Add Environment properties to make fixed and volumetric fog not affect sky rendering

[Calinou]

Describe the project you are working on

The Godot editor 🙂

Describe the problem or limitation you are having in your project

Fog rendering in Godot 4.0 always affects the sky rendering. This may not always desired, especially when you are using fog as an atmospheric effect (without intending it to be used for open world fog fading). For fixed (non-volumetric) fog, setting Aerial Perspective to 1 will let you see the background sky again, but it has a performance cost.

In comparison, fog in Godot 3.x never affects sky rendering.

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

Add a way to make fog rendering not fully affect sky rendering, so that the background sky can still be partially visible.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Add Fog Sky Affect and Volumetric Fog Sky Affect properties to Environment. By default, both would default to 1.0. If set to 0.0, then fog rendering will not affect the sky at all. Intermediate values will only partially affect sky rendering.

I have a WIP pull request implementing this: https://github.com/Calinou/godot/tree/environment-fog-add-sky-affect

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

No, as this is core rendering functionality (implemented in the sky GLSL shader, which is not user-overwritable).

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

See above.

[clayjohn]

Keep in mind, sky shaders have a disable_fog render mode and they can override the fog value using the FOG built-in. This allows the sky to show through the fog even if the fog is quite thick.

[Calinou]

Keep in mind, sky shaders have a disable_fog render mode and they can override the fog value using the FOG built-in. This allows the sky to show through the fog even if the fog is quite thick.

I see, I wasn't aware about that. Maybe disable_fog should be exposed in the inspector for the various default Sky resources (like godotengine/godot#55551 did for filter modes)?

[clayjohn]

@Calinou I think thats the way to do it if this is desired! We could also expose the FOG builtin as a uniform as well if users desire it.

[mrjustaguy]

Having that option is very much desired, it greatly increases the flexibility of the fog and is very straight forward.

[clayjohn]

Related to this proposal, I noticed that the current dependency on z-far distance also breaks when using reflection probes as the z-far distance changes between probes. Unless you set the probe's max_distance to be the same as the camera's z-far you end up with a different amount of fog between the sky and the reflected sky.