Add a volumetric cull mask to lights for volumetric fog (and implement an "additive fog" option for volumetric lighting)

[Calinou]

Note: This feature was discussed with reduz. It's probably good to implement if someone finds a way to do it.

Describe the project you are working on:

The Godot editor 🙂

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

With the newly-added volumetric fog effect, it's not possible to prevent individual lights from contributing to the volumetric fog's color.

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

A volumetric cull mask property could be added to prevent individual lights from contributing to the volumetric fog's color.

Alternatively, this can be done by adding a Volumetric Fog Energy property to Light3D. (It's probably better as it gives more control to the user, rather than an all-or-nothing solution.)

In addition, there should probably be a way to disable fog completely in areas not lit by anything (instead of having a black fog). This way, it can be used purely as a volumetric lighting solution, not adding any fog unless desired by the artist.

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

There could be a Volumetric Cull Mask property mimicking Light's Cull Mask property. It would only affect volumetric fog. Direct and indirect lighting would be left unaffected by this mask.

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

No, as this is core renderer functionality.

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

This is core renderer functionality.

[SIsilicon]

Volumetrics has been added to 4.0? That's pretty neat! Now I don't have to worry about porting my volumetrics plugin. 😄

On a more related note, how will this work? Like have a cull mask in the volumetric fog settings? Then the mask will just cull away lights that aren't in the same layer.

[Calinou]

I started work on this here: https://github.com/Calinou/godot/tree/volumetric-fog-add-per-light-energy

The "additive fog" part works great, but it needs per-light fog energy to be really useful. Unfortunately, I haven't got that part to work yet due to clustering artifacts being present ever since I added the volumetric_fog_energy property to Light3D.

For DirectionalLight3D, the default volumetric fog energy is too strong when used with additive fog. Make sure to set volumetric_fog_energy to a value between 0.01 and 0.05 to get a subtle "godrays" effect.