KHR_materials_clearcoat and KHR_materials_specular

extensions/2.0/Khronos/KHR_materials_clearcoat/README.md

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+# KHR\_materials\_clearcoat
+
+## Khronos 3D Formats Working Group
+
+* Alexey Knyazev
+* Don McCurdy
+* Ed Mackey
+* Norbert Nopper, UX3D [@UX3DGpuSoftware](https://twitter.com/UX3DGpuSoftware)
+* Romain Guy
+* Tobias Haeussler
+
+## Acknowledgments
+
+* TODO
+
+## Status
+
+Experimental
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+TODO  
+
+## Extending Materials
+
+The PBR clearcoat materials are defined by adding the `KHR_materials_clearcoat` extension to any glTF material. 
+For example, the following defines a material like varnish using clearcoat parameters.
+
+```json
+{
+    "materials": [
+        {
+            "name": "varnish",
+            "extensions": {
+                "KHR_materials_clearcoat": {
+                    "clearcoatFactor": 1.0
+                }
+            }
+        }
+    ]
+}
+```
+
+### Clearcoat
+
+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](/specification/2.0/README.md#appendix-b-brdf-implementation) for more details on the BRDF calculations.
+
+|                                  | Type                                                                            | Description                            | Required             |
+|----------------------------------|---------------------------------------------------------------------------------|----------------------------------------|----------------------|
+|**clearcoatFactor**               | `number`                                                                        | The clearcoat layer intensity.         | No, default: `0.0`   |
+|**clearcoatTexture**              | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo)             | The clearcoat layer intensity texture. | No                   |
+|**clearcoatRoughnessFactor**      | `number`                                                                        | The clearcoat layer roughness.         | No, default: `0.0`   |
+|**clearcoatRoughnessTexture**     | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo)             | The clearcoat layer roughness texture. | No                   |
+|**clearcoatNormalTexture**        | [`normalTextureInfo`](/specification/2.0/README.md#reference-normaltextureinfo) | The clearcoat normal map texture.      | No                   |
+
+The clearcoat formula `f_clearcoat` is the same as the specular term from the Metallic-Roughness material, using F0 equal 0.04.
+The following abstract code describes how the base and clearcoat layers should be blended together:  
+
+```
+blendFactor = clearcoatFactor * fresnel(0.04, NdotV)
+f = (f_emissive + f_diffuse) * (1.0 - blendFactor) + mix(f_specular, f_clearcoat, blendFactor)
+```
+
+If `clearcoatFactor = 0`, the clearcoat layer is disabled and the material is behaving like the core Metallic-Roughness material:
+
+```
+f = f_emissive + f_diffuse + f_specular
+```
+
+If textures are not set, the default values of the clearcoat layer textures are used and the values are not inherited from the underlying Metallic-Roughness (or other) material. If one wants to have the same textures, one have to explicitly set the same texture sources.
+
+## Appendix
+
+TODO
+
+## Reference
+
+### Theory, Documentation and Implementations
+
+[Autodesk Standard Surface - Coating](https://autodesk.github.io/standard-surface/#closures/coating)  
+[AxF - Appearance exchange Format](https://www.xrite.com/-/media/xrite/files/whitepaper_pdfs/axf/axf_whitepaper_en.pdf)  
+[Blender Principled BSDF](https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html)  
+[Disney BRDF Explorer - disney.brdf](https://github.com/wdas/brdf/blob/master/src/brdfs/disney.brdf)  
+[Enterprise PBR Shading Model - Clearcoat](https://dassaultsystemes-technology.github.io/EnterprisePBRShadingModel/spec.md.html#components/clearcoat)  
+[Filament Material models - Clear coat](https://google.github.io/filament/Materials.md.html#materialmodels/litmodel/clearcoat)   
+[Physically-Based Shading at Disney](https://disney-animation.s3.amazonaws.com/library/s2012_pbs_disney_brdf_notes_v2.pdf)  
+[Substance Painter - Updated Clear Coat Shader](https://docs.substance3d.com/spdoc/version-2018-3-172823522.html#Version2018.3-UpdatedClearCoatShader)  
+[THE PBR GUIDE BY ALLEGORITHMIC - PART 1](https://academy.substance3d.com/courses/the-pbr-guide-part-1)  
+[THE PBR GUIDE BY ALLEGORITHMIC - PART 2](https://academy.substance3d.com/courses/the-pbr-guide-part-2)  
+[Unreal Engine 4 Material - Clear Coat](https://docs.unrealengine.com/en-US/Engine/Rendering/Materials/MaterialInputs/index.html#clearcoat)  

