View Full Version : Ambient occlusion with ceiling: how?

11-13-2006, 12:41 PM
Hi all,
i'm working about an interior lighting and i'm tryng to achieve an illumination.
My question is: with the ambient occlusion material i obtein this image only without the ceiling:


The same image computed in FG and GI (very long time after so many many many trials....):

Now if i want composit (to sharp the shadows.....and other stuff) is not possible because the ceiling is missing, but if i put the ceiling the image is begin very dark....
Any tips?

11-13-2006, 01:45 PM
So from what I understand you want to composite the Ao image over another one, but the black makes unnecessary parts dark, right ?

Just add an output shader to the pass to set the background color to white instead of black, or put a big sphere with a constant white material around your scene (you might wanna disable the secondary rays on that in case it has influence over the rest of the AO)

11-13-2006, 01:57 PM
Hi Marciani,
My first advice to you is not to get too frustrated with Mental Ray. It really wasn't built for doing Archviz easily - all the amazing looking archviz you see around is created with renderers that can create great pictures with a single click (and relatively no R&D/CGI knowledge). (Mental ray is set up to produce many, many frames of photorealistic character animation at high speed - so the drawback is that it ends up being like a science experiment.)

I noticed in your last thread (Archiviz) you pointed people (me included) to the global illumination tutorial - I'm suprised you didn't go into depth with it, because it's really good. I just spent all weekend playing with it (sad I know!) and got so good results. For interiors, GI seems much more realistic and faster than just FG. (FG looks great for exteriors where you can do HDRI or use a physical sky - check out the shader at XSIbase.)

In fact, I've tried using FG with GI on an interior and it just triples render time and makes it look far worse, so currently I'm sticking with GI only.

My advice would be to show show a wireframe that indicates where your spot lights are (there doesn't appear to be any). It looks like a record shop or similar, which should have many small spot lights highlighting the floor and merchandise and thus bouncing light back into the scene and casting shadows. You can't use 1 pointlight and expect a realistic result.

As for AO - don't stick to the settings set by the sliders in the AO shaders dialog box, type larger numbers in, they go much further than that. With AO, the ceiling will appear dark. But in photoshop you can overlay/multiply the AO over the FG pass and then mask out/fade areas where you choose. Don't expect the final product to come out of the renderer - you need to experiment with it afterwards. Maybe do one AO pass with a ceiling and one without, then experiment with them in photoshop, you may find both together compliment each other.

11-14-2006, 05:48 AM
Hi all, thx for reply.

I've spent my weekend playng with setup, but no picture are great, or probably i'm not able to do this.
So much unpredictable result.........
Different light change drastically the scene......
I'll try again

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11-14-2006, 05:48 AM
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