KHR_materials_specular

extensions/2.0/Khronos/KHR_materials_specular/README.md

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+# KHR\_materials\_specular
+
+## Khronos 3D Formats Working Group
+
+* TODO
+
+## Acknowledgments
+
+* TODO
+
+## Status
+
+Experimental
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+This extension adds two parameters to the metallic-roughness material: `specular` and `specularColor`.
+
+`specular` allows users to configure the strength of the specular reflection in the dielectric BRDF. A value of zero disables the specular reflection, resulting in a pure diffuse material. The metal BRDF is not affected by the parameter.
+
+`specularColor` changes the F0 color of the specular reflection in the dielectric BRDF, allowing artists to use effects known from the specular-glossiness material (`KHR_materials_pbrSpecularGlossiness`) in the metallic-roughness material.
+
+
+## Extending Materials
+
+The strength of the specular reflection is defined by adding the `KHR_materials_specular` extension to any glTF material.
+
+```json
+{
+    "materials": [
+        {
+            "extensions": {
+                "KHR_materials_specular": {
+                    "specularFactor": 1.0,
+                    "specularTexture": 0,
+                    "specularColorFactor": [1.0, 1.0, 1.0],
+                    "specularColorTexture": 0
+                }
+            }
+        }
+    ]
+}
+```
+
+Factor and texture are combined by multiplication to describe a single value.
+
+| |Type|Description|Required|
+|-|----|-----------|--------|
+| **specularFactor** | `number` | The strength of the specular reflection. | No, default: `1.0`|
+| **specularTexture** | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | A grayscale texture that defines the specular factor. Will be multiplied by specularFactor. | No |
+| **specularColorFactor** | `number[3]` | The F0 color of the specular reflection (RGB). | No, default: `[1.0, 1.0, 1.0]`|
+| **specularColorTexture** | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | A 1-channel luminance or 3-channel RGB texture that defines the F0 color of the specular reflection. Will be multiplied by specularColorFactor. | No |
+
+### Conversions
+
+Material models that define f0 in terms of reflectance at normal incidence (with 0 meaning 0%, 0.5 meaning 4%, and 1 meaning 8%) can be converted to `specularColorFactor` and `specularColorTexture` as follows:
+
+```
+specularColorFactor = 2 * reflectanceFactor
+specularColorTexture = reflectanceTexture
+```
+
+Alternatively, the index of refraction defined in `KHR_materials_ior` can be set to the value 1.788789, resulting in a f0 scale factor of 0.08.
+
+In addition, it is possible to convert specular-glossiness materials (`KHR_materials_pbrSpecularGlossiness`) to metallic-roughness by combining `KHR_materials_specular` and `KHR_materials_ior`. The conversion is lossless and makes effects like sheen (`KHR_materials_sheen`) and clearcoat (`KHR_materials_clearcoat`) available to specular-glossiness materials.
+
+```
+ior = inf
+specularColorFactor = specularFactor_specgloss
+specularColorTexture = specularGlossinessTexture_specgloss[0:2]
+```
+
+The conversion is not compatible with transmissive materials defined via `KHR_materials_transmission` and `KHR_materials_volume`.
+
+## Implementation
+
+The specular factor scales the microfacet BRDF in the dielectric BRDF. It also affects the diffuse BRDF; the less energy is reflected by the microfacet BRDF, the more can be shifted to the diffuse BRDF.
+
+The specular color changes the F0 color of the Fresnel that is multiplied to the microfacet BRDF. The color at grazing angles (F90) is not changed.