Backport new 3D point light attenuation as an option (3.x)

[Calinou]

This provides more realistic lighting with a very small performance cost. The option is available in both GLES3 and GLES2, and can be enabled in the Project Settings. This goes well with the ACES Fitted tonemapping mode that was recently added.

When enabled, this also makes upgrading Godot 3.x projects to Godot 4.0 easier, since lighting in 3.x will better match how it'll look in Godot 4.0.

Testing project: test_light_attenuation_3.x.zip

Testing project 2 (use for testing CPU lightmapper attenuation backport): test_light_attenuation_lightmap_3.x.zip

Preview

Note: The difference in brightness between the GLES3 and GLES2 images is expected, as GLES2 does not use HDR internally.

Lights in the middle have their attenuation set to 1.0 (the default). Lights on the right have their attenuation set to 0.5 ("Out" preset). The red light in the distance has its attenuation set to 2 ("In" preset).

GLES3 fragment shading

Before	After

GLES2 fragment shading

Before	After

GLES3 vertex shading

Note: Vertex shading looks broken here due to the floor's low vertex count.

Before	After

GLES2 vertex shading

Note: Vertex shading looks broken here due to the floor's low vertex count.

Before	After

TODO

• Adapt the CPU lightmapper code to take the new option into account.
• Test on mobile and HTML5 to make it sure it looks as expected. Check if there's a noticeable performance impact on mobile (and if so, document it).

[lawnjelly]

Have looked through the code and tested it in GLES2 and GLES3. It looks a shedload better than the old attenuation imo (or at least more realistic). 👍

I don't know whether @clayjohn wants to give it a once over too.

[clayjohn]

Looks great! I'm sure many users will find this very useful.

[Calinou]

The CPU lightmapper now takes the new light attenuation option into account.

Fully baked light (with bounces disabled) on the left, real-time light on the right:

GLES3

Note: The lightmap's colors appear washed out unless Hdr As Srgb is manually enabled on the texture's import settings. Enabling this import setting makes the colors correct, but the resulting lightmap becomes too dark compared to the real-time rendering. This doesn't happen in GLES2. We should look into this in a separate issue.

Old attenuation	New attenuation

GLES2

Old attenuation	New attenuation

[Calinou]

As for the mobile part, I tested this PR on a OnePlus 6. It looks correct, and the performance is pretty much identical.

Visual comparison

GLES3

Note: HDR was enabled on mobile to get a similar appearance as on desktop platforms.*

GLES2

Performance comparison

GLES3

Note: HDR was enabled on mobile to get a similar appearance as on desktop platforms.

Old attenuation	New attenuation

GLES2

Don't mind the different image sizes. The rendering resolution is identical (2280×1080), but I accidentally downsized one of the images.

Old attenuation	New attenuation

FPS fluctuates between 46 and 48 with both settings.

[JFonS]

I think there are a couple of leftover debug prints, other than that it looks good :)

[akien-mga]

Thanks!

[Giwayume]

Unreal 4 allows both physically based and legacy light attenuation to be used in the same scene, because it is recognized that some lights may require an artistic approach even if most are physically based. Can you make it a setting per-light instead of a project-wide setting?

[clayjohn]

@Giwayume this still allows tweaking the falloff curve for artistic purposes just like it used to! The difference is now the default curve uses proper inverse squared law.

[Giwayume]

I'm aware,

https://docs.unrealengine.com/4.27/en-US/Resources/ContentExamples/Lighting/4_1/

Exponential Falloff can often get close to realism, but will generally be best for stylized visuals.

https://docs.unrealengine.com/4.27/en-US/Resources/ContentExamples/Lighting/4_2/

Inverse Square Falloff is a different type of light falloff that most closely replicates the behavior of light in the real world.

Unreal intentionally keeps both falloff implementations because they recognize some games need it. Indeed I have seen Unreal 4 games that liberally use a combination of both. It's just a check box on the individual light to switch between the two types.

[Giwayume]

"Disabling inverse squared falloff can be useful when placing fill lights (don't want a super bright spot near the light)."

[Calinou]

Adding a per-light toggle will inevitably increase the performance demands (and configuration complexity), so I'd prefer not to do it.

You can use a zero or slightly negative attenuation curve for fill lights (right-click the easing curve to show presets). This will prevent them from reaching high levels of brightness 🙂

[Giwayume]

I think if Unreal 4 can manage it, it's not a big performance it.

[Giwayume]

light_params[3] (shadow enabled) is basically used as a boolean right now. Could easily store extra data in that field. Though I know float comparison in glsl has its quirks.