Implement GI Reflections (SDFGI, VoxelGI) for LightmapGI

[guerro323]

This complete a part of godotengine/godot-proposals#3012 (it doesn't address disabling VoxelGI reflections for SDFGI).
Depends on #86102 for Specular Occlusion.

(greatly exaggerated for showing the reflections, look in screenshots for the normal ones)
(scene from https://sketchfab.com/3d-models/japanese-street-at-night-fb1bdcd71a5544d699379d2d13dd1171)

This add support for reflections from GI such as SDFGI or VoxelGI onto lightmapped objects.

Benefits and constraints

• It help level designers dealing with less reflection probes, especially in complex scenes (such as rounded walls, complex corridors that aren't straight, etc...)
• Reflection probes can still be used and blends well with the GI Reflections and should be used for cube shaped interiors or when you need a mirror-like reflection which can be ruined by SDFGI/VoxelGI voxels look.
• There is a cost of using LightmapGI + SDFGI, but it's less than SDFGI only as we only apply the reflection buffer (further optimizations could be possible).
• VoxelGI + LightmapGI has a smaller cost than VoxelGI Only as we only trace for reflection.
• Without specular occlusion it doesn't work well with simple shapes and no textures if you crank the roughness to 0 and metallic to 100.
• Specular occlusion will help making the GI reflections look more right and occluded when it should be.

Screenshots and Comparisons

'Reflection' scene

Source: https://github.com/Calinou/godot-reflection

Lightmap 4.2	SDFGI	Lightmap + Specular Occlusion	Lightmap + SO + SDFGI	Lightmap + SO + VoxelGI

Cube scene

Lightmap 4.2	SDFGI	Lightmap + Specular Occlusion	Lightmap + SO + SDFGI	Lightmap + SO + VoxelGI

Japanese Street

Source: https://sketchfab.com/3d-models/japanese-street-at-night-fb1bdcd71a5544d699379d2d13dd1171

Lightmap 4.2	Lightmap + Specular Occlusion	Lightmap + SO + SDFGI	Lightmap + SO + VoxelGI

Simple scene:

• The left part of the floor is a standard default material, the other part is a metallic with roughness set to 0.
• The black cubes are material with roughness set to 0 and black color.
• The grey cube is a 100% metallic white material.
• The white cube is a 100% specular white material.

Lightmap Only	Lightmap + SDFGI

Caveats:

• You may need to modify the values of SDFGI or VoxelGI as they may be darker when lightmaps are on. A good way to check if everything is right is to have a fully reflective surface and check if the GI reflections match the same brightness/color as your scene.

Open Questions:

• Optimizations? It is possible to optimize SDFGI to only care about reflection data?
• An option to disable this effect? Would there be an use case to only have GI reflections only on non-lightmapped objects?

---

¹ I posted some more screenshots (without specular occlusion) and benchmarks here godotengine/godot-proposals#3012 (comment).

[clayjohn]

Great work so far!

I've added this to the agenda for the next rendering meeting. This idea has popped up a few times, but I think it will require some discussion and a lot more work. Particularly we need to discuss the workflow issues here and how to optimize out the calculation of diffuse lighting from VoxelGI/SDFGI.

[atirut-w]

Testing this out on my project, SDFGI reflections are abnormally dark on lightmapped meshes.

Details

No lightmap on sphere:

Lightmap on sphere:

This issue is also apparent in a newly created project:

[guerro323]

@atirut-w This seems to come from the mix of a lightmap with a dynamic directional light and the default specular occlusion mode.
It seems that the lightmap is a little bit darkened when computed with a dynamic light which incorrect result to the specular occlusion.
You can remove this effect by switching to Project Settings → Lightmapping → Specular Occlusion Mode → Conservative (Fast) (I'll make this option the default as Reduce is too much problematic with dynamic lights; you can also disable specular occlusion)

[atirut-w]

you can also disable specular occlusion

Apparently, turning it off also darken reflections?

[atirut-w]

(Moved to #86102)

[guerro323]

Apparently, turning it off also darken reflections?

I accidentally included an old commit of the specular occlusion in it which made disabling it buggy.
It should be fine now.

[atirut-w]

It should be fine now.

Well, about that... disabling specular occlusion now turn reflections black...

[guerro323]

Would it be possible to have a simple scene that reproduce this? I'm not able to get black reflections :/

[atirut-w]

I managed to reproduce a slightly different behavior where everything except the sky is black: Lightmap.zip

[guerro323]

It is fine on my side.
I'm not sure why it doesn't work for you, maybe it is related to the shader cache?

[atirut-w]

Weird. Clearing the shader cache doesn't solve it either.

Here's the system information anyway, so that it may help: Godot v4.3.dev (1bc749129) - Windows 10.0.22621 - Vulkan (Forward+) - dedicated AMD Radeon RX 6700 XT (Advanced Micro Devices, Inc.; 31.0.23013.1023) - AMD Ryzen 7 5700X 8-Core Processor (16 Threads)

[Calinou]

Tested locally (rebased against master 17e7f85), it works as expected.

LightmapGI only

LightmapGI + SDFGI

Non-reflective materials appear darker because the material isn't fully rough (it's the fallback 3D material).

SDFGI only

No GI

[atirut-w]

It is fine on my side. I'm not sure why it doesn't work for you, maybe it is related to the shader cache?

I narrowed it down to something about AMD GPUs. I tested the MRP on my laptop which has Intel Xe graphics and the issure was not occuring there. I think this needs more testing on AMD GPUs like mine (RX 6700 XT) because it's a pretty breaking bug.

One silver lining is that it only appears when specular occlusion is disabled, I guess?