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Archive Notice: This thread is old and no longer active. It is here for reference purposes. This thread was created on an older version of the flipcode forums, before the site closed in 2005. Please keep that in mind as you view this thread, as many of the topics and opinions may be outdated.
 
tetsifly

March 18, 2005, 06:51 AM

Does anybody know what exactly final gather and ambient occlusion are? I've been working on a global illumination renderer (www.igneus.co.uk) for a while now and I've not seen either term included in any of my text. Yet many commercial renderes now support them. Are they just fancy names for basic GI techniques like radiosity, path tracing, photon mapping, irradiance caching etc.

Any ideas?

 
playmesumch00ns

March 18, 2005, 07:43 AM

Final Gather is basically QMC hemisphere irradiance sampling with irradiance caching. You use it to calculate diffuse interreflection. It's better than radiosity as your brdf doesn't have to be isotropic.

Ambient Occlusion is exactly the same just without the irradiance. You essentially sample the hemisphere and record how many rays are occluded by geometry, then average the result to get your ambient occlusion value. Occlusion is good for adding contact shadows between objects without going the whole hog and doing full diffuse interreflection with colour bleeding. It's also good for doing a fairly good approximation to IBL since you can do a quick hdr lookup, then mult the result with your pre-calculated occlusion value et viola. Kinda like cheap and cheerful PRT.

 
bssrdf

March 20, 2005, 03:36 AM

So basically, Ambient Occlusion is a trick. It does not fit into a physically-based readering framwork,right?

playmesumch00ns wrote: Final Gather is basically QMC hemisphere irradiance sampling with irradiance caching. You use it to calculate diffuse interreflection. It's better than radiosity as your brdf doesn't have to be isotropic. Ambient Occlusion is exactly the same just without the irradiance. You essentially sample the hemisphere and record how many rays are occluded by geometry, then average the result to get your ambient occlusion value. Occlusion is good for adding contact shadows between objects without going the whole hog and doing full diffuse interreflection with colour bleeding. It's also good for doing a fairly good approximation to IBL since you can do a quick hdr lookup, then mult the result with your pre-calculated occlusion value et viola. Kinda like cheap and cheerful PRT.

 
playmesumch00ns

March 22, 2005, 07:21 AM

Yep, basically. But then, it's all a trick isn't it. And ambient occlusion is a very useful one.

 
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