Have noticed this happening quite often. If I switch the Refraction setting to “Color + Alpha”, it gives me an alpha with transparent areas even if there’s a solid background and should be a pure white alpha.
Providing an example with basic material test scene. If I create a glass material with “Color + Alpha”, I get this incorrect alpha. Am I doing something wrong or is this a consistent bug in Vray?
Thanks - Chris
Hi Chris,
No, this is not a bug , it happens because some of the refraction rays hit the environment/alpha and this affects the render alpha channel. You will get the same result for Affect channels set to all channels.
You can use multimatte render element to get more options later in post. For an example :
*) Alpha element with Affect alpha to Color + Alpha
*) Multimatte element to get complete alpha on that object
I don’t understand how this is not a bug. That Alpha channel represents the opacity of the pixel. Those pixels are fully opaque, there’s a plane behind the glass. You would not be able to see through the plane just because there’s glass in front of it.
Yes, that would be the case if you use the opacity to make the object transparent, but when the refractive rays are used to affect the alpha channel then all data collected by those rays will be transferred to the final alpha. You can see this in your image as well, the black spots in the alpha represent the “white” environment (where actually the ray hit the alpha) in the Beauty pass.
We have options to turn this behavior On end Off from the Affect Channels menu.
Please let me know if you have any difficulties to get specific render results from this scene. I would be glad to assist you.
