GLTFLoader: Fix color space for specular map.

[Mugen87]

Related issue: #23627

Description

This PR defines the color space for specularMap. Only relevant for KHR_materials_pbrSpecularGlossiness. SpecGlossVsMetalRough.glb now renders as expected.

[donmccurdy]

Thanks! I wonder how this wasn't broken before. 🤔

[Mugen87]

The previously automatic inline decode only worked for specific textures of built-in materials. For all others (like the specularMap of GLTFMeshStandardSGMaterial) Texture.encoding could not be used and the custom shaders had to perform the inline decode manaully.

[WestLangley]

So, KHR_materials_pbrSpecularGlossiness is still a thing?...

See KhronosGroup/glTF#2078

[donmccurdy]

New glTF material features (like sheen, transmission, refraction, IOR, iridescence, subsurface scattering, ...) are only added to the metal/rough PBR workflow by Khronos now and moving forward, so the spec/gloss workflow in glTF provided by KHR_materials_pbrSpecularGlossiness is basically deprecated.

With extensions like KHR_materials_specular and KHR_materials_ior, the metal/rough workflow can losslessly represent all of the same materials as the spec/gloss workflow anyway, and there are some tools (like glTF Transform) to convert spec/gloss to metal/rough.

I think we could consider removing support for spec/gloss.

[elalish]

Thanks @Mugen87! Yeah, Spec-Gloss is deprecated, but only for about a year now and there are lots of assets floating around with the extension because many authoring tools didn't like the idea of a lossy conversion. I would wait until there is a high quality free tool for converting Spec-Gloss GLBs to MR before we remove support.

[donmccurdy]

Not intending to rush the process, but -

gltf-transform metalrough input.glb output.glb

The conversion is not lossy if the viewer supports the specular and IOR extensions.

[elalish]

Awesome, thanks @donmccurdy! Is this part of a web GUI anywhere yet? I don't know if you want to help build out the functionality of the model-viewer editor or build your own, but either way it would be awesome. I know, I know, in our copious spare time...

[donmccurdy]

At some point I'm going to have https://gltf.report/ start doing the conversion by default (it'll need metal/rough for other reasons), but for now it can be accomplished by running this in the sidebar script tab:

import { metalRough } from '@gltf-transform/functions';
await document.transform(metalRough());

[mrdoob]

Thanks!

[mrdoob]

Cherry-picked into the master branch and included in 0.138.3.