How do I get solid reflection alpha from matte objects that are not 100% reflective?

[Alex_M]

I have a sphere and a semi-reflective ground plane and I want to get solid reflection alpha on the floor from the sphere.

The ground plane has a semi-reflective material (Reflection of 150 gray color + fresnel) and in the Vray properties menu of the ground object I've enabled Matte object, set Alpha contribution to -1 and enabled Affect alpha. Everything works perfectly apart from the fact that the alpha of the reflections onto the ground plane is not 100% solid white but something like 50% grey (apparently because it is not a perfect mirror with white reflections and no fresnel) which is not what I want.

What I want is to get solid white reflection alpha onto the semi-reflective matte object (the floor) in a single render. Is this possible or do I have to make 2 renders - one with the aforementioned setup and then another one with a 100% mirror material applied to the floor to get the solid white alpha I need?

To get this in a single render I thought that it would be possible with VrayMtlWrapper + VrayOverrideMtl. I applied a VrayMtlWrapper material to the ground and set its options in such a way to get alpha of the reflections. Then I plugged in a VrayOverrideMtl into it so that I can add a 100% reflective material in its Reflect Mtl slot while having the original semi-reflective material in the Base mtl slot. Unfortunately it seems that VrayOverrideMtl doesn't work with VrayMtlWrapper. The alpha on the ground plane disappears if I plug VrayOverrideMtl into VrayMtlWrapper.

[joconnell]

It's a bit fiddly but you can render an extratex pass with a vray dirt in reflection occlusion mode - you can match your gloss value to whatever the ground plane. Swap the black and white swatches around so you're getting your white alpha. Next thing to do is limit the pass to only the objects you want. Dirt has two different object lists, "exclude" controls the objects that the dirt effect will be seen on (in this case you want to put your plane in here since this is where the matte will be visible, set the mode in the window that appears to "Include") and then "result affect" controls which objects in the scene are allowed to cast or create a matte, in this case your teapot. Again set your list to include and put your teapot in here. Last thing is do a few test renders and increase your radius in the vray dirt until it's big enough to see all of the matte without cropping it off - this is camera view dependent so beware on animations!

[Alex_M]

Thanks for the tip, I'll try!

[Alex_M]

This seem to work but I'm only getting a mask of the reflection. I would like to have a mask that features both the reflection and the object generating the reflection. If that's even possible. Sucks that the VrayOverrideMtl doesn't seem to work with matte objects, otherwise I think that would have done the trick by using a different reflective material in its Reflect mtl slot. Not sure if this is a bug or if this material is currently unsupported with matte. I would love to hear from Chaos support.