Light cache incredibly slow with CPU rendering and displacement

[leuey]

I'm having hard time understanding the results I'm getting. I have a scene with a single object (sort of lamp sized, so tall and narrow and just in the middle of the screen) lit by a single HDR. This renders very fast (as expected) with both GPU and CPU in about 20 seconds and looks great. If I turn on turn on displacement (which is really minor, btw...just some bumps) and it takes V-Ray CPU 6.5 minutes just to generate the light cache...and when it finally starts rendering it renders about for about 2 minutes (4x as long without displacement) and looks *terrible*. Meanwhile RT GPU calculates the light cache in about 30 seconds and renders the scene in 30 seconds and looks great. My CPU and GPU are pretty similar in power, MODO will render this scene with displacement in 2min...V-Ray is only 1/3 of the way through the light cache phase at that point. What's up? I have plenty of memory, it's not that.

Turning 'Secondary GI' to Brute Force doesn't seem to help with anything.

Any way for the light cache to ignore displacement? The effect is so subtle I can't imagine it actually effects the lighting. Or any other suggestions? Some simple displacement shouldn't make for unusable render times.

I can't post the scene but maybe I can share through email.

-Greg

[leuey]

I downloaded the Maya 3.6 demo and I am not seeing the same problem. Rendering in CPU mode with Light Cache as secondary and displacement happening in the scene doesn't have any problems. About 20 seconds of Light Cache calculation vs. 6.5 minutes in MODO. I am using **the exact same settings** (I double-checked). Again, MODO V-Ray RT GPU has no problem here..but RT CPU and 'regular' CPU take FOREVER with the light cache when the displacement layer is active. Am I missing something obvious?

Trying to render out this project with V-Ray standalone option and RT GPU CUDA, but it crashes about every 50 frames. This is a very simple scene...1 object, 5 image maps, 1 hdr.....

[vladimir.nedev]

Can you send me the scene (preferably the Maya one too) to vladimir.nedev@chaosgroup.com ?

You could check the static displacement and subdivision option in the V-Ray Geometry Properties.
Are you using the official 3.52.01 build or a nightly ?
In the official build, this option is disabled by default, and it is also considered disabled when the V-Ray Geometry Properties are missing.

Both V-Ray for Maya and the V-Ray for MODO nightly builds have the option enabled by default (in Maya it's called "pre-tessellated" geometry and displacement), and it is also considered enabled when the V-Ray Geometry Properties are missing.

V-Ray GPU ignores the option and always uses static displacement, and since it works, I am guessing this is the issue.

As for why the dynamic displacement is slow.

It could be caused by a very low dynamic memory limit. The limit is 0 by default, which means no limit. So it should be fine unless you modified it.

Of course, it's possible that there is a performance bug with the dynamic displacement.
I think at this point it is disabled by default in all integrations, so it's not getting tested regularly.

There is also a slight chance that a V-Ray for MODO specific texture used for displacement is causing the slowdown.
But I am guessing you are using just a bitmap, since you could transfer the scene to Maya ?

Greetings,
Vladimir Nedev

[leuey]

Hi Vladimir - thanks for the response. I will try and get that to you - I did some more tests with a simple object and I'm not getting the slow down. In the trouble scene adding a V-Ray mesh package and using those displacement options results in displacement not being rendered at all. But in a simple test scene those package options work..so I'm not sure what's up. I did notice MODO RT displacement subdiv defaults to 256 while in Maya it's 4...seems like a big difference...but that's just RT. Anyway, it may be tomorrow as I've got some other deadlines today.

thanks,

Greg