

I hope this question makes sense.

[Svetlozar Draganov]

I can't see any reason why the work-flow you mentioned shouldn't work correctly - I've just run a simple test and everything worked as expected.

[davision]

Can you explain how you set it up? I have also tried to test this and the VrayDirt "shadow" was not showing up in alpha.

[tashko.zashev]

Hi Davision,

can I do fake GI using a VrayAmbientLight with VrayDirt and use a Matte to catch sun shadows AND catch the AO related shading in the alpha channel

This approach will allows you to catch the shadows casted by direct light sources, but unfortunately not the shadows produced by the AO (Ambient light).
Here are few screenshots:


Probably you will not get the same result with this workflow as the one with GI, but you can test and use it if the final animation is acceptable for you.
Note that you can always add additional AO render element and use it later at the compositing stage.

[Joelaff]

Could you catch the Occlusion shadows separately from the light shadows? Then composite these together in the post? This would be my preferred method anyway, because then I could tweak the colors independently, etc.

[Svetlozar Draganov]

Unfortunately this is not possible on a single pass - you have to render two separate passes if you want to have a separate control for the light shadows and occlusion.