View Full Version : GI photons & particle instanced geometry

09 September 2010, 07:34 PM
Hi everyone, I wanted to know how I can fix this issue with rendering particle instanced geometry with GI.

Basically I've been breaking my scene up into passes in Maya and once I started doing my test renders I noticed my instanced geo wasnt being illuminated on my GI passes. My scene has large debris that I modeled and positioned manually and some smaller debris that were dynamically placed.

green - modeled geometry
red - particle instanced geometry

The pics below show that the instanced geo is behaving as expected in my other passes

I'm not sure what could be causing this. I've been trying to fix this for a while now. I dont know if this topic has already been covered. I looked through the forums and didn't come across any info but if i missed let me know. Thanks for any advice.

09 September 2010, 08:01 PM
OK so I still been working at it with out a good solution yet. I tried converting my instance to geo using nimbletool's unsinstancer. Very cool tool by the way but not the route I should go. The added geometry drastically increased render times after conversion. I think it's just too much added information during render time. That's why I wanted to use the particle instance since it already has most the info piped into it from the original source geometry.

I put together a sample scene here with the basics of what I'm trying to do. I think all I need is to figure out how to get photons to interact with the particle instance.

test scene (

Much appreciation to anyone who can help me troubleshoot this or find a workaround.

09 September 2010, 01:41 AM
Awesome! I think I got it figured out. Sometimes you keep digging through the docs and keep testing. What I did was go under the mental ray render settings towards the end of the list under "Render Options>Overrides>Caustics/Global Illum...", there's some drop downs that control how mental ray behaves with elements in your scene. I'm not sure to what extent and have to read more into it but basically after I changed the GI generating/GI receiving to "on" mental ray started to treat the particle instance geometry like i needed it to dealing with photon interaction.

I'm gonna do some testing in my main scene to see how my render times are looking. For a while I was stuck with converting the geometry and doing GI overrides on a per object basis just to bring render times back down. testing...

09 September 2010, 06:35 PM
This is what I got out of the GI pass once I got the photons to interact with the particle instance.

The model geometry(grn) and particle instance geometry(rd) are interacting with photons now.

Getting this to work and using the instance for debris instead of actual geometry brought my render times back down to 2-5min range instead of the 10+ I was getting. The pass is only GI. No FG contribution and I'm using a photon map for every GI pass that is using a different light source. Also the base shapes that I'm using for my particle instance have a subdivision approximation applied and displacement map in the shading group to add some irregularity. Trying to keep the displacement on with the actual geometry wasn't even happening. :)

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09 September 2010, 06:35 PM
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