per-prim depth bias ?

[xum-adsk]

Description of Issue

USD allows depth bias in a single pass. Is there any chance to apply depth bias to a single prim?
A use case is to apply mesh decals to surfaces. The decal and the surface are co-planar. The decal needs to win depth test if both are drawn together in a single pass.
Unity allows to add "Offset Factor, Units" to a shader. Unreal allows to apply a custom offset along camera direction. Any chance in USD to support similar feature?
Unity: https://docs.unity3d.com/2021.2/Documentation/Manual/SL-Offset.html
Unreal: https://docs.unrealengine.com/4.26/en-US/WorkingWithContent/MeshDecals/

Steps to Reproduce

1. Create 2 coplanar meshes.
2. z-fighting.

System Information (OS, Hardware)

Windows 10, MSVC 2019

Package Versions

v21.05

Build Flags

N/A (default)

[xum-adsk]

Actually, Storm already adds an epsilon to Peye.z to the selection. It's a little different from the second "units" parameter of glPolygonOffset. glPolygonOffset adds after projection. But they are similar.
Is there any chance to extend this to each prim? For example, a bulitin primvar similar to displayColor and displayOpacity. Or get it from the material if there is one. Then, modify gl_Position.z in the vertex shader.
https://github.com/PixarAnimationStudios/USD/blob/090ef0d849ced875b3b99a15e454b69148ccd8e1/pxr/imaging/hdx/shaders/selection.glslfx#L401-L408

[jilliene]

Filed as internal issue #USD-6775

[xum-adsk]

Thank you! I am applying CLA. Hopefully I can contribute later.