please add support for "KHR_materials_clearcoat", "KHR_materials_transmission", "KHR_materials_sheen"

[rawwerks]

Description

There are three new PBR extensions that Khronos recently released that dramatically improve the ability to create photo-realistic GLTF files. https://gfxspeak.com/2020/12/03/physically-rendering-extensions/

Given that model-viewer supports GLTF, and that these 3 extensions are official extensions for GLTF 2.0, it would seem within the scope of this application to include the functionality to correctly display these new extensions: "KHR_materials_clearcoat", "KHR_materials_transmission", "KHR_materials_sheen"

https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos

Comparison

Comparison between Model Viewer (Left, extensions not supported) and Babylon (Right, extensions supported)

See example file at:
https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/ToyCar

Version

Whatever version is live at https://modelviewer.dev/editor/

Browser Affected

This is not a browser-specific issue.

OS

This is not an OS-specific issue.

[rawwerks]

@bsdorra @elalish - any implementation tips on KHR_materials_transmission since these extensions are hot off the press?

edit: this implementation tip from @MiiBond may be useful here as well: mrdoob/three.js#16977 (comment)

[elalish]

Yes, you've already linked to the right place in three.js. Support in model-viewer will come via support in three. I believe clearcoat is already working, and sheen is partially there, but not hooked up to GLTFLoader. We definitely intend to support these extensions.

[donmccurdy]

2/3 of these (clearcoat, transmission) have been in three.js for a bit now and I assume are working in model-viewer already? Although I'd add Volume and IOR to the list now. 😉

EDIT: Correction, it doesn't look like transmission is working just yet:

[elalish]

They should in the next release when we update three.js versions. You can test the current state of development with our editor. We're also working on getting some tests going for these extensions before we go trumpeting support.

[elalish]

I'm going to call this fixed, as we have not just these extensions now with r131, but also ior, specular, and volume. I think sheen may not be totally supported yet and there could easily be some bugs in the others, but I'd rather track those as individual issues.