Make decals contribute to global illumination

[novemberist]

Describe the project you are working on:

a 3d game that makes heavy use of emissive lights and global illumination

Describe the problem or limitation you are having in your project:

decals that use emissive textures currently don't contribute to global illumination

Describe the feature / enhancement and how it helps to overcome the problem or limitation:

when decals with emissive textures are applied to an object that is set to contribute to GI, the decals should too, this would be great for easily placing light panels, neon signs etc.

Describe how your proposal will work, with code, pseudocode, mockups, and/or diagrams:

unfortunately i don't know the intricacies of the engine code for GI, so I have no solution to offer here

If this enhancement will not be used often, can it be worked around with a few lines of script?:

it can only be worked around by not using decals as light emitters, if you want them to contribute to GI

Is there a reason why this should be core and not an add-on in the asset library?:

it's a rendering feature and thus needs changes in the core engine

Bugsquad edit: Reopened from godotengine/godot#40125.

[clayjohn]

Thanks for re-opening this! :)

[Calinou]

In the meantime, you could add a OmniLight or SpotLight as a child of the Decal node.

[Calinou]

See also godotengine/godot#51299, which is impacted by a similar limitation. With VoxelGI or SDFGI, it may not be feasible to implement this without adding a lot of complexity and reducing performance.

If you need lightmaps to work with GI, you can add lights with their bake mode set to Static near the emissive decals in question, then bake lightmaps again. This will not have much of a performance cost as the light will be fully baked (both direct and indirect light).

However, there's still some good news. In 4.0alpha, you can now use screen-space indirect lighting (SSIL) which reads the viewport's emission buffer, and therefore decals' emission channel 🙂
This works in real-time, so it updates instantly when a decal is changed.

SSIL disabled

SSIL enabled

Testing project (master): test_decal_ssil.zip