ensure that indirect sheen is shadowed by specular ambient occlusion.

[bhouston]

Fixed #26939

Description

This fixes sheen so that that when it is lit by indirect light (IBL) it will be shadowed by ambient occlusion.

Before:

After:

Caveat: My reuse of the specularOcclusion function is an approximation. I am not sure if it is appropriate for indirect specular, rather I think it was designed for direct specular. It is close anyhow and better than not occluding indirect sheen.

Fun fact: I think that clear coat has the same bug. I will make a PR for occluding indirect clearcoat separately.

This contribution is funded by Threekit

[github-actions]

📦 Bundle size

Full ESM build, minified and gzipped.

Filesize dev	Filesize PR	Diff
651 kB (161.3 kB)	651.2 kB (161.4 kB)	+179 B

🌳 Bundle size after tree-shaking

Minimal build including a renderer, camera, empty scene, and dependencies.

Filesize dev	Filesize PR	Diff
443.7 kB (107.3 kB)	443.9 kB (107.4 kB)	+179 B

[mrdoob]

/ping @elalish @WestLangley

[WestLangley]

Related: #8107.

And, the paper describing the specular occlusion term:
https://seblagarde.files.wordpress.com/2015/07/course_notes_moving_frostbite_to_pbr_v32.pdf

[bhouston]

Thanks @WestLangley! I remembered wrong, the specular occlusion is for indirect specular. Thus I think this is mostly correct.

Review is still appreciated though!

[elalish]

Thanks, this looks good to me. I should have thought of this when I added support.