[WIP] OMI_materials_blend

[aaronfranke]

This is a work-in-progress extension to add additional blending modes to glTF materials.

Preview: https://github.com/aaronfranke/gltf-extensions/tree/OMI_materials_blend/extensions/2.0/OMI_materials_blend

Godot implementation: omigroup/omi-godot#13

See icosa-mirror/UnityGLTF#1, KhronosGroup/glTF#1189, and KhronosGroup/glTF#1302 for the existing discussions.

(also, I deleted duplicate ### JSON Schema sections in the audio extensions)

[hybridherbst]

Great comparison table!

I think the part about blending is a bit misleading, and maybe there need to be additional clarifications for transmission:

"BLEND"
Simple translucency for materials such as glass. The behavior is the same as in the base glTF specification.

"Blend" does not actually simulate glass materials; it is a magical opacity property which makes objects disappear into thin air. "Transmission" on the other hand simulates glass materials –

• at opacity=0 with "blend", objects are completely invisible, there are no reflections visible either
• at transmission=1, objects are fully transparent but their reflections still show up (like glass)

[aaronfranke]

@hybridherbst Thanks, good catch! I actually took that from the PlayCanvas docs, so I guess their docs are wrong. I've modified the description of BLEND to simply match what Khronos has for glTF.

[mikeskydev]

Does it make sense to add new alphaMode values directly on the material property instead of storing in the extensions property?

[aaronfranke]

@mikeskydev Either is allowed per the glTF material schema (one possible value is "type": "string" for any string).

{
    "alphaMode": "ADD" // Directly setting the value.
}

However, if the only option was to use the existing "alphaMode" field, then this would make it impossible to provide a custom fallback when an importer doesn't support this extension. Therefore, we can and should allow for both.

{
    "alphaMode": "BLEND", // Fallback.
    "extensions": {
        "OMI_materials_blend": {
            "alphaMode": "ADD" // Override when clients support OMI_materials_blend.
        }
    }
}

I made a quick implementation in Godot and it can handle importing both of these cases: omigroup/omi-godot#13