Solve 'self overlap' with transparent 3D objects

Describe the problem

Usually semitransparent shaders do not write into the depth buffer. However, this can create draw order problems, especially with complex non-convex meshes. If you want to fade in & out meshes like that, then using a shader that fills in the depth buffer before rendering transparency might be useful.

Semitransparent object; left: standard Transparent/Diffuse shader; right: shader that writes to depth buffer.

Describe the solution you'd like

Unity recomends a solution like below: https://docs.unity3d.com/2017.2/Documentation/Manual/SL-CullAndDepth.html

Shader "Transparent/Diffuse ZWrite" {
Properties {
    _Color ("Main Color", Color) = (1,1,1,1)
    _MainTex ("Base (RGB) Trans (A)", 2D) = "white" {}
}
SubShader {
    Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
    LOD 200

    // extra pass that renders to depth buffer only
    Pass {
        ZWrite On
        ColorMask 0
    }

    // paste in forward rendering passes from Transparent/Diffuse
    UsePass "Transparent/Diffuse/FORWARD"
}
Fallback "Transparent/VertexLit"
}

But! Multi-pass shaders are not supported by URP. There are some alternatives like using the draw objects render pass with light tags. But we'll need to find a solution that works for both URP and built-in pipelines.

Describe alternatives you've considered

n/a

Additional context

Original issue: https://github.com/microsoft/MixedRealityToolkit-Unity/issues/6285

If adding an extra depth pass we need to ensure it's not enabled by default. This API might be able to do that? But it is still unclear.

Start by reviewing the linked Unity depth, transparent shader, and SetShaderPassEnabled documentation, then compare the URP and built-in pipeline constraints described in the issue. Done means a transparent non-convex mesh avoids self-overlap in both pipelines, while any extra depth pass remains disabled by default.