Occlusion Material or Hold-Out Shader in ARKit and SceneKit

Question

I have seen some examples demo of ARKit where material A is blocking material B, kind of creating occlusion effect, or black hole, or masking. But all of them seems to be using Unity engine (I might be wrong). I wanted to do this using what Apple provide already. Or maybe using Metal Shader.

Wondering if anyone knows a trick that allows this for ARKit or even in SceneKit term, when 2 objects are overlapping, but wanting one object to occlude the other object, leaving the other object empty. It cannot be CSG right, because that would be too expensive?

Answer 1

To generate a hold-out mask, also known as Occlusion Material, use.colorBufferWriteMask, instance property that writes depth data when rendering the material.

sphere.geometry?.firstMaterial?.colorBufferWriteMask = []

Then assign an appropriate object's rendering order:

sphere.renderingOrder = -100  

And, at last, allow SceneKit to read from / write to depth buffer when rendering the material:

sphere.geometry?.firstMaterial?.writesToDepthBuffer = true
sphere.geometry?.firstMaterial?.readsFromDepthBuffer = true

sphere.geometry?.firstMaterial?.isDoubleSided = true