"matte/knockout/DepthMask" material

"matte/knockout/DepthMask" material

I'm looking for a "matte" material which is a special material (distantly related to Shadowmat) that colors everything through it black/transparent.

Basically it's used to combine CG objects into live action. For instance, a sphere cutting through this material would be visible above but disappear below.

Here's a mockup I've faked using chroma key.

Sometimes this is called "matte", "knockout" or in Unity/Vuforia "Depth Mask".

Thoughts?

If you’re okay doing a few render passes, you can set things up so you have a black and white pass that is used for some additional masking as a post process using After Effects, Final Cut Pro X, Premier etc.

So, you’d set up one pass for RGBA. Then do another where all objects are either black or white, and that’s used as a mask to paint some more transparent areas in the first RGBA pass. It’s a bit of a mess, but using layers and some other tricks could help manage it.

And, if using that method means there’s antialiasing issues, you could render to larger sizes without AA (at 4× the size or something), then bilinear scaled down. Depends on the project requirements, really. It would be nice to have this as a specific feature though!