Fix LOD selection for image-based lighting

[jjhembd]

Description

This PR fixes two errors in LOD sampling of environment maps for image-based lighting:

1. In czm_sampleOctahedralProjection, the interpolation of mip levels was sometimes starting from the wrong lower-bound mip level.
2. In OctahedralProjectedCubeMap, the number of mip levels was limited to 6. This limit meant that the lowest-resolution versions of the environment map were not used. For rough materials lit by high-res environment maps, this could sometimes result in specular reflections with too much detail.

The improvement in LOD sampling from this PR is most visible on the Barn Lamp sample model, which has a lot of variation in the material roughness.

Before this PR:

After fixing mip level interpolation:

After allowing more mip levels:

Clearcoat Wicker model, before this PR:

After fixing mip level interpolation:

After allowing more mip levels:

Note how the specular reflection is slightly broader and brighter when an appropriately low-res version of the environment map is used.

Issue number and link

Fixes #12069.

Testing plan

Load the glTF PBR Extensions Sandcastle and compare the results to the same Sandcastle in main. Most models should show some improvement, but the effect is most dramatic on the "Barn Lamp" model.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Awesome, thanks @jjhembd! Just a few questions from me.

[ggetz]

I agree that the fix is resulting in better appearances for sure. But any idea why there was a 0.5 increment in the first place?

[jjhembd]

I think this dates back to before we were interpolating the mip level. In this previous version we were simply rounding to the nearest mip, which required the 0.5 increment. 006d859 added trilinear filtering but neglected to remove the 0.5 when selecting the "currentLod".

[ggetz]

Would it be worth capping this to a higher number than 6?

[jjhembd]

I tried a few values with mixed results. My choice of no cap was based on 2 factors:

1. The higher-numbered LODs are the lower-resolution ones, so including them adds very little memory overhead.
2. If a user tries the same environment map at different resolutions, they would expect to see differences only in the high-res details of the reflections (mip level 0). However, if we cap the max LOD, different input resolutions will result in a different mip being used for the background low-res part of the reflection.

[jjhembd]

@ggetz I think I answered your questions. Let me know if any further clarification is needed.

[ggetz]

Great, all makes sense.

The only thing I noticed when testing this out is some fine white lines around highlights that are new and don't look quite right. Any idea what is causing this?

[jjhembd]

Quick check of the Water Bottle sample model.
Before this PR:

After this PR:

Note the double reflection for the sun. This is caused by #12027

[jjhembd]

some fine white lines around highlights that are new and don't look quite right. Any idea what is causing this?

@ggetz These are clipping artifacts where the scene.light and the IBL sum to > 1.0. They go away if I set scene.light.intensity = 0.0, as in this Sandcastle. Unfortunately this makes the rest of the globe disappear, since the globe uses the scene.light and no IBL.

I tried setting scene.light.intensity = 0.5 and imageBasedLighting.imageBasedLightingFactor = new Cartesian2(0.5, 0.5). But this doesn't work, because imageBasedLightingFactor only applies to the procedural IBL, not environment maps.

I think these issues will be resolved when we address #12027.

[ggetz]

Got it. Thanks @jjhembd!