extensions/2.0/Khronos/KHR_materials_clearcoat/schema/glTF.KHR_materials_clearcoat.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_clearcoat glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the clearcoat material layer.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "clearcoatFactor": {
+            "type": "number",
+            "description": "The clearcoat layer intensity.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "The clearcoat layer intensity (aka opacity) of the material. A value of 0.0 means the material has no clearcoat layer enabled."
+        },        
+        "clearcoatTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The clearcoat layer intensity texture.",
+            "gltf_detailedDescription": "The clearcoat layer intensity texture. Stored in channel R with default linear value 1.0."
+        },
+        "clearcoatRoughnessFactor": {
+            "type": "number",
+            "description": "The clearcoat layer roughness.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "The clearcoat layer roughness of the material."
+        },        
+        "clearcoatRoughnessTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The clearcoat layer roughness texture.",
+            "gltf_detailedDescription": "The clearcoat layer roughness texture. Stored in channel G with default linear value 1.0."
+        },
+        "clearcoatNormalTexture": {
+            "allOf": [ { "$ref": "material.normalTextureInfo.schema.json" } ],
+            "description": "The clearcoat normal map texture.",
+            "gltf_detailedDescription": "A tangent space normal map for the clearcoat layer."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/2.0/Khronos/KHR_materials_specular/README.md

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+# KHR\_materials\_specular
+
+## Khronos 3D Formats Working Group
+
+* Norbert Nopper, UX3D [@UX3DGpuSoftware](https://twitter.com/UX3DGpuSoftware)
+
+## Acknowledgments
+
+* TODO
+
+## Status
+
+Experimental
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+TODO  
+
+## Extending Materials
+
+The specular materials are defined by adding the `KHR_materials_specular` extension to any glTF material. 
+
+```json
+{
+    "materials": [
+        {
+            "extensions": {
+                "KHR_materials_specular": {
+                    "specularFactor": 0.5
+                }
+            }
+        }
+    ]
+}
+```
+
+### Specular
+
+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](/specification/2.0/README.md#appendix-b-brdf-implementation) for more details on the BRDF calculations.
+
+|                     | Type                                                                | Description            | Required             |
+|---------------------|---------------------------------------------------------------------|------------------------|----------------------|
+|**specularFactor**   | `number`                                                            | The specular.          | No, default: `0.5`   |
+|**specularTexture**  | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | The specular  texture. | No                   |
+
+This extension is overwriting the default `F0 = 0.04` for non-metallic materials in the Metallic-Roughness workflow. Instead, `F0 = 0.08 * specularFactor * specularTexture` is used.
+
+## Appendix
+
+TODO
+
+## Reference
+
+### Theory, Documentation and Implementations
+
+[Blender Principled BSDF](https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html)  
+[Disney BRDF Explorer - disney.brdf](https://github.com/wdas/brdf/blob/master/src/brdfs/disney.brdf)  
+[Physically-Based Shading at Disney](https://disney-animation.s3.amazonaws.com/library/s2012_pbs_disney_brdf_notes_v2.pdf)  
+[THE PBR GUIDE BY ALLEGORITHMIC - PART 1](https://academy.substance3d.com/courses/the-pbr-guide-part-1)  
+[THE PBR GUIDE BY ALLEGORITHMIC - PART 2](https://academy.substance3d.com/courses/the-pbr-guide-part-2)  

extensions/2.0/Khronos/KHR_materials_specular/schema/glTF.KHR_materials_specular.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_specular glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the specular channel for the Metallic-Roughness layer.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "specularFactor": {
+            "type": "number",
+            "description": "The specular channel.",
+            "default": 0.5,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": ""
+        },        
+        "specularTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The specular channel texture.",
+            "gltf_detailedDescription": "The specular channel texture. Stored in channel A with default linear value 1.0 of the AO-Roughness-Metallic texture."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/2.0/Khronos/KHR_materials_thinfilm/README.md

