View Full Version : question about occlusion pass


bonestructure
03 March 2008, 02:42 PM
I'm working in Max 7. I've never used occlusion before, but in the scene I'm working on, I'd like to try it. But I have a couple of problems I don't know the answers to. I have the scene in a seperate file set up with the occlusion materials and all, but the scene originally has several lights in it, including one negative light to darken a corner under a table. The scene itself is a night time scene, interior, with the primary light coming from moonlight through a window. Accent light coming from near the camera, set low, as if from another room. Another accent light for rim lighting as coming from a radio dial. Not a strong light, just enough. And another couple of lights just to lighten up an object here and there in the scene that's too dark without it. Apart from the moonlight spot casting shadows through a window tinted slightly blue and set at .8, most of the other lights are set between .5 and .1. It's not an extremely dark scene, but it is night. I don't want to lighten the scene up, so how do I composite the occlusion pass over the scene in order to get the detail and shadow the occlusion provides, without making the scene brighter? The final scene has already been rendered, and post worked, so it's just the way I want it. I just want the shadows and details occlusion provides.

As well, is it absolutely necessary to use Final Gather on an occlusion pass, or can I get along without it?

As I say, I've never done an occlusion pass before, so please excuse me if these are stupid questions. I've looked at online tutprials to find out how to set it up, but none of them answered these questions for me. I should mention that the original scene is rendered with the scanline renderer because I simply do not like the antialiasing mental ray provides. The maps I make never come out with the sharpness I prefer. I wish I had Cebas, but finances aren't there and not likely to be. I should mention that I'm about half blind, so a lack of sharpness in texture maps really irritates the hell out of me. It's hard enough doing art with impaired eyesight as it is.

Hopefully someone here can give me info on these questions. Thanks ahead of time for any help.

ienrdna
03 March 2008, 07:45 PM
Final Gather if form of global illumination not occlusion in it self.
The way I set up, is to create ao material. Ambient/Reflective Occlusion or VrayDirt map in self illumination channel of a material. Apply to every thing and do not use eny other lights, it all self illuminated. And in post multiply ambient occlusion pass over your image.

MikeBracken
03 March 2008, 02:07 AM
In Mentalray this is very easy and renders VERY fast. Just create a MentalRay material. Put the AO shader in the surface slot. Assign to all objects. Turn OFF any GI/FG and any exposure control. As already mentioned here, Multiply the AO pass with your Final pass. In the latest versions there is a material overide slot in the render options.

Regards,
Mike

ctrl.studio
04 April 2008, 08:25 PM
is it absolutely necessary to use Final Gather on an occlusion pass, or can I get along without it?take care. ambient occlusion is so called because it should work on the 'ambient' contribution. ie. multipling an ao pass x your ambient pass, you should limit ambient contribution in zones where there's a lot of occlusion. the same should go if your 'ambient' is actually your global illumination solution. ie. you should multiply the ao x the fg contribution alone. then add the whole to the scene.

max

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