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 JDex03-10-2006, 05:37 AMSo I've been trying to get a better technical handle on how FG is working in Mental Ray. You've got the Minimum and Maximum radii, and they define the size of the hemisphere around the interection point where the camera ray intersects and object using a FG enabled shader. The hemisphere is aligned with the object's surface normal and any surface within an intersecting point's minimum radius will use the samples from the hemisphere. So when the camera ray hits the object and forms the hemisphere, what exactly is the renderer looking at within the maximum radius and outside of the maximum radius? I think I'm just having a fundamental brain fart, but for some reason I can't get my head around this.
JDex
03-11-2006, 02:20 AM
No one else have a handle on this?

Micro26
03-12-2006, 09:41 AM
I m not quite sure , but i think that its a sampling thing,the rays hits an object forms hemispheres and if there arent enought rays(accuracy?) that fall inside the hemisphere another bounce/spread occurs.
Please correct me if i m wrong.
:)

jeremybirn
03-12-2006, 02:37 PM
So I've been trying to get a better technical handle on how FG is working in Mental Ray.

You've got the Minimum and Maximum radii, and they define the size of the hemisphere around the interection point where the camera ray intersects and object using a FG enabled shader. The hemisphere is aligned with the object's surface normal and any surface within an intersecting point's minimum radius will use the samples from the hemisphere.

So when the camera ray hits the object and forms the hemisphere, what exactly is the renderer looking at within the maximum radius and outside of the maximum radius? I think I'm just having a fundamental brain fart, but for some reason I can't get my head around this.

I could be wrong, I'm not an expert in this, but I think you could be confusing two things. First there's the Falloff Start and Falloff Stop distances (using parameter names from Maya) that determines how far the rays will shoot from each final gather point to sample the scene. These really determine the size of the hemisphere around each point that gets sampled, how far the indirect light will be transported by a FG bounce.

Then there's the Min and Max radius setting, which are more about getting smooth results and reducing noise. MR can't make every pixel in the scene be a final gather point because it would take too long, and the min and max radius set how much space there will be between FG points. Lower numbers put them closer together, so there are more of them, and the rendering will be slower but more accurate.

Any of these numbers can be a 0 which would automatically choose distances for you.

-jeremy

reven
03-12-2006, 03:05 PM
http://www.jozvex.com/tutorials.html

was just working on a tutorial from his mental ray tutorials...had the way to calculate the min and max values...and he is a member on cgtalk, so you might be able to PM for a more direct explanation! hope that helps!

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