Alpha channel of Reflected objects?

[Greg3D]

Setup:

-Reflective Ground plane, using VrayMat to reflect within a VrayWrapper to make it a matte object...alpha contribution -1

-Sphere floating above the ground plane.

The rendered RGB result is the sphere and it's reflection, but in the alpha channel only the alpha for the sphere exists, and not the alpha for the reflection.

Am I missing something in this setup? I'd like to have the alpha channel for the reflection as well naturally.

[vance3d]

I tried something similar to this and could not get an alpha for the reflections either.

Also I had an object inside a mirrored room lit with gi and even though the walls were not part of the gi calculation, they still blocked the sky light from hitting the object. I though that excluding things from receiving gi make them kind of invisible to the light sources.

any thoughts on either of these issues would be helpful.

vance

[v_wrangler]

Re: Alpha channel of Reflected objects?

Setup:

-Reflective Ground plane, using VrayMat to reflect within a VrayWrapper to make it a matte object...alpha contribution -1

-Sphere floating above the ground plane.

The rendered RGB result is the sphere and it's reflection, but in the alpha channel only the alpha for the sphere exists, and not the alpha for the reflection.

Am I missing something in this setup? I'd like to have the alpha channel for the reflection as well naturally.

Greg,

I don't think it would be possible as Reflect/refract
is dependent on the object behind it. You best bet could be
to isolate the reflection (Blur's utility Material) nd rerender
reflection only. Then additive/whatever composite it in post...

But it is a real saver if Vlado can pull up something in his sleeves

BTW, theres some talk over at disceets webboard regarding a
cool rendering utility script that allows you to render layers/elements.
Just canT remember the name and the URL right now . I think Grant Adam wrote it...

Regards,

Roy

[Greg3D]

Re: Alpha channel of Reflected objects?

I don't think it would be possible as Reflect/refract
is dependent on the object behind it. You best bet could be
to isolate the reflection (Blur's utility Material) nd rerender
reflection only. Then additive/whatever composite it in post...

Roy, I think an alpha for the reflection of an object in a matte material makes sense. If you have the ability to isolate the RGB of a reflection in a matte material or matte object type, then of what use is it if there is no alpha associated with that reflected image? I can understand this being more of a problem with glossy reflections, which I would accept, but I would love to have a SOLID alpha of the reflection.

If VRay offered the render to layers options that Max's scanline renderer or that Brazil offers, I'd be a happy camper.

[vlado]

Greg, things are not that simple, as the reflective object may be transparent thus having an alpha channel of its own. I'm not quite sure what the final alpha should be in that case. Also imagine that the object is both relfective and refractive. How would the alpha be computed in that case? From the reflection, from the refraction, from the object itself, or some complicated mix of those?

Best regards,
Vlado

[Greg3D]

Greg, things are not that simple, as the reflective object may be transparent thus having an alpha channel of its own. I'm not quite sure what the final alpha should be in that case. Also imagine that the object is both relfective and refractive. How would the alpha be computed in that case? From the reflection, from the refraction, from the object itself, or some complicated mix of those?

Vlado,

I understand, but I am looking at it this way:

The reflective object may be transparent and have an alpha of it's own as you say. Great, when it's rendered in IT'S RGB pass, all is fine, it uses it's alpha. When I make it a MATTE material/object, my pupose for doing this is to isolate either a shadow or a reflection of another object on it. At this point, I don't care about the alpha of the reflecting object, I only care about the alpha of the object being reflected in it...which is why I am trying to isolate it so I can use it seperately.

This would allow me to place a cg element into live footage, and with an isolated reflection pass, let me have it reflecting in reflective objects in the live footage. The alpha should be solid, allowing me the ability to adjust how, and how much, it's composited into the final image