Adding the KHR_materials_iridescence effect

[UX3D-nopper]

Replaces #1742

[PascalSchoen]

@sebavan this is defined in the specular extension and adopted by this extension (see KHR_materials_specular #Implementation).

The issue is, that if we simply use 1 - iridescence_fresnel for the weight of the base, you get inverse colors there. If you don't weight base at all, you gain energy. I updated the PR with the same description and a reference to KHR_materials_specular.

Another approach would've been to use the grayscale representation of iridescence_fresnel as this would reduce the energy-loss but could gain a bit of energy as well. It was thus decided by the group to use the approach, that is currently documented.

[emackey]

On the call today it was suggested we rename "iridescenceIOR" to "iridescenceIor" to fix the camelCase. @lexaknyazev has some additional nonbreaking corrections to some of the description fields, to align them with language used in the core spec.

[sebavan]

All good for me I changed it in Babylon and it should be up in the next couple hours :-)

[UX3D-nopper]

We can also make the changes in Gestaltor.

[UX3D-nopper]

However, did we think about the camel case rule for OpenGL. or OpenXR?
IOR is Index Of Refraction and IOR in short. So, in this case, it should stay iridescenceIOR, correct?
Otherwise it should be OpenGl or OpenXr, right?

[lexaknyazev]

@UX3D-nopper
Existing glTF JSON properties follow lowerCamelCase convention. The KHR_materials_ior extension did not make an exception for its ior property despite the latter being an acronym. So for consistency, we should call iridescence IOR as iridescenceIor.

[UX3D-nopper]

So the rule is like the one here: https://google.github.io/styleguide/jsguide.html#naming-camel-case-defined
Or do we have a better document for this?

[UX3D-kanzler]

I have opened a pull request for the sample models (iridescenceIOR->iridescenceIor): KhronosGroup/glTF-Sample-Models#349

[donmccurdy]

PR for gltf-transform: donmccurdy/glTF-Transform#570

[echadwick-artist]

It would be helpful if the code block in the Extending Materials section included each of the Properties, to illustrate all their formatting, e.g.:

{
    "materials": [
        {
            "extensions": {
                "KHR_materials_iridescence": {
                    "iridescenceFactor": 1.0,
                    "iridescenceTexture": {
                      "index": 0
                    },
                    "iridescenceIor": 1.3,
                    "iridescenceThicknessMinimum": 200.0,
                    "iridescenceThicknessMaximum": 400.0,
                    "iridescenceThicknessTexture": {
                      "index": 0
                    }
                }
            }
        }
    ]
}

[emackey]

This was ratified by the Khronos Board on July 22, 2022.