bevy_pbr: Make choosing of diffuse indirect lighting explicit.

[cryscan]

Objective

Make choosing of diffuse indirect lighting explicit, instead of using numerical conditions like all(indirect_light == vec3(0.0f)), as using that may lead to unwanted light leakage.

Solution

Use an explicit found_diffuse_indirect condition to indicate the found indirect lighting source.

Testing

I have tested examples lightmaps, irradiance_volumes and reflection_probes, there are no visual changes. For further testing, consider a "cave" scene with lightmaps and irradiance volumes. In the cave there are some purly dark occluded area, those dark area will sample the irradiance volume, and that is easy to leak light.

[alice-i-cecile]

Much clearer, thank you.

[mgi388]

@cryscan re:

as using that may lead to unwanted light leakage

do you mind explaining to me what this means and how it manifests as a bug in a game eg with an example if you have one? I’m using light maps myself, albeit not entirely successfully, and I’m trying to understand exactly what you mean by light leakage and also why the old code has the problem.

[cryscan]

@cryscan re:

as using that may lead to unwanted light leakage

do you mind explaining to me what this means and how it manifests as a bug in a game eg with an example if you have one? I’m using light maps myself, albeit not entirely successfully, and I’m trying to understand exactly what you mean by light leakage and also why the old code has the problem.

Irradiance volumes (or light probes) are prone to light leakage. One common case is an indoor area with thin walls: light from outside of the wall is very likely to leak into the inside area which are supposed to be dark. On the other hand, lightmaps are less likely to leak light. In terms of this particular PR, the original code checks whether to use lightmaps or fallback to irradiance volumes by checking whether the data read is zero. This is unrelieable because for some highly occluded pixels where the lightmap is intended to be completely dark, the shader picks irradiance volumes, and that will very likely to make the pixel accidentally lit up, and you will see artefacts in very dark areas.