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Update clearcoat extension spec #2415

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Jul 19, 2024
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Update clearcoat extension spec #2415

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6 changes: 6 additions & 0 deletions extensions/2.0/Khronos/KHR_materials_clearcoat/README.md
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Expand Up @@ -77,6 +77,12 @@ clearcoatRoughness = clearcoatRoughnessFactor * clearcoatRoughnessTexture.g

If `clearcoatNormalTexture` is not given, no normal mapping is applied to the clear coat layer, even if normal mapping is applied to the base material. Otherwise, `clearcoatNormalTexture` may be a reference to the same normal map used by the base material, or any other compatible normal map.

A mesh primitive using a clearcoat material with a clearcoat normal texture **MUST** have a defined tangent space, i.e., it **MUST** have `NORMAL` and `TANGENT` attributes or its base material **MUST** have a normal texture. When the mesh primitive does not have `NORMAL` or `TANGENT` vectors, they are computed as defined in the glTF 2.0 specification.

Since the glTF 2.0 specification does not mandate any particular tangent space derivation algorithm, mesh primitives using clearcoat materials with clearcoat normal textures **SHOULD** always provide their `NORMAL` and `TANGENT` vectors.

When the material has both `normalTexture` and `clearcoatNormalTexture` properties defined, these textures **SHOULD** use the same texture coordinates because they operate in the same tangent space and their texel values are usually correlated to each other.

The clearcoat effect is modeled via a microfacet BRDF. The BRDF is layered on top of the glTF 2.0 Metallic-Roughness material, including emission and all extensions, using a new `fresnel_coat` function:

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