V-ray Shadow Catcher / Matte Shadow / Reflections - WORKFLOW?

[mediarobotics]

I'm trying to render a scene on alpha channel composite it later on on a live-action 3D tracked footage.

I'm using the v-ray shadow catcher option and sometimes the v-ray mtl wrapper to catch the shadows, but for now I also need reflections and sometimes refractions on the ground or glass next to the element I'm rendering, is there a good workflow for this? can someone suggest what should we do here to catch realistic reflections on the ground for example? and play with its glossiness amount, etc...

[Joelaff]

Matte for refl/refr in VrayMtlWrapper works for this. Set Affect Alpha on and amount to -1.0

Best to make separate passes (renders) for each layer (shadows, reflections). This makes it easy to tweak in post. Ideal to get the glossiness correct in the render for maximum realism. You can always blur a hint more in the post.

[mediarobotics]

Thanks will give it a try and get back to you, what about caustics and refractions?

[Joelaff]

You can add or subtract the caustics pass from the beauty pass and get close enough usually. This lets you dial caustics up or down. Honestly, we don’t do a lot with caustics, but just did a project and used that method.

Refraction is tough because you really need it as close as possible in the render for realism, rather than faking it. You can enable affect all channels (or at least apha) in the material and also set up refraction passes. It really depends on what you are trying to do to it in post.

For VFX we sometimes have to layer multiple passes in various ways. But any way you can get the look in the render the better the result (e.g. but putting footage on stand in geo behind the refractive surface).

Also you can do tricks like putting a UV ramp image behind the refractive object to let this get refracted, and then using this to map your original footage with the same refraction, like with the Texture node in Fusion or RE:Map, etc.

I do find that Occam and his razor prevail, and usually the best method is to get things as close as possible in beauty. This also makes sure you get automatic proper energy conservation. (Though fresnel VRayExtraTex passes and judicious use of them in comp can help, but most comper-only artists don’t have the same understanding of energy conservation as 3d artists; so you may have to pass on that info if someone else is comping.) Also VrayRefractionFilter (or reflection— along with their Raw counterparts) can be of use sometimes.

Especially with refraction, it is all very shot dependent, and we make a shot by shot decision on what approach to use.

I am sure others have different approaches/perspectives, and hopefully they will share as well. My general philosophy is to get the beauty pass as close as possible to a direct merge in. The less you have to do in post the better. Of course there are always a ton of things to do in post anyway. Not saying our comps are simple by any means. They are a spaghetti mess, but 3d is the best place to make major changes and comp should be used only for fitting the renders into the scene.

[mediarobotics]

Thank you so much for your detailed reply