Vulkan Mobile: LightmapGI baking makes environment light twice as dark as it should be

[Calinou]

Tested versions

• Reproducible in: 4.3.beta 92c8e87

System information

Godot v4.3.beta (92c8e87) - Fedora Linux 40 (KDE Plasma) - X11 - Vulkan (Forward+) - dedicated NVIDIA GeForce RTX 4090 (nvidia; 555.58.02) - 13th Gen Intel(R) Core(TM) i9-13900K (32 Threads)

Issue description

LightmapGI baking in Forward Mobile makes environment light twice as dark as it should be.
This means that the visual outcome of lightmap baking is different, even if you view the lightmaps in the same rendering method:

Forward+ bake (seen in Compatibility)	Mobile bake (seen in Compatibility)1	Compatibility bake2

Environment baking in Mobile was added in #85793. I assume it's missing the 2.0 exposure multiplier that we use elsewhere, e.g. for real-time sky reflections: #55878

cc @Lasuch69

Steps to reproduce

• Bake lightmaps in Forward+. Run the scene and look at the results.
• Switch to the Mobile rendering method. Restart the editor and bake lightmaps.
• Switch to the Forward+ rendering method. Run the scene without baking lightmaps and look at the results. Notice how the scene now looks darker overall, particularly in areas that only receive baked environment lighting.

Minimal reproduction project (MRP)

test_lightmapper_rendering_methods.zip

Footnotes

1. Glowing material emission is missing due to https://github.com/godotengine/godot/issues/94406. ↩

2. This uses Forward+ for baking on desktop platforms, Mobile on mobile platforms if I'm not mistaken. ↩

[Lasuch69]

Where this exposure multiplier is exactly?

I cannot find it, even in forward clustered.

[clayjohn]

Its defined here:

godot/servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile.glsl

Line 537 in b6223c0

layout(constant_id = 15) const float sc_luminance_multiplier = 2.0;

Here is where it is used for skies:

godot/servers/rendering/renderer_rd/shaders/forward_mobile/scene_forward_mobile.glsl

Line 1173 in b6223c0

specular_light *= sc_luminance_multiplier;