KHR_materials_clearcoat extension

extensions/2.0/Khronos/KHR_materials_clearcoat/README.md

@@ -0,0 +1,96 @@
+# KHR\_materials\_clearcoat
+
+## Khronos 3D Formats Working Group
+
+* Norbert Nopper, UX3D [@UX3DGpuSoftware](https://twitter.com/UX3DGpuSoftware)
+* Tobias Haeussler, Dassault Systemes [@proog128](https://github.com/proog128)
+* Alexey Knyazev [@lexaknyazev](https://github.com/lexaknyazev)
+* Don McCurdy, Google [@donrmccurdy](https://twitter.com/donrmccurdy)
+* Sebastien Vandenberghe, Microsoft [@sebavanjs](https://twitter.com/sebavanjs)
+* Romain Guy, Google [@romainguy](https://twitter.com/romainguy)
+* Ed Mackey, AGI [@emackey](https://twitter.com/emackey)
+* Alex Wood, AGI [@abwood](https://twitter.com/abwood)
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## 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 clear coating that can be layered on top of an existing glTF material definition.  A clear coat is a common technique used in Physically-Based Rendering to represent a protective layer applied to a base material.  See [Theory, Documentation and Implementations](#theory-documentation-and-implementations)
+
+## Extending Materials
+
+The PBR clearcoat materials are defined by adding the `KHR_materials_clearcoat` extension to any compatible glTF material (excluding those listed above).  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 B](/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 computed using the specular term from the glTF 2.0 Metallic-Roughness material, defined in [Appendix B](/specification/2.0/README.md#appendix-b-brdf-implementation).  Use F0 equal `0.04`, base color white `1.0, 1.0, 1.0`, metallic value `0.0`, and the clearcoat roughness value defined in this extension as follows:
+
+```
+clearcoatRoughness = clearcoatRoughnessFactor * clearcoatRoughnessTexture.g
+```
+
+The following abstract code describes how the base and clearcoat layers should be blended together:
+
+```
+clearcoatBlendFactor = clearcoatTexture.r * clearcoatFactor * fresnel(0.04, NdotV)
+
+color = mix(f_emissive + f_diffuse + f_specular, f_clearcoat, clearcoatBlendFactor)
+```
+
+If `clearcoatFactor` (in the extension) is zero, the whole clearcoat layer is disabled.
+
+The values for clearcoat layer intensity and clearcoat roughness can be defined using factors, textures, or both. If the `clearcoatTexture` or `clearcoatRoughnessTexture` is not given, respective texture components are assumed to have a value of 1.0. If both factors and textures are present, the factor value acts as a linear multiplier for the corresponding texture values.
+
+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.
+
+## 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-2020x.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": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The clearcoat layer intensity texture.",
+            "gltf_detailedDescription": "The clearcoat layer intensity texture. These values are sampled from the R channel.  The values are linear.  Use value 1.0 if no texture is supplied."
+        },
+        "clearcoatRoughnessFactor": {
+            "type": "number",
+            "description": "The clearcoat layer roughness.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "The clearcoat layer roughness of the material."
+        },
+        "clearcoatRoughnessTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The clearcoat layer roughness texture.",
+            "gltf_detailedDescription": "The clearcoat layer roughness texture. These values are sampled from the G channel.  The values are linear.  Use value 1.0 if no texture is supplied."
+        },
+        "clearcoatNormalTexture": {
+            "allOf": [ { "$ref": "material.normalTextureInfo.schema.json" } ],
+            "description": "The clearcoat normal map texture.",
+            "gltf_detailedDescription": "A tangent space normal map for the clearcoat layer.  If desired, this may be a reference to the same normal map used by the base material.  If not supplied, no normal mapping is applied to the clear coat layer."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/README.md

@@ -32,7 +32,7 @@ but may still change before they are complete._
 |-----------|--------|
 | [KHR_mesh_instancing](https://github.com/KhronosGroup/glTF/pull/1691) | Early discussions. |
 | [KHR_mesh_quantization](https://github.com/KhronosGroup/glTF/tree/master/extensions/2.0/Khronos/KHR_mesh_quantization/README.md) | Ready for implementation and feedback. |
-| [KHR_materials_clearcoat](https://github.com/KhronosGroup/glTF/pull/1756) | Ready for implementation and feedback. |
+| [KHR_materials_clearcoat](2.0/Khronos/KHR_materials_clearcoat/README.md) | Ready for implementation. Awaiting ratification. |
 | [KHR_materials_ior](https://github.com/KhronosGroup/glTF/pull/1718) | In development. |
 | [KHR_materials_thinfilm](https://github.com/KhronosGroup/glTF/pull/1742) | In development. |
 | [KHR_materials_transmission](https://github.com/KhronosGroup/glTF/pull/1698) | In development. |