Add Environment linear fog

[briansemrau]

Proposed solution to godotengine/godot-proposals#3429

This adds Environment properties for linear fog.

This nicely combines with exponential fog, allowing linear fog to completely obscure objects beyond a distance while exponential fog provides a more realistic effect in the interim.

The two are combined using the formula: `fog_amount = max(fog_exp, fog_linear)`

Blue: linear
Green: exponential
Dashed Red: combined

There is no need for any fog_mode enum property, as linear and exponential fog are combined and can be individually disabled (fog_density = 0 or fog_linear_end = 0).

---

Only exponential fog (current fog default):

Only linear fog:
(note: linear fog does not (yet?) effect the sky cubemap like exponential fog)

Combined linear and exponential:

Combined linear and exponential with aerial perspective:

---

Remaining tasks:

• Exponential fog modifies the sky cubemap. Should linear fog do the same?
• Maybe add a linear_fog_amount property to control the maximum linear fog density
• Some kind of warning when fog_linear_start >= fog_linear_end

[briansemrau]

@Calinou @clayjohn I'd appreciate your opinions on this method, if you'd like to take a look at the comparison screenshots.

[clayjohn]

The results looks good and implementation looks good, but I am not sure it is needed.

To me, the purpose of #54222 is to allow fading in objects for open-world style games, I don't know how many games use fog to fade in objects at a distance. But I imagine it is not many as it will mostly only really work when using a flat sky color.

Ultimately, we also allow writing custom fog values in spatial shaders, so I don't really see why this needs to be builtin to the scene shader.

[briansemrau]

To me, the purpose of #54222 is to allow fading in objects for open-world style games

Just to note, using per-instance distance fade from that PR with chunk-based loading will result in obvious chunk boundaries. A per-fragment approach is needed for that use case, whether it be fog, transparency, dithering or otherwise.

I don't know how many games use fog to fade in objects at a distance. But I imagine it is not many as it will mostly only really work when using a flat sky color.

A classic example where this is used is Minecraft. IMO, fog is a very easy and elegant solution to handling draw distance limits. It fades over all geometry evenly and at the same distance, just by configuring one or two properties. And with fog_aerial_perspective, it blends into the sky beautifully.

Ultimately, we also allow writing custom fog values in spatial shaders

As described in godotengine/godot-proposals#3429, implementing the same custom fog shader code for each material in a scene can quickly become tedious to create and maintain. It also requires converting every material into a shader material. I'll concede that that issue should be addressed directly, or perhaps there's a better way with the current system that I'm not aware of.
Regardless, I believe that built-in linear fog is a feature that some people will want to have as an option, even if not for obscuring draw distance limits.

[clayjohn]

Don't forget, we also have fading built in to the StandardMaterial https://docs.godotengine.org/en/latest/classes/class_basematerial3d.html#class-basematerial3d-property-distance-fade-mode

Perhaps this could be a built in option in the StandardMaterial?

It is a tough thing to balance. On its own the PR is quite small. But it will have an impact on most 3D shaders in the engine. Remember, adding more variables into a shader increases VGPR pressure and reduces occupancy (and occupancy is a bit of a problem in the scene shader right now). So the tradeoff is between performance for all scene shaders and ease of use for people who want this feature (it would be a total pain to enable for every material/shader).

[briansemrau]

Performance concerns are very real. Can't argue with that. I'll see if I can find some time to exploring those options you mentioned, as well as some other ideas I've got.

[briansemrau]

I'll close this, but this should be marked salvageable should the feature be desired in the future.

With regards to custom fog, in its current state I think it is insufficient without godotengine/godot-proposals#944 or godotengine/godot-proposals#2595. Fog shouldn't be per-material, but per environment, even though it executes per material rather than as a post-process effect. Otherwise, I can't see many people using custom fog in a reasonably sized project, as maintainability becomes immensely challenging.