Vulkan: Indirectly lit surfaces show up black in reflections when using SDFGI with Bounce Feedback set to 0.0

[DKesserich]

Godot version

4.0 dev (af451ff)

System information

Windows 10, Vulkan, Radeon 5700XT

Issue description

Surfaces that are being lit by bounced lighting when SDFGI is active are reflected as if they are receiving no lighting at all.

Steps to reproduce

Extreme Example:
Create a 3D scene with a World Environment and enable SDFGI.
Build an enclosed volume out of MeshInstance3Ds (either planes or cubes)
Place a sphere inside the enclosed volume. Set the sphere material to have a metalness of 1 and a roughness of 0.
Place an OmniLight or a Spotlight inside the volume. Set the Range (if OmniLight) or the Angle (if Spotlight) to a low amount.
Set the Energy high enough for bounce lighting to fill the interior.
Observe that only the parts of the surface that are receiving direct lighting will be reflected in the sphere.

Minimal reproduction project

SDFGITest.zip

[DKesserich]

SDFGI doesn't seem to work at all on latest. I had to go back two weeks to find a commit where it worked.

[Calinou]

I'm not sure if this is a bug or a design decision to improve performance. The generated SDF may not be able to store indirect light to display it in reflections in the first place.

There are plans to refactor SDFGI to add more features to it and improve performance, but this may be done aftere 4.0 is released.

SDFGI doesn't seem to work at all on latest. I had to go back two weeks to find a commit where it worked.

This is a known issue.

[Calinou]

It seems that increasing SDFGI's Bounce Feedback above 0.0 seems to resolve this (try 0.5). Don't set it too high or infinite feedback look will happen, making surfaces overly bright.

Therefore, #39965 would resolve this out of the box if it's merged.

It's also interesting to note that changing Bounce Feedback updates reflections instantly, but diffuse light in the 3D world will take a while to update. Increasing Bounce Feedback above 1.0 will not further brighten reflections as well.

Bounce Feedback 0.0 (default)

Bounce Feedback 0.5

Bounce Feedback 0.5, Energy 0.5

[Calinou]

Thinking about it, this may actually be done by design. If Bounce Feedback is set to 0, then lighting will only bounce one time (as the second bounce's energy is multiplied by 0). This means there is no way for the resulting indirect lighting to show up in reflections, since it would have to bounce twice (or more) to achieve this.

#39965 will resolve this by default, but this is still something that will happen if you manually decrease Bounce Feedback to 0.

[Calinou]

This is now addressed by default by #39965, closing. #59418 will also document this in case you manually decrease sdfgi_bounce_feedback to 0.0.