Depth mask

This mask relies on the depth-buffer which contains depth information associated to each pixel. Each Compo Composite 2D Layer, Shape layer and 3D Layer together. [Ctrl] + [Shift] + [N] to create a new composition after opening Smode Read More using 3D elements have such a depth-buffer.
Either the Depth Map is set to "Current", which means that the used depth will be of the current compo, or this parameter can be customized to point to external Compo Composite 2D Layer, Shape layer and 3D Layer together. [Ctrl] + [Shift] + [N] to create a new composition after opening Smode Read More such as compos inside Shared Element A shared element that can be reused at several places like an After Effects precomposition Read More .
Here is the "Mask - Depth.compo" example that applies a Mask TextureMask a part of your image Read More with a depth mask as a post-fx of a 3d compo:


Do not forget inside the depth mask parameters to choose the depth buffer it will be based on. In the next picture the depth mask depth map has been chosen from a compostion having a 3D object and so, a depth buffer :
Also remember than to actually generate a depth buffer you must write your 3D objects inside of it. To do so, go check the 3D objects 3D Renderer Displays a 3D geometry Read More parameters and change the Renderer Depth Settings Settings that affect how a renderer uses the depth-buffer Read More as on the image below (it is like that by default):