KHR_materials_sheen

extensions/2.0/Khronos/KHR_materials_sheen/README.md

@@ -0,0 +1,109 @@
+# KHR\_materials\_sheen
+
+## Khronos 3D Formats Working Group
+
+* TODO
+
+## Acknowledgments
+
+* TODO
+
+## Status
+
+Experimental
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+TODO
+
+## Extending Materials
+
+The PBR sheen materials are defined by adding the `KHR_materials_sheen` extension to any glTF material. 
+For example, the following defines a material like velvet.
+
+```json
+{
+    "materials": [
+        {
+            "name": "velvet",
+            "extensions": {
+                "KHR_materials_sheen": {
+                    "sheenFactor": 1.0
+                }
+            }
+        }
+    ]
+}
+```
+
+### Sheen
+
+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                       |
+|----------------------------------|---------------------------------------------------------------------------------|----------------------------------------|--------------------------------|
+|**sheenFactor**                   | `number`                                                                        | The sheen intensity.                   | No, default: `1.0`             |
+|**sheenColor**                    | `array`                                                                         | The sheen color.                       | No, default: `[1.0, 1.0, 1.0]` |
+|**sheenTexture**                  | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo)             | The sheen color and intensity texture. | No                             |
+|
+
+The sheen roughness is identical to the material roughness to simplify the configuration. `sheenRoughness = materialRoughness`
+
+The sheen formula `f_sheen` is different from the specular and clear coat BRDF.
+```
+sheenTerm = sheenColor * sheenIntensity * sheenDistribution * sheenVisibility;
+```
+
+As you notice there is no fresnel for the sheen layer.
+
+The sheen distribution follows the "Charlie" sheen definition from ImageWorks (Estevez and Kulla):
+```
+alphaG = sheenRoughness * sheenRoughness
+invR = 1 / alphaG
+cos2h = NdotH * NdotH
+sin2h = 1 - cos2h
+sheenDistribution = (2 + invR) * pow(sin2h, invR * 0.5) / (2 * PI);
+```
+
+The sheen visibility is following the one defined by Ashkimin:
+```
+sheenVisibility = 1 / (4 * (ndotl + ndotv - ndotl * ndotv))
+```
+
+The full layer would then be:
+```
+f_sheen = lightColor * sheenTerm * PI * ndotl
+```
+
+The following abstract code describes how the base and sheen layers should be blended together:
+```
+specularity = mix(metallicF0, baseColor, metallic)
+reflectance = max(max(specularity.r, specularity.g), specularity.b)
+
+final = f_emissive + f_diffuse + f_specular + (1 - reflectance) * f_sheen
+```
+
+If `sheenFactor = 0`, the sheen 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
+
+[NP13 David Neubelt, Matt Pettineo – “Crafting a Next-Gen Material Pipeline for The Order: 1886”, SIGGRAPH 2013](https://blog.selfshadow.com/publications/s2013-shading-course/rad/s2013_pbs_rad_notes.pdf)
+[EK17 Alejandro Conty Estevez, Christopher Kulla – “Production Friendly Microfacet Sheen BRDF”, SIGGRAPH 2017](https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_sheen.pdf)
+[AS07 Michael Ashikhmin, Simon Premoze – “Distribution-based BRDFs”, 2007](http://www.cs.utah.edu/~premoze/dbrdf/dBRDF.pdf)
+[cloth-shading](https://knarkowicz.wordpress.com/2018/01/04/cloth-shading/)

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

@@ -0,0 +1,36 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_sheen glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the sheen material model.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "sheenFactor": {
+            "type": "number",
+            "description": "The sheen layer intensity.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "The sheen layer intensity of the material. A value of 0.0 means the material has no sheen layer enabled."
+        },
+        "sheenColor": {
+            "type": "array",
+            "items": {
+                "type": "number",
+                "minimum": 0.0,
+                "maximum": 1.0
+            },
+            "minItems": 3,
+            "maxItems": 3,
+            "description": "Color of the sheen layer.",
+            "default": [1, 1, 1]
+        },
+        "sheenTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "The sheen layer texture.",
+            "gltf_detailedDescription": "The sheen layer texture. Stored in channel RGB the sheen color in Gamma and in A the linear factor."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}