Shadow catcher alpha not sampled well

[Alex_M]

It seems that there are problems with sampling the alpha of shadow catchers in RT CUDA. They render way noisier than Adv even at noise threshold of 0.005. Is there something that I can do to make the shadow catcher's alpha smoother?

Please check the screeenshot below. This is a sphere on a plane and a Vray plane light. All render settings are at default.

[savage309]

Does it gets cleaner if you add lower noise treshold (comparing noise thresholds between Adv and V-Ray GPU sometimes can have those effect)?

Best,
Blago.

[Alex_M]

Hi Blago,

No, the alpha looks exactly the same down to the individual pixels, even if I lower Max noise to 0.001. The RGB channel is silky smooth though at this noise level.

[tashko.zashev]

Thank you for this report Alex.

It looks like the alpha noise level is not considered for the current noise threshold level in GPU mode and I'll make a note in our system about this.
Until we have this improved in the builds you can slightly increase the Ray per pixel parameter until you get the desired image quality. I also found that setting the Max noise thresh. to 0 make the alpha very smooth but this is not really an option since the render time will go up a lot.

[Alex_M]

Setting Rays per pixel to 64-128 worked, thanks! It's still not as smooth as I want it but it's much better.

[maxwood]

Looks like this is the same issue I posted about yesterday morning - although much better explained by Alex. (Thanks Alex) https://forums.chaosgroup.com/forum/...shadow-catcher

Indeed, increasing rays per pixel helps a lot but doesn't solve it.

[maxwood]

Minor bug I've noticed, when right clicking to reset to the default rays per pixel value it doesn't respect the engine type chosen.

Resetting rays per pixel when using CUDA changes to 4 (should be 16).
Looks like it's taking the default value from CPU instead.

[tashko.zashev]

Minor bug I've noticed, when right clicking to reset to the default rays per pixel value it doesn't respect the engine type chosen.

Resetting rays per pixel when using CUDA changes to 4 (should be 16).
Looks like it's taking the default value from CPU instead.

I'll forward this to our developers. Thanks!

[tashko.zashev]

We have fixed the noisy alpha issue for the next update of V-Ray 4.

[maxwood]

We have fixed the noisy alpha issue for the next update of V-Ray 4.

Brilliant, thanks Tashko