Hair renders differently in 3.1 & 3.2

[gjwegner]

Hello, I apologize if this issue has already been discussed. I'm getting very different results when rendering hair in vray 3.1 & 3.2 in 3dsmax 2014. I've attached an image for comparison. Render and VrayHairMtl settings for both the renders are the same.

3dsmax also crashes when using the 'include' on any vray light with hair in the scene during rendering. Even if the 'include' set is empty it crashes if I simply check the 'include' and render. I'm pretty sure I've seen a similar topic discussed about the 'include/exclude' but not sure if there's a work-around.

[jacksc02]

Are you using Embree?
Try turning it on/off to see if you get the same results

[vlado]

What Chris said. Still, it would be very useful to take a look at the scene.

Best regards,
Vlado

[gjwegner]

Thanks for the reply. Your solution worked. I tried simplifying the scene for upload as it was very dense and contained a ton of point cache data. However, now when I turn off 'use embree for hair' it crashes. Using 3ds Max 2014 and Vray 3.2.02

Here's the max file: https://dl.dropboxusercontent.com/u/24381117/hair.max

[yordan.madzhunkov]

Hi gjwegner,
I will try to briefly explain why the render is different. The embree developers assumed that individual hair stands are considered to be subpixel sized. Althoght, this is true most of the time, it is not always the case. That's why we modified the embree intersection model to be more accurate. With improved hair intersection model, V-Ray can render fat hair more accurate and there is a visual difference that can be spoted even by inexperienced users. There is almost no visual difference for slim hair.

About performance: The new intersection model is slightly slower, compared to the classical embree model. However, we manage to overcome the slowdown with other optimiziations. That's why most of V-Ray users didn't recognize that the embree geometry model have changed.

I attached a simple curly fat hair scene that illustrate some of the artifacts that we observed, when we use the classical embree intersection.

Best,
Yordan

[gjwegner]

Thanks you so much for the explanation. It's well appreciated.

[yordan.madzhunkov]

Hi gjwegner,
Again thank you for providing as with the scene. We fixed the crash when embree is not used. You can get the fixed version from today's nightly builds (nightlies.chaosgroup.com).