Faster refraction on gpu

[nikand]

I have been wondering about this for quite some time. Why is rendering refraction so much slower on GPU than CPU?
In many scenes I feel this is holding GPU back. Everything is faster but the refraction takes ages to refine...

What are your thoughts about this?

best regards Nikolaj

[savage309]

What V-Ray version and for what host app are you using? Do you have a sample scene?

Best,
Blago.

[nikand]

Hi Blago
I will create a scene for you. I have noticed this from the very beginning of Rt gpu.
3ds max and vray 35004

[savage309]

Hi Blago
I will create a scene for you. I have noticed this from the very beginning of Rt gpu.
3ds max and vray 35004

Make sure to compare for the same amount of render time, not for the same amount of samples. For the same amount of samples, V-Ray GPU will produce noisier results, it is just that it is expected to calculate samples from camera much faster.

Best,
Blago.

[nikand]

okay then here is test with a tv on a wall(solid glass plate in front of tvpanel) . 1m render brute force each.
Gpu is clean compared to cpu, except for the refraction/caustics. These are noisier on gpu.

scene is lit with a single hdri

[savage309]

Is that V-Ray mtl, or the refraction is part of blend material ?

Best,
Blago.

[nikand]

its a vray mat inside a multimat.
I will try to leave out the multimat to see if there is any difference

[savage309]

It will not make a difference.
Can you send the scene to us to debug it ?

Best,
Blago.

[nikand]

I will send it later today Blago
/Nikolaj

[Alex_M]

Seems that I have the same problem in a scene I'm working on now. I found that the reason is the HDRI I use for environment. If I disable the reflections coming from it by using a black color in "Reflection/refraction environement" the problem goes away but the rendered image is not as realistic. Please check the screenshot below. This was rendered for 4 min. on 2 x GTX 980

UPDATE: Seems that I get these fireflies with Adv as well so it seems it's not isolated to RT only.
UPDATE 2: I put the HDRI in a Vray Dome light instead of the environment slots and the noise is now gone!