Wireframe Mesh Performance on Quest

[Vavassor]

Geometry shaders are used to draw wireframe meshes on PC. But Meta Quest headsets don't support geometry shaders in VR. So for Quest we use a wireframe shader using Vertex IDs. This requires a copy of the mesh that has split faces.

This copy is both slow and annoying because it requires your meshes to have "read/write" enabled in import settings. Caching is used to mitigate the performance cost. But it can cause framerate stutters or freezing if meshes are big. Or if the cache is filled.

Investigate alternative methods of wireframe rendering.

[Vavassor]

Rejected Ideas

• GL.wireframe is not supported on mobile. And not whitelisted in Udon.
• Storing the barycentric coordinates or triangle center per vertex is another common method of wireframe rendering. But it also requires mesh modification, which we're trying to avoid.
• Use _CameraDepthTexture to estimate the edges of triangles. Use of the depth texture is not supported on Quest in VRChat. (although it's not a hardware limitation)

Ideas to Investigate

• Conservative Rasterization can be used for wireframe rendering. Don't know whether this feature is supported on Quest.

[Vavassor]

Having a retained mode option for drawing meshes would help. Because those meshes could be copied up front instead of doing on-demand copies and caching. But it wouldn't help meshes that truly need to change frequently.