KHR_materials_specular extension branch.

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,
+            "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": { }
+    }
+}