Roughness value and rendering speed

[marcatore]

Hi,
I've noted that, using a glossy material, the roughness values have impact on rendering speed (samples/sec).
With extreme low roughness values you get slower rendering speed than medium one.
For example.
Roughness 0.001


Roughness 0.1


I always expected the reverse...more roughness..more time to sample the result.

[Dade]

I always expected the reverse...more roughness..more time to sample the result.

Have you tried on CPU ?

This may be consequence of GPU being faster at tracing "near" (aka coherent) rays (at low roughness, most rays are fired in about the same direction).

P.S. this effect is also a consequence of the very simple scene, I doubt you can notice this effect on a normal scene.

[marcatore]

Have you tried on CPU ?

Tried and it render at the same speed.

P.S. this effect is also a consequence of the very simple scene, I doubt you can notice this effect on a normal scene.

Actually it seems that it envolves also a "normal scene".
Look at this (GPU viewport render)
Roughness of the external light fixture shell: 0.05


Roughness: 0.001 (the lower value possible)


Consider also that, in this scene, it crashed blender after some seconds of viewport rendering when I use that low roughness value

[Dade]

Consider also that, in this scene, it crashed blender after some seconds of viewport rendering when I use that low roughness value

10x times slower, no, that is not normal. Sometime, GPU handles errors like NaNs or memory fault very slowly. If you post a test scene I can check.

[marcatore]

I've sent you a PM.
the wetransfer link it's working for a week..let me know if you go over that time...I'll send you again.

[marcatore]

it seems that the glossy coating material is not affected by this issue.
with 0.001 as roughness, it renders fast as higher values.

[Dade]

it seems that the glossy coating material is not affected by this issue.
with 0.001 as roughness, it renders fast as higher values.

I'm testing the scene and the problem seems to affect only the combination of GTX980 with Optix on my hardware. Every other combination (i.e. GTX980 without Optix and RTX2070 with/without Optix) works fine. This looks like an Optix bug.

Do you have a GTX 9xx GPU ?

It starts to be pretty clear that, while Optix is supposed to work on old GTX GPUs, it is both slower than my BVH code and has problems. We will change the default behavior as soon as B.Y.O.B. is back in town and Optix will be used only on RTX GPUs.

In the mean time, you can just disable Optix in BlendLuxCore preferences and it should fix the problem.

P.S. this is standard corporate programmate obsolescence at work

[Dade]

BTW, another workaround is probably to use Diseny material instead of Glossy2.

[marcatore]

Do you have a GTX 9xx GPU ?

In the mean time, you can just disable Optix in BlendLuxCore preferences and it should fix the problem.

I have a GTX1070.

Tried disabling Optix and no more problem with roughness.
With Optix on, Disney or Glossy Coating (with a matte as base) material renders fine.

[Dade]

I have a GTX1070.

Yes, right, GTX10xx has no RTX hardware too.