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+# KHR\_materials\_thinfilm
+
+## Khronos 3D Formats Working Group
+
+* Norbert Nopper, UX3D [@UX3DGpuSoftware](https://twitter.com/UX3DGpuSoftware)
+
+## Acknowledgments
+
+* TODO
+
+## Status
+
+Experimental
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+TODO  
+
+## Extending Materials
+
+The thin film materials are defined by adding the `KHR_materials_thinfilm` extension to any glTF material. 
+
+```json
+{
+    "materials": [
+        {
+            "extensions": {
+                "KHR_materials_thinfilm": {
+                    "thinfilmFactor": 1.0
+                }
+            }
+        }
+    ]
+}
+```
+
+### Thinfilm
+
+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](/specification/2.0/README.md#appendix-b-brdf-implementation) for more details on the BRDF calculations.
+
+|                             | Type                                                                | Description                      | Required              |
+|-----------------------------|---------------------------------------------------------------------|----------------------------------|-----------------------|
+|**thinfilmFactor**           | `number`                                                            | The thin film intensity.         | No, default: `0.0`    |
+|**thinfilmTexture**          | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | The thin film intensity texture. | No                    |
+|**thinfilmThicknessMinimum** | `number`                                                            | The thin film minimum thickness. | No, default: `400.0`  |
+|**thinfilmThicknessMaximum** | `number`                                                            | The thin film maximum thickness. | No, default: `1200.0` |
+|**thinfilmThicknessTexture** | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | The thin film thickness texture. | No                    |
+
+```
+float thinfilmThicknessNormalized = (thinfilmTexture.g * (thinfilmThicknessMaximum - thinfilmThicknessMinimum) + thinfilmThicknessMinimum) / 1200.0;  
+float thinfilmIntensity = thinfilmTexture.r * thinfilmFactor;
+```
+
+The thin film effect is overwriting the F (Surface Reflection Ratio) from the specular glossiness effect.
+```
+vec3 thinfilmSpecularReflection(vec3 F0, float VdotH, float thinfilmIntensity, float thinfilmThicknessNormalized)
+{
+    vec3 F = fresnelReflection(F0, VdotH);
+
+    if (thinfilmIntensity == 0.0)
+    {
+        return F;
+    }
+
+    vec3 lutSample = texture(lutTexture, thinfilmThicknessNormalized, VdotH)).rgb - 0.5;
+
+	vec3 intensity = thinfilmIntensity * 4.0 * F0 * (1.0 - F0);
+
+	return clamp(lutSample * intensity + F, 0.0, 1.0);    
+}
+```
+
+
+## Appendix
+
+TODO
+
+## Reference
+
+### Theory, Documentation and Implementations
+
+[A Practical Extension to Microfacet Theory for the Modeling of Varying Iridescence](https://belcour.github.io/blog/research/2017/05/01/brdf-thin-film.html)  
+[Practical Multilayered Materials - Call of Duty - page 62](https://blog.selfshadow.com/publications/s2017-shading-course/drobot/s2017_pbs_multilayered.pdf)  

extensions/2.0/Khronos/KHR_materials_thinfilm/schema/glTF.KHR_materials_thinfilm.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_thinfilm glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the thin film effect.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "thinfilmFactor": {
+            "type": "number",
+            "description": "The thin film intensity.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": ""
+        },        
+        "thinfilmTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The thin film intensity texture.",
+            "gltf_detailedDescription": "The thin film intensity texture. Stored in channel R with default linear value 1.0 of the thin film texture."
+        },
+        "thinfilmThicknessMinimum": {
+            "type": "number",
+            "description": "The thin film minimum thickness.",
+            "default": 400.0,
+            "minimum": 0.0,
+            "gltf_detailedDescription": ""
+        },        
+        "thinfilmThicknessMaximum": {
+            "type": "number",
+            "description": "The thin film maximum thickness.",
+            "default": 1200.0,
+            "maximum": 1200.0,
+            "gltf_detailedDescription": ""
+        },        
+        "thinfilmThicknessTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The thin film thickness texture.",
+            "gltf_detailedDescription": "The thin film thickness texture. Stored in channel G with default linear value 1.0 of the thin film texture."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}