HoloLens and DepthMask for Invisible Planes

While working on a recent project I ran into a situation where I needed an invisible plane for a container object that housed other objects inside and needed to appear as if we were looking inside of the wall through a cut-out.

Trying out different solutions provided on the internet didn't work. The object would always come out black:

With the black box around the cutout, it takes away from the experience that the pipes are actually inside the wall.

The solution? Adding a DepthMask shader to the container object. Following this guide, I was able to create a shader that worked well with the HoloLens. Here's the full code:

Shader "DepthMask" {
   
    SubShader {
        // Render the mask after regular geometry, but before masked geometry and
        // transparent things.
       
        Tags {"Queue" = "Geometry-10" }
       
        // Turn off lighting, because it's expensive and the thing is supposed to be
        // invisible anyway.
       
        Lighting Off

        // Draw into the depth buffer in the usual way.  This is probably the default,
        // but it doesn't hurt to be explicit.

        ZTest LEqual
        ZWrite On

        // Don't draw anything into the RGBA channels. This is an undocumented
        // argument to ColorMask which lets us avoid writing to anything except
        // the depth buffer.

        ColorMask 0

        // Do nothing specific in the pass:

        Pass {}
    }
}

Here is the result with the DepthMask shader applied to the container object:

This looks much better! Now it really looks like we're looking inside the wall. I hope this helps you if you were getting stuck on how to make invisible planes for your own HoloLens project.