Add Environment Min Light property to LightmapGI

[Calinou]

This property works the same as BakedLightmap's Environment Min Light property in 3.x. It can be used to avoid overly dark areas in the lightmap, or to tweak the shadow color for artistic reasons.

This closes #50568.

Testing project: test-lightmapgi-environment-min-light.zip

Preview

The color is exaggerated for previewing purposes. Typically, you'll use much darker colors like #050505.

TODO

• Make lightmap probes affected by Environment Min Light so that dynamic objects are lit correctly.
• Fix Environment Min Light being too bright when Directional is enabled.

[reduz]

I do not see the point of this given you have the ambient constant color, its quite redundant.

[Calinou]

I do not see the point of this given you have the ambient constant color, its quite redundant.

The constant ambient color does not spread to shaded areas when using LightmapGI. While this is physically accurate, it can result in scenes looking too dark unless you manually add a bunch of OmniLight3Ds in dark spots (also known as "unmotivated light sources").

This PR is also in the interest of avoiding a feature regression for users moving from 3.x to 4.0. Several users seem to be relying on this feature in 3.x already, and we should ensure there are as few roadblocks as possible for upgrading to 4.0.

[ScarfKat]

This would be super helpful. I just ran into a relevant issue today, with lightmapping being darker than I anticipated. Fingers crossed this makes it into 4.4.

[Calinou]

Rebased and tested again, it works as expected.

This would be super helpful. I just ran into a relevant issue today, with lightmapping being darker than I anticipated. Fingers crossed this makes it into 4.4.

I'm afraid there are several outstanding issues I need to address before this is in a mergeable state (see TODO in the pull request description). These are nontrivial to fix as I'll need to modify the lightmap baking shader to bake the correct data on the GPU, instead of modifying the final image data on the CPU (which can't affect lightmap probes, or deal with directional lightmaps correctly). This is more involved since I'm not familiar with the lightmap baking shader's code.

In the meantime, there are a few things you can do to address lightmaps being too dark:

• Increase the Bounce Indirect Energy property in your LightmapGI node.
• Increase Indirect Energy on specific Light3D nodes.
• Add some unmotivated light sources (i.e. lights with Static bake mode, relatively high range and a low energy) in specific dark areas.
• Change the tonemapping method used in the Environment node. ACES has a tendency to overdarken scenes, so try using Filmic instead. AgX can be used as an alternative that displays bright colors in a more accurate fashion (similar to ACES) but without the overdarkening issues.

[filipworksdev]

This is very helpful! 👍