Question: Caustics CPU render is blotchy, but with GPU or IPR fine - why?

[BeneZ]

Hi,
I am rendering a simple test scene.
I get a good looking result when I render with IPR or CUDA.

But with CPU render it looks blotchy.
What's the reason? How to use the CPU rendering?

The render time was almost the same (between 22 and 29 seconds).
All using denoiser.

Thanks

[Muhammed_Hamed]

The CPU/GPU IPR always uses BF+BF for GI
But the production rendering uses BF+LC depending on the settings you use in your GI tab
You can try to use BF+BF in your GI tab and see if this fixes the issue

[Muhammed_Hamed]

And change the max render time to 0 in your render settings

[vlado]

Are you using the irradiance map for GI? If yes, then it causes the blotchiness in the production rendering. Switch to brute force for primary bounces and light cache or brute force for the secondary bounces and it should be fine.

Best regards,
Vlado

[vlado]

The CPU/GPU IPR always uses BF+BF for GI
But the production rendering uses BF+LC depending on the settings you use in your GI tab
You can try to use BF+BF in your GI tab and see if this fixes the issue

BF+LC is not a problem. The irradiance map is.

Best regards,
Vlado

[BeneZ]

Hi, thanks,

but it didn't solve to switch to BF+BF.
Attached here the scene file and screenshots. I'm quite curious what could be the reason...

The blotchiness comes not from rendertime, it comes up even in the first second. I set the max. rendertime also at 0 now.

Just a related question:
Is it o.k. to scale the LightRect? In old Vray versions I heard you need must only scale under VRayLightRectShape: Size > UV,
but it seems not to make a difference anymore. (The RectLight in this scene, which causes the blotchiness, wasn't scaled).

Best regards,
Ben

[Muhammed_Hamed]

but it didn't solve to switch to BF+BF.

In Caustic tab, uncheck on... and under GI, check Reflective Caustics.. it will render fine with BF+BF or BF+LC just like with Cuda engine
Thing is that Caustics options are all hidden for GPU ui
I'm not able to download your scene for some reason, but I made one that is very similar to yours and yeah having caustics on from Caustics tab produces this kind of noise
I don't know if this is the intended behavior by devs or not.. you will have to wait for confirmation

Is it o.k. to scale the LightRect?

Scaling lights should be ok I think

[BeneZ]

Thanks Muhammed, yes it nearly is the same, just that CPU is a bit darker. And that CPU takes 1.44min and CUDA only 33 seconds.
I didn't realize, that caustics were hidden in CUDA, which means that they aren't raytraced at the moment.

I can imagine, that to fight the blotchiness needs a lot more rendertime and try&error.
But so far my main question is solved, I just needed to figure out what's the difference and why this happens.

If someone has anything else to mention that we didn't see, I' would be glad.

[Muhammed_Hamed]

The difference between CPU and Cuda engine is to be expected. They are 2 separate engines, the GPU code with written from scratch and optimized for GPU rendering
You will find other differences like normal mapping, displacement, SSS and such
The noise you got when you enabled the direct caustics doesn't seem to be a sampling issue. probably a bug, you will need to wait for confirmation from devs
Also keep in mind that the affect shadow options in your glass material is on by default, unlike Vray for 3Ds Max and Vray for Modo
So if you want to light an interior scene you will need to turn off affect shadows to get enough light into your interior.