ambient occlusion question

# 1 15-08-2006 , 01:02 PM

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Join Date: May 2004

Location: Utah, USA

Posts: 369

Here is a link to a test video clip I rendered
1.45 meg quicktime

I used a Maya blinn material along with the mental ray mib_amb_occ texture which I plugged into the color and ambient channels of the blinn.

Here are the amb_occ setting I used:
64 samples
spread: .8
Max dist: 30
reflective: on
Mode: 0
Falloff: 1

I rendered using mental ray production settings. There are no lights in the scene. I set the max dist to 30 because I didn't want the floor to create too much occlusion on the underside of the model. At 0 the underside of the model was too dark. I set the reflection to ON because it gave a more interesting look.

As you can see in the video, the camera is doing a simple turntable loop around the model. The occlusion texture is working well except that it is strobing. I don't know why it is strobing. I want to use occlusion passed for compositing but it won't work with the present results I'm getting.

Any ideas on how I can eliminate the strobing?

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# 2 15-08-2006 , 06:05 PM

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Join Date: Oct 2002

Location: London, UK

Posts: 2,800

it looks like a sampling error. Try upping the samples, also turn reflectiions off so that you can elimate that as the problem. Also I'm not 100% on your occlusion render. Shouldn't there be occlusion on the ground below the object too?

Try increasing the number of rays etc to up the samples (i'm no MR expert though! user added image

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# 3 15-08-2006 , 09:19 PM

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Join Date: Jun 2003

Location: stuck in the 90's boston, USA

Posts: 1,871

do you have FG on?

u also might want to increase the spread close to 1. i ususally use .9-.95

it might also be because its calculating the occlusion for every frame. so since you have max distance not at 0, at each frame the mesh is slightly farther or closer to the camera center of interest, so its calculating the occlusion differently on each frame.

yous hould, bake an occlusion map using transfer surface info [i think in maya. ] and use that map. it will increase render speed by 1000x user added image

or leave max distance to 0 user added image

Image dump

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# 4 16-08-2006 , 07:41 AM

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Join Date: May 2004

Location: Utah, USA

Posts: 369

Thanks guys. The strobing went away when I turned the reflective switch off. It makes sense since the reflective switch cause the rays to bounce off the surface at an angle rather than along the normals. Also, I turned the max distance up to 300 which gave much better results without occluding the bottom of the model too much. I also played with the spread a bit and finally settled on .9... I messed around with the fall off too just to see what it does but I think that in the end, I left it at 1.

Right now I'm just trying to get comfortable with the mib_amb_occ texture because I have a lot of rendering to do and I want to use it as one of my render layers.

Question: Do you think I should use the occ texture just to generate the occlusions created my the model inself (self occlusion) or should I include all other geometry in the scene in the occlusion? I'm wondering if it is better to do a normal shadow pass for the model to capture the shadows that other geometry in the scene might be casting onto it... I"m not sure I'm stating this correctly. Basically I'm wondering if the model's self occlusion should be one pass and all other shadows on the model should be a seperate pass.

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ambient occlusion question

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