It might help if I knew how to do this in max without using Vray. I only know how to do it there.
I've got a box, with items behind it, and I want the box to override the alpha to 0 no matter what. Setting it to transparent I'll still see objects behind it. I want to ensure it renders the alpha as black.
I'll be fine on this project going back after the fact and rendering a pass to get this done (unless someone answers within a couple hours) but I'd definitely like to know, if not to just save some time and hard drive space so I don't have to render 2 sets of files.
If you know how to do this in mental ray it will most likely work in iRay as well, but I'm not so sure.
Why am I using iRay you ask? Because I have 1 processor and 2 video cards. It's a ton faster, and gives instant preview of lighting and materials when you render. If only they'd put iRay in active shade, I'd be a happy man.
If you haven't tried it, it's well worth the hassle.