GLTFLoader: Support KHR_materials_transmission physically-based transparency

[Doerge]

Describe the bug

The extension KHR_materials_transmission does not semi transparent materials from a .glb.

To Reproduce

Steps to reproduce the behavior:

1. Get the reference glb from here https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/TransmissionTest
2. Load it with threejs gltf loader.
3. The parts of the geometry that are supposed to be semi-transparent are instead 100% opaque. Only 100% transparency is respected. See screenshots below.

Live example

https://jsfiddle.net/jt6ywhs5/12/

Expected behavior

Semi-transparency and transmission works.

Screenshots

Reference:

Threejs:

Platform:

• Device: Desktop
• OS: MacOS
• Browser: Chrome 87.0.4280.88 (Official Build) (x86_64)
• Three.js version: latest stable from https://threejs.org/build/three.module.js

[donmccurdy]

three.js only implements a small part of the KHR_materials_transmission extension today, which is why we haven't listed the extension in the docs yet. There are more transmission features left to implement. See #16977 for a bit more about this. Also note that using an environment map is critical for physically-based reflections and transparency.

[Doerge]

Ah I see. I was a bit too fast then. I landed on the source code through Google, and thought it was already there. Sorry!
Yeah, I didn't bother with an env map, since it was clear the semi transparency wasn't working. Was hoping it was just a setting but alas! Feel free to close this issue if it's already tracked elsewhere.

[donmccurdy]

Let's keep this open — #16977 is a pretty broad issue so I think it's good to have something specifically tracking KHR_materials_transmission.

[yaroslavnikiforov]

Is it possible to implement this in three.js using MeshPhysicalMaterial without extension? (like Rough Transmission yellow ball on the reference image)

[donmccurdy]

No, some changes to MeshPhysicalMaterial would need to be made to support these features.

[yaroslavnikiforov]

Hi, what is the status of implementing this, is it work in progress? Will it be included in the next release? Thanks.