GPU Shadow catcher

[Garryclarke]

Hi,

I've been trying to get a shadow catcher to work properly.

I found 1 error where if the material on the shadow catcher is black (0,0,0) it will switch off the shadow.

But I like to set my shadows brightness to .5 so that the shadow is a little bit lighter when I comp it on a coloured background. This works fine in CPU but using the GPU I always get solid black shadows with a completely white alpha.

Is there a way to get lighter shadows using the GPU?

[Garryclarke]

Here's a file of the above scene, Max 2019 latest VRay Next update 3.2 (4.30.02 build 00001)

[Garryclarke]

I've been trying to lighten the shadows by adding another light that just includes the ground plane, and this works up to a point but this new light will reflect GI off the ground onto the box. I don't want the ground plane to generate any GI but it seems I can't switch it off using the VRay properties.

Is this something I'm doing wrong?

[Micha]

What light sources do you use? Looks like the sun or dir light is taken in account for the GPU calculation. So, the question could be - why is the other sky(?) light ignored?

[Garryclarke]

Just a spotlight and a domelight and I've tried adding different lights to 'lighten' the shadow but the all do the same thing, bouncing GI onto the box.

I switched off GI in the end as I couldn't stop the shadow catcher bouncing GI. By the time I tweaked the scene to work reasonably well it was quicker to render on the CPU.

[Micha]

So, looks like you have a problem with the dome light. I used GPU, dome light and shadow catcher at V-Ray for Rhino and it worked fine. You could make your test with a dome light only and try to find the reason
..

[hermit.crab]

Adding a dome light to the attached scene seems to produce the effect you want. Otherwise, I've logged a request for Shadow Brightness support (internal bug-tracker id: VGPU-4800). The Receive/Generate GI parameters in the V-Ray Properties and MtlWrapper are currently not supported.

[Garryclarke]

Thanks for looking into this for me.

Adding a dome light, even excluding it from the box still adds GI to the box. Also it obviously starts going slower to the point where it's easier to use the CPU where I'll get what I want without any problems.

Unfortunately the fact that the shadow catcher doesn't work properly or support switching off GI etc is a bit of a deal breaker for me at the moment. I'll check back when there's been a few more updates.

[vlado]

You can check the matte implementation in V-Ray GPU 5 beta, it has been reworked and should match the CPU quite closely.

Best regards,
Vlado