Sometimes when you do Augmented reality apps, you need hide parts of 3D which pass behind user body or head. Easiest way to make this – use hidden 3d mannequin or 3d head with special shader which cull all pixel data behind it.

358b2eefaa239355f0d5718942baf63c

I just wrote small one for Unity3D.

Shader "Custom/CullMaterial" {
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
	}
	 SubShader {
	 Tags { "Queue" = "Background-1" }
        Pass {
            Material {
                Diffuse (1,1,1,1)
            }
            Lighting On
            ColorMask 0
        }
    }
	FallBack "Diffuse"
}