Treat specular less than 0.02 as occlusion

[clayjohn]

Closes: godotengine/godot-proposals#4818

This is a very common hack used in almost all PBR renderers to allow removing specular contribution in dielectric materials.

Filament calls it "Pre-baked Specular Occlusion". Essentially, a specular value of less than 0.02 is outside the realm of physical quantities, so that range is reused to signify occlusion. In effect, this makes the specular fade out when very low values are used and it does nothing when normal range values are used.

Importantly, this PR matches the behaviour with Blender, Unreal, and Unity.

Specular = 1.0

Before

After

Specular = 0.5

Before

After

Specular = 0.0

Before

After

[mindinsomnia]

You have my eternal gratitude!

[Calinou]

Nice work 🙂

The class reference should be updated to mention two things:

• metallic_specular's behavior when its value is lower than 0.02, disabling both specular lobes and IBL. The documentation should also mention that for most realistic use cases, you don't need to change metallic_specular.
• How the Disabled specular mode differs from setting metallic_specular below 0.02. IIRC, the Disabled specular mode will only disable the specular lobes, not IBL.

Also, it might be worth discussing for a future PR whether we should add a specular map property to BaseMaterial3D.

[BraqueDown]

The image for Specular = 0.0 Before is wrong. It's set to 1.0 instead.

[clayjohn]

The class reference should be updated to mention two things:

• metallic_specular's behavior when its value is lower than 0.02, disabling both specular lobes and IBL. The documentation should also mention that for most realistic use cases, you don't need to change metallic_specular.
• How the Disabled specular mode differs from setting metallic_specular below 0.02. IIRC, the Disabled specular mode will only disable the specular lobes, not IBL.

Done!

The image for Specular = 0.0 Before is wrong. It's set to 1.0 instead.

Fixed :) thanks for letting me know

[akien-mga]

Thanks!