ADOBE_materials_clearcoat_tint (KhronosGroup#1810)

* First version of clearcoat tint extension

* Change ADOBE_materials_clearcoat_tint schema to make tint factor array of 3, not 4.

* Small fixes

* Slight update to tint formula in README.md

* Updated specification links to use the glTF Registry.

* Fix schema link.

* Change Markdown bullets from `*` to `-` where applicable.

* Updates from review

---------

Co-authored-by: Ed Mackey <elm19087@gmail.com>

extensions/2.0/Vendor/ADOBE_materials_clearcoat_tint/README.md

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+# ADOBE\_materials\_clearcoat\_tint
+
+## Contributors
+
+- Mike Bond, Adobe, [@miibond](https://twitter.com/miibond)
+
+## Status
+
+Complete
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+- Requires the `KHR_materials_clearcoat` extension to also be defined on the material
+
+## Exclusions
+
+- This extension must not be used on a material that also uses `KHR_materials_pbrSpecularGlossiness`.
+- This extension must not be used on a material that also uses `KHR_materials_unlit`.
+
+## Overview
+
+This extension defines a way to tint the clear coating provided by the `KHR_materials_clearcoat` extension.
+
+## Extending Materials
+
+Adding tint to clearcoat materials can be done by adding the `ADOBE_materials_clearcoat_tint` extension to any glTF material that already has the `KHR_materials_clearcoat` extension defined on it.  For example, the following defines a material with a dark red varnish.
+
+```json
+{
+    "materials": [
+        {
+            "name": "varnish",
+            "extensions": {
+                "KHR_materials_clearcoat": {
+                    "clearcoatFactor": 1.0
+                },
+                "ADOBE_materials_clearcoat_tint": {
+                    "clearcoatTintFactor": [
+                      0.4,
+                      0.1,
+                      0.1
+                    ]
+                }
+            }
+        }
+    ]
+}
+```
+
+### Clearcoat Tint
+
+All implementations should use the same calculations for the BRDF inputs. Implementations of the BRDF itself can vary based on device performance and resource constraints. See [Appendix B](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#appendix-b-brdf-implementation) for more details on the BRDF calculations.
+
+As with the core glTF 2.0 spec, all coloured textures are assumed to be sRGB and all coloured factors are assumed to be linear.
+
+|                                  | Type                                                                            | Description                            | Required             |
+|----------------------------------|---------------------------------------------------------------------------------|----------------------------------------|----------------------|
+|**clearcoatTintFactor**    | `number[3]`                                                                                       | The clearcoat tint's colour factor.  | No, default: `[1.0, 1.0, 1.0]`   |
+|**clearcoatTintTexture**   | [`textureInfo`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-textureinfo)  | The clearcoat layer's tint texture.  | No                               |
+
+The clearcoat tint defines the transmittance of light through the coat. That is, it defines what colour of light is allowed to pass through. As with other factor/texture combinations, these two values are multiplied.
+```
+clearcoat_tint.rgb = clearcoatTintFactor.rgb * clearcoatTintTexture.rgb
+```
+The `clearcoat_tint` modifies the formula for `f_clearcoat` (as defined in [KHR_materials_clearcoat](/extensions/2.0/Khronos/KHR_materials_clearcoat/README.md)) only slightly. The `(f_emissive + f_diffuse + f_specular)` term is simply multiplied by the clearcoat tint to represent the absorption of all light coming from the layer below.
+
+```
+clearcoatBlendFactor = clearcoatTexture.r * clearcoatFactor
+clearcoatFresnel = fresnel(0.04, NdotV)
+
+color = mix(vec3(1.0), clearcoat_tint, clearcoatBlendFactor) * (f_emissive + f_diffuse + f_specular) * (1.0 - clearcoatBlendFactor * clearcoatFresnel) +
+        f_clearcoat * clearcoatBlendFactor
+```
+
+## Schema
+
+- [material.ADOBE_materials_clearcoat_tint.schema.json](schema/material.ADOBE_materials_clearcoat_tint.schema.json)
+

extensions/2.0/Vendor/ADOBE_materials_clearcoat_tint/schema/material.ADOBE_materials_clearcoat_tint.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "ADOBE_materials_clearcoat_tint glTF Material Extension",
+    "type": "object",
+    "description": "glTF extension that defines the colour tint of the clearcoat.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "clearcoatTintFactor": {
+            "type": "array",
+            "items": {
+                "type": "number",
+                "minimum": 0.0,
+                "maximum": 1.0
+            },
+            "description": "The transmittance of the clearcoat layer.",
+            "default": [ 1.0, 1.0, 1.0 ],
+            "minItems": 3,
+            "maxItems": 3,
+            "gltf_detailedDescription": "The colour of light allowed to be transmitted through the clearcoat layer of the material. A value of black means no light passes through. A value of white means all light passes through. These values are linear."
+        },
+        "clearcoatTintTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The clearcoat layer tint texture.",
+            "gltf_detailedDescription": "The clearcoat layer tint texture.  The values are stored in sRGB.  Assume white colour if no texture is supplied."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}