GLTFExporter: Fixed alphaMode handling

[mrdoob]

I think the current logic is broken.

@fernandojsg @donmccurdy let me know if this looks good to you.

[donmccurdy]

Agreed, this change is more intuitive and supports cases where material.transparent = true but material.opacity = 1.0 (e.g. the transparency is in .map instead).

[donmccurdy]

Aside, I do like the idea of having an .alphaMode setting on the threejs material, similar to glTF or Blender's equivalents, to make this more explicit and perhaps support future cases like Alpha Hash or TRAA. Maybe just in NodeMaterial for starters..

[mrdoob]

Agreed, this change is more intuitive and supports cases where material.transparent = true but material.opacity = 1.0 (e.g. the transparency is in .map instead).

Yeah, I was re-exporting the Flight Helmet and it was setting alphaTest to 0.5 incorrectly.

Aside, I do like the idea of having an .alphaMode setting on the threejs material, similar to glTF or Blender's equivalents, to make this more explicit and perhaps support future cases like Alpha Hash or TRAA. Maybe just in NodeMaterial for starters..

Hmm, yeah. We may have to refactor a few things.

[mrdoob]

Another aside... In the GLTFLoader, what do you think about setting material.premultipliedAlpha to true when alphaMode is set to BLEND?

I don't know if there is anything in the spec about it, but it will make three/model-viewer look closer to Filament.

I think the glass will look more similar here:

[donmccurdy]

... what do you think about setting material.premultipliedAlpha to true when alphaMode is set to BLEND?

glTF doesn't specify, although it probably should. In the meantime I have no objection to changing it. 😇

[mrdoob]

Hmm... Just tried locally, doesn't seem to change that much... 🤔

Before:

After:

[mrdoob]

Seems better though.

[mrdoob]

I'll continue testing. Merging this for now.