Compatibility renderer has lower-quality (ir)radiance map generation than Forward+ and Mobile

[Chaosus]

Tested versions

4.3 rc2

System information

Windows 11

Issue description

On forward renderer the simple MeshInstance with a SphereMesh looks like this (which is correct):

On compatibility renderer it has blurpy artifacts:

IDK if this is already reported, feel free to close if then.

Steps to reproduce

Create a MeshInstance3D and assign a SphereMesh to it, and run that scene on Compatibility Renderer.

Minimal reproduction project (MRP)

TestSphere.zip

[Calinou]

This is probably just the radiance map generation being lower quality, particularly for higher roughness mipmaps. I'm not sure if much can be done about this.

Forward+ and Mobile can achieve higher quality as they use texture arrays for reflections (if enabled in the project settings), while Compatibility can't. Try disabling Texture Array Reflections in the Project Settings, restart the editor and look at how the ambient light looks when using Forward+.

[F3der1co]

Try disabling Texture Array Reflections in the Project Settings, restart the editor and look at how the ambient light looks when using Forward+.

Forward+ with Texture Array Reflections turned off:

While worse still looks worlds better than on Compatibility.
To me it looks like it uses a much lower resolution mip-map than compatibility, which hides the blobbiness. Just a guess.

[clayjohn]

edit PR incoming tomorrow

To confirm, this comes from calculating Radiance directly within the mipmaps of a single cubemap texture. In the other RD rendering methods we using a Texture array so the quality can be significantly higher.

I suspect that we can get the same quality just by taking more samples at the lower mip levels. Effectively the reason it looks so much better is we take more samples when using texture arrays (its the same number of samples per pixel, but we have more pixels, therefore more samples).

I'll run a quick test and see

edit: It's not just sample count, there is a bug in how the texture is sampled