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# 1 01-02-2007 , 04:12 AM
Radical Edward's Avatar
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problem with caustics and GI

When I render a scene in mental ray with a directional light and final gather on, I get this image (I turn the final gather on mainly because the scene is on a white plane, which appears grey if FG is off...

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# 2 01-02-2007 , 04:13 AM
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however the same scene with all the same settings, except with final gather off and global illumination on, and it looks like this.... any idea what on earth is going on?

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# 3 01-02-2007 , 04:48 AM
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a blizzard? j/k can't help you, sorry.

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# 4 01-02-2007 , 04:57 AM
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Not really too sure off the top of my head, have you got Caustics on? asthe causitc photons might be too large and causing the problem.

I know its a bit of a lame answer but the help files go over mental rag GI and causitcs problems and how to sor them out, and I think that this type of problem is one example.

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# 5 01-02-2007 , 05:40 AM
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This is from the Maya Help Files on the topic below

Global illumination and caustics workflow

Generally, the workflow is as follows:

Make sure that at least one light source in the scene emits photons.
See Turn on photon emission for a light source.

Make sure that at least one surface is casting or receiving caustics.
By default, all objects cast and receive caustics, though typically not all have to in order for you to achieve the look you want. To fine-tune the look of global illumination or caustics or to reduce render times, you can specify exactly which objects should cast and, or receive photons. See Flag objects to cast and receive photons.

To see the distribution of photons for global illumination, you can generate a 3-dimensional visualization map in shaded or wireframe scene views.

Make sure raytracing and the desired global illumination and, or caustic effects are enabled.
See Render with global illumination.

Render with mental ray for Maya.

Combine global illumination with Final Gather for the most physically accurate lighting effects. See Combine global illumination with final gather.

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# 6 01-02-2007 , 06:31 AM
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what does your render look like if you turn off the directional light (or turn the intensity to zero)?

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# 7 01-02-2007 , 08:15 AM
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It was the directional light causing the problem I got rid of it and replaced it with a spotlight... the help file says don't use directional lights for caustics user added image

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