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Old 05-14-2014, 08:12 AM   #1
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Nick Knatchbull
London, United Kingdom
Join Date: Dec 2013
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AO just inverse GI???

Dear Forumers,

I am just a lay-physicist/lighting knowledgeable guy, and a thought occurred to me last night. Please can someone point out the difference for me, if there is one...

Isn't ambient occlusion just inverse global illumination and vice versa???



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Old 05-14-2014, 10:03 AM   #2
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Nope, not even close.

AO only uses occluded parts of an object.
Sometimes it is used to get a GI'ish look, but its not the same thing and its absolutely not a physical correct way of doing this.

Global illumination truelly simulates light bouncing arround.
Old 05-14-2014, 10:32 AM   #3
J. Schreyer
ARRI Berlin
Berlin, Germany
Join Date: Aug 2003
Posts: 7,983
AO is not lighting. It's a simulation where object would oclude themselve from lighting in an absolutly evenly lit environment(which doesn't exist).

However, you can utilize AO for lighting purposes or even fake GI shadows with it, after all CGI is more often than not operating on a "if it looks right than it is right" principle instead of 110% physical accuracy.
Old 05-14-2014, 02:28 PM   #4
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Nick Knatchbull
London, United Kingdom
Join Date: Dec 2013
Posts: 16
Cool. Thanks for clearing that up. My visual cortex and imagination were not operating well this morning!!! Image of light bouncing in my head, after your explanation makes perfect sense now.


Love the Doom avatar btw CHRiTTeR.

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Old 06-03-2014, 07:25 PM   #5
Raphael Protti
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To give a bit more low level answer:

GI: rays shot from light source, bounced on surfaces. The resulting solution can be stored as a point cache, or directly added to the lighting... This is an emulation of what real light does.

AO: rays are shot from geometry surface vertex, outward. The closer the rays terminate from origin (by hitting another surface), the greater the occlusion interpretation. This is a way to calculation ambient shadow, without practical lights.

The main difference is the point of origin. AO can do color bleeding as well. GI has one extra dimension in that there is usually a variable of light energy conservation.

I can see why they would seem as inverse in a certain logic... AO is for calculating shadowing, whereas GI adds lighting. But there is no inverse relationship in a mathematical sense.
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