Hi guys,

Finished with the Brace dojiggermabob thingy and tossed the fender together.

I wanted to play with compositing in PS and AO. So I made a AO, WIRE, and DIF render of each shot and composited them in PS. I did not use FG at all since it just blows out the scene with the all white background.

The renders are without lights. I wanted to examine the affect of the distance value of the MR AO texture node. As you can see from the setup there is a plane that has no visibility in the scene and receives shadows and is not visible in reflection. I believe distance is the max distance from each point can be affected by occlusion. But I am not 100% sure because reading the mental ray documentation is like reading stereo instructions, written in Japanese, and then transcribed to English by a French speaking Chinese person.

Anyway, this is what I have gleaned so far.

The Spread controls size of the hemisphere projecting rays to each point. It has a range from 0.0 to 1.0. Think of a half sphere with a cutting plane. When the value is 1.0 you get the entire hemisphere. As the value gets smaller the cutting plane moves up and reduces the size of the dome, which narrows the cone of rays that affect the occlusion of each point. The documentation says the range is 0.0-1.0 but I have seen people using 1.1 so I am assuming that this might allow the cutting plane to move down below the mid-line of the dome and thus begin to cast rays at an upward angle?

The distance is the maximum distance from each point that rays can affect the occlusion of that point. Think of it as the radius of the hemisphere above each point. I am assuming it is in scene units. I used inches in my scene. With a value of 0 the dome is the size of the entire scene. I tried values of 0, 57 (the distance of the camera from the center of the object), 22 (the diagonal of a cube surround the object in the scene), and 14 (the approximate diameter of a sphere surrounding the objects in the scene).

The first affect I notice is that the greater the distance value the longer the render and the darker and more spread out the shadows. The render times for the same preview section of my scene were as follow...

value time
------ ------------
0............~4 min+
57..........~3 min+
22..........~2 min+
14..........~1:50
7...........~1:50 and some of the parts farthest from the camera had no occlusion

Here is the scene setup....



These are the renders at a distance of 22 and a spread of 1.0....