Vulkan: LightmapGI with Directional enabled exhibits negative-colored specular reflections on metallic materials

[Calinou]

• Related to Vulkan: Directional LightmapGI does not look correct with emissive surface light #64645.

Godot version

4.2.dev 221884e

System information

Godot v4.2.dev (221884e) - Fedora Linux 38 (KDE Plasma) - X11 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4090 (nvidia; 535.104.05) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

Issue description

LightmapGI with Directional enabled exhibits negative-colored specular reflections on metallic materials:

This doesn't occur at low metallic values, even if roughness is kept to 0.0:

Disabling environment baking in LightmapGI doesn't resolve the issue.

Steps to reproduce

• Create a MeshInstance3D with a metallic smooth material, and another one with a rough emissive material (or a bright material that reflects light).
• Enable Directional in LightmapGI. Bake lightmaps.
• Notice how reflections will take the opposite color of the light reflected by the other node.

Minimal reproduction project

test_lightmap_directional_metallic.zip

[clayjohn]

This comes from our fake code to add a little bit of specular from the lightmaps. The code produces invalid values when used with a directional light. I would just remove it entirely