Reflection mattes

[Dubbie99]

One thing that has bugged me for years is that as far as I know, there doesn't seem to be a way to do a single render out of V-Rray and have some sort of AOV that includes an alpha for reflections.

Think of a dark sphere that you plan to composite onto a brightly lit floor photographic plate.

The dark reflections of the sphere need to be composited OVER the existing bright light on the floor.
I know its not 100% mathmatically correct because the floor plate includes both diffuse and reflection mixed together. However way you look at though, you need an alpha of the reflection to start compositing.

Currently we make a seperate render where the sphere is made not visible to primary rays and assigned a 100% white self illum texture.

It would be amazing if this effect could be done with an LPE or something. Does anyone know if it is possible?

[Dubbie99]

Any comment from Chaos on this?

[Gosho.Genchev]

Hey Dubbie99 ,

I also failed to find a way to do this with LPEs, and I can't think of another solution besides having a separate ROP setup as you've already been doing. There are plans to improve LPE support in the near future - I'm taking note of this thread and will consider it a feature request for improving plate workflows with LPEs.

Best regards!

[Dubbie99]

Thanks very much for looking into this. It could be a very useful feature for VFX work.

[Gosho.Genchev]

Yup, it absolutely will. As you've undoubtedly seen in the forums here, people have come forward with shadow select requests and better matte workflow suggestions and we're thinking of ways to improve the workflow.
If there are other areas where your plate workflow could be simplified, please don't hesitate to tell us so we can keep that in mind when implementing new features!

Best regards!

[david.anastacio]

Hey,

This sounds like the same request I made almost a year ago
https://forums.chaos.com/forum/v-ray...ction-mask-aov

The suggestion at the time was to use LPEs which I never found a way to do, and LPEs in my mind where never the solution as they can extract data currently in the render and for these mask new data is required.

Best,
David.

[LeoYfver]

To jump on this thread too i have made a proposal of how i would like to have from the shadowcatcher :P

Its basically exactly how the background plate node works in karma


1. It has a shadow catcher that is unlit but still catches gi/reflection/refraction/emission(from geo).
2. LPE:s extra tags like holdout; shadow; that makes it possible to customize your own aovs and choose how granular you want your aovs. For example catch the shadow on the shadow_catcher geo from a specific light


https://www.sidefx.com/docs/houdini/...oundplate.html
https://dl.acm.org/doi/10.1145/3214745.3214746 (this paper describes the prelit shading. It mean the shader will behave like a normal shader except that it wont pickup lights on itself because it assumes the light is baked in to the plate, you want to have a way to opt out of it too if its a cg light which could be a checkbox on the light)
this seams to exist in clarisse and 3delight atleast.

Regards Alexis

[ben_martin1]

One way we have worked around this is taking the ground material you are using with a wrapper mtl attached to it and by changing the refl amount to a high value it should make the alpha to nearly 1. Might work in your case.

[anthonyvfx]

Yup, it absolutely will. As you've undoubtedly seen in the forums here, people have come forward with shadow select requests and better matte workflow suggestions and we're thinking of ways to improve the workflow.
If there are other areas where your plate workflow could be simplified, please don't hesitate to tell us so we can keep that in mind when implementing new features!

Best regards!

If people are requesting this, and clearly from what I've seen, have been requesting it for years.........why isn't this done yet? Not to sound combative, I'm just extremely curious why this seems to not have been an added feature by now. Vray 6 should have this at this point, it's some of the most basic and most asked-for features when working in VFX.

[Gosho.Genchev]

Hey anthonyvfx ,

When we get a feature request, more often than not an R&D developer would look into it and often find potential issues or areas that would need to be improved before we roll it out for actual production use. For shadow mattes specifically, I know from colleagues that there was some research into this - at the time the conclusion was that it would be rather slow and some optimizations need to be done under the hood before it can be tackled. The R&D team is looking into this already so I hope we'd have a solution to offer in the near future.

Best regards!

[vlado]

On the original question about reflections on matte objects, ben_martin1 provided the correct solution - the reflections are picked up from the base material of the matte object.

On the subject of shadow mattes per light, as Gosho mentioned, this is on the “to do” list.

Best regards,
Vlado