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ambient occlusion .... say what?
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ambient occlusion .... say what? // Archive: Tech Forum
Post by Burnart // Oct 26, 2006, 5:48pm
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Burnart
Total Posts: 839
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Over the past several weeks I keep coming across the term "ambient occlusion" but I have no idea what it means. Could someone with much more knowledge enlighten me on the following;
What is it?
What is it used for?
Can you do it in tS? |
Post by trueBlue // Oct 26, 2006, 6:15pm
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trueBlue
Total Posts: 1761
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Ambient Occlusion:
http://en.wikipedia.org/wiki/Ambient_occlusion (http://en.wikipedia.org/wiki/Ambient_occlusion)
Occlusion:
In computer graphics (http://en.wikipedia.org/wiki/Computer_graphics), the term is used to describe the manner in which an object closer to the viewport masks (or occludes) an object further away from the viewport. In the graphics pipeline (http://en.wikipedia.org/wiki/Graphics_pipeline) one implements a form of occlusion culling (http://en.wikipedia.org/wiki/Occlusion_culling) to remove hidden surfaces before shading (http://en.wikipedia.org/wiki/Shading) and rasterizing (http://en.wikipedia.org/wiki/Rasterizing) take place. |
Post by Jack Edwards // Oct 26, 2006, 7:07pm
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Jack Edwards
Total Posts: 4062
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It's an important part of Global Illumination or GI. Basically it means that objects are shaded based on how much of the surface is exposed to the possible 180 degree arc of ambient light. The more angles blocked the darker the shading.
Here's an abient occlusion example (no lights in scene):
http://www.motbc.com/ForumStuff/06-10/AO.jpg
Here's the same scene with 2 spot lights and an infinite light in Lightworks:
http://www.motbc.com/ForumStuff/06-10/noGI.jpg
Here's the scene with ambient occlusion and full GI with light bouncing (no lights in scene):
http://www.motbc.com/ForumStuff/06-10/GInolights.jpg
Here's the scene with AO, full GI, and lights:
http://www.motbc.com/ForumStuff/06-10/GI.jpg
Hope that helps. :-)
-Jack. |
Post by parva // Oct 26, 2006, 10:17pm
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parva
Total Posts: 822
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2739
What is it used for?
As Ambient Occlusion pass (texture map) it is used for enhance details (corners etc.). It enhance the shape of the object. Or in composition program to use for animations (or in games).
Can you do it in tS?
no. Ambient Occlusion can't be rendered as pass or as a Shader. Vray 1.5 has a dirt Shader which can do also ambient occlusion like results but this is not part of tS vray yet. |
Post by Jack Edwards // Oct 27, 2006, 12:31am
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Jack Edwards
Total Posts: 4062
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Hi Parva,
Very cool example. Much more illustrative than mine.
However, setting VRay's QMC depth to 1 and 2nd bounce value to zero would give you an ambient occlusion render, just as I did above. Any GI rendering engine should be able to generate ambient occlusion renders because it's a basic part of GI -- that is calculating the first ray. So unless we're talking about two different things, ts7 w/ vray can do ambient occlusion. Granted it can't bake it into texture maps like you did for you char though or implement it on a shader basis like you also noted.
It'd be very cool if we can get the dirt shader added to our version of VRay.
-Jack. |
Post by parva // Oct 27, 2006, 1:04am
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parva
Total Posts: 822
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I'm not sure. From my experiences GI and AO are two different things which give different results.
GI does the overall lighting situation too much and using more bounces increase the rendertime. AO alows to shadow only the small parts and details without effecting the overall lighting too much. The problem with AO is also that you don't get color bleeding.
The good thing with AO is that you don't need a lightsource. |
Post by phlewp // Oct 27, 2006, 7:34pm
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phlewp
Total Posts: 9
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Ambient occlusion gives GI like renders, but they are not necessarily the same thing, nor is AO an important part of GI renders. GI is the process of calculating how light diffuses and reflects off surfaces, whereas AO renders cast rays off the surface of the model - those that hit the "sky" appear brighter. Another simple way of picturing it: if you picture rays at specific point on a model, radiating out in every direction, the points where all the rays reach the sky will be brightest. Those in which some of the rays hit another object (or the ground) will be darker, and those that don't reach the sky at all will be darkest.
If a package doesn't support AO, but it does support GI, you can create AO-like renders using a 180 dome that has some sort of luminosity associated with it. |
Post by Burnart // Oct 28, 2006, 5:51pm
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Burnart
Total Posts: 839
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Thanks for the responses people! Today I am marginally less ignorant than I was yesterday - which is always a good thing. Thanks for the eg.s and links.:jumpy:
BTW phlewp or anyone else - I get the GI 180 degrees from above but is the setup such that its brightest from directly above and then dims to zero intensity by the time it reaches 90 degrees from the apex? Ie. I could use an IBL light based on an image which is white at the top and black by halfway down down its vertical size? Is that right or have I got the wrong end of the stick? |
Post by Jack Edwards // Oct 28, 2006, 8:46pm
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Jack Edwards
Total Posts: 4062
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Hi Burnart,
The 180 degrees is in relation to the surface itself. So the shading is dependant on other nearby objects as well as the object itself which may have geometry that would occlude rays to that surface. You could basically think of the shading value of each pixel as the sumation of light hitting that surface from all angles. So basically in artist speak, the concept is that the more angles blocked by objects and surfaces the darker the surface. ;-)
In my example above the objects apear to be lit from above because I put a plane on the ground to block light from below. If I put the plane above them then it would block light from above.
-Jack. |
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