Mental Ray Passes Impact Render Times

#1

Hi there,
I am having a problem with Mental Ray Render Passes for Maya and I can’t figure out what the deal is. Whenever I start adding contribution maps and passes, even just for a beauty pass, my render times skyrocket. They will double if I don’t create any contribution maps and break out normal diffuse, indirect, spec etc. But adding a contribution maps will take as much as 3 times as long. What is going on here?

I’ll try and post a link to an example of my setup, but in the meantime, does anyone know why this is happening or have similar results? I’m in Maya 2011 SP1, almost always using mia_material_x_passes and all I’m doing is:

  1. Select objects and lights > Assign to new render layer.
  2. Select objects > Create Pass contribution map and add selected.
  3. In Render Settings > Create corresponding passes i.e. foreground_beauty/diffuse whatever.
  4. Assign those passes to their respective contribution maps
  5. Add naming tokens to the filename structure to keep everything in a separate directory.
  6. Render

I’ve been doing this for a long time and I don’t ever remember seeing passes impact render times so much. It’s really frustrating. In the meantime I’ve been using the Deex shaders which are nice, but I miss my contribution map workflow! I really appreciate your help.

Cheers

#2

I can tell you that I’ve recently had problems using a similar setup with 2011 sp1. The biggest problem I had was that the alpha channels were not rendering correctly and nParticles were also not rendering properly if at all. I would say in the meantime if you are in a crunch, use a different approach. Unfortunately I think that there are some bugs and/or limitations using contribution maps at the moment ( someone please correct me if I’m wrong. ) My workaround was going back to an older workflow which was using render layers with rgb mattes. It’s not ideal but if something’s just not working I tend to take a different path. If I’m not under a time contraint I will do everything I can to figure it out. Hope this helps…

#3

MrJustin

Thank you so much for your reply, it’s just nice to know others are having issues with this setup. I have been doing the RenderLayer and RGB Matte thing in the meantime which has been an sufficient workaround for now.

For all, since then I’ve done a bunch of tests and found some interesting things about where in the process my render times are increasing the most and just thought I’d share.

Simply adding pass contribution maps and the most common passes (with the exception of AO) have been costing about a 20% increase in render time, which is significant but not necessarily a killer.

Adding AO, or even having it turned on in the layer has averaged to about a 60% increase. It seems beneficial to render AO on a separate layer if possible.

Finally, the real killer was turning off holdout on any of the passes. It make sense that this would be taxing (more information to process) but I was getting like a 300% increase which is ridiculous. It’s shown to be a lot more faster to use a render layer setup with primary vis turned off to achieve a similar result, especially for background layers or whatever.

Still, if anyone has any insight into why this happens I’d be interested to understand what Maya/MR is doing differently that is so time consuming.

Thanks again :slight_smile:

#4

David,

Thanks for doing some benchmarking, it does seem like the significant increase in render time means that something is wrong. I too would love for somebody to shed some light on this for us, although something tells me there could be a (or some) bug(s) involved. Since I am running 2011 I cannot say if these problems persist in 2012. Anyone?

Ohh I also wanted to comment on AO. I understand why some people like breaking the AO out into it’s own pass and it is handy to have. However, I am a big fan of baking it in using the built in AO in the MIA shader. It is physically accurate and therefore (if done correctly) will produce better results in my opinion if you are going for a photorealistic look, with less work on the compositing end. By using the built in MIA AO, you get the additional features of detail distance and color bleed. Eventhough these features increase render time, it does result in a more physically accurate render.

This is probably a more of a question of do you want to do the work in your compositing software to get the AO to look right or do you want to spend the time to get it right straight out of maya. You can produce the similar results using both methods. Just my 2 cents…

  • J
#5

After all of my tests, I’ve decided to abandon PCM’s for now. I think my workflow is just going to be a simple set of passes, perhaps splitting the foreground and background on separate render layers, and using RGB Mattes. I had a talk with my compositor and we both agreed that something akin to the whole contribution map thing can be handled nearly as easily in post with mattes, especially since it saves all this headache. Also, I agree that the mia_material’s built in AO has been worth the extra render time.

Thanks again for you help with everything.

#6

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