KHR_materials_clearcoat_ior

extensions/2.0/Khronos/KHR_materials_clearcoat_ior/README.md

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+<!--
+SPDX-License-Identifier: LicenseRef-KhronosSpecCopyright
+-->
+
+# KHR\_materials\_clearcoat\_ior
+
+## Contributors
+
+- Mike Bond, Adobe, [@miibond](https://github.com/MiiBond)
+
+## Status
+
+Draft
+
+## 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
+
+The clearcoat BRDF in `KHR_materials_clearcoat` uses a fixed value of 1.5 for the index of refraction (IOR). This extension allows users to override that default and set the IOR for the clearcoat layer explicitly, analogous to how `KHR_materials_ior` sets the IOR for the base layer.
+
+## Extending Materials
+
+The index of refraction of the clearcoat layer is configured by adding the `KHR_materials_clearcoat_ior` extension as a sub-extension to `KHR_materials_clearcoat`.
+
+```json
+{
+  "materials": [
+    {
+      "extensions": {
+        "KHR_materials_clearcoat": {
+          "clearcoatFactor": 1.0,
+          "extensions": {
+            "KHR_materials_clearcoat_ior": {
+              "clearcoatIor": 1.4
+            }
+          }
+        }
+      }
+    }
+  ]
+}
+```
+
+| |Type|Description|Required|
+|-|----|-----------|--------|
+| **clearcoatIor** | `number` | The index of refraction of the clearcoat layer. | No, default: `1.5` |
+
+Typical values for the index of refraction range from 1 to 2. In rare cases values greater than 2 are possible.
+
+### Layering
+
+The clearcoat effect is modeled as a microfacet BRDF layered on top of the base material using `fresnel_coat` as defined in `KHR_materials_clearcoat`. This extension updates the IOR parameter of that Fresnel term to use `clearcoatIor` instead of the default `1.5`.
+
+```
+# clearcoat
+clearcoat_brdf = specular_brdf(
+  normal = clearcoatNormal,
+  α = clearcoatRoughness^2)
+
+# layering
+coated_material =
+  fresnel_coat(
+    normal = clearcoatNormal,
+    ior = clearcoatIor,
+    weight = clearcoat,
+    base = material,
+    layer = clearcoat_brdf)
+```
+
+### Implementation
+
+This extension changes only the computation of the Fresnel term for the clearcoat layer:
+
+```
+dielectric_f0_clearcoat = ((clearcoatIor - 1)/(clearcoatIor + 1))^2
+```
+
+Note that for the default index of refraction `clearcoatIor = 1.5` this term evaluates to `0.04`.
+
+## Schema
+
+- [material.KHR_materials_clearcoat_ior.schema.json](schema/material.KHR_materials_clearcoat_ior.schema.json)

extensions/2.0/Khronos/KHR_materials_clearcoat_ior/schema/material.KHR_materials_clearcoat_ior.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_clearcoat_ior glTF Material Extension",
+    "type": "object",
+    "description": "glTF extension that defines the index of refraction of the clearcoat layer.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "clearcoatIor": {
+            "type": "number",
+            "description": "The index of refraction of the clearcoat layer.",
+            "default": 1.5,
+            "oneOf": [
+                {
+                    "minimum": 0.0,
+                    "maximum": 0.0
+                },
+                {
+                    "minimum": 1.0
+                }
+            ],
+            "gltf_detailedDescription": "The index of refraction (IOR) of the clearcoat layer is a measured physical number usually in the range between 1 and 2 that determines how much the path of light is bent, or refracted, when entering the clearcoat material. It also influences the ratio between reflected and transmitted light in the clearcoat layer, calculated from the Fresnel equations."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}