occluding objects on render

Laughing_Bun

this isn't real time related but I am doing a scene with gems falling and I need to know how to render only a certain selection of objects while still using the "hidden" objects to occlude the rendered geometries alpha.

In the image I attached you can see what I want to do. I want the Green gems to be rendered by themselves but I want the Blue, Red, Orange objects to be hidden on render but still subtract from the green objects alpha channel.

I know you polycounters can help me!

Im using max 10 and After effects

Warheart

Hmmm... In Maya I would use the "use background" shader I think it does what you are looking for (punches a hole in the alpha but does not render itself). There must be an equivalent in Max. Possibly the "Matte/Shadow" material is what you are looking for?

The only problem with this is that you wouldn't be able to see the green jems refracted through the other jems. What is the reason that you want to render the green jems as a separate pass? If it's to do separate colour correction on them or something like that then you might want to just render a separate mask pass that you can use to mask your filters in the comp.

sprunghunt

you need to put a "matte/shadow" material on the blue, red, and orange gems. This will make those gems only show up in the alpha channel.

Laughing_Bun

Thanks guys for your reply. Your method worked like a charm. In the end I went with a material ID pass and keyed stuff out in after effects. Rendering separate passes was just not Ideal, But its good to know for the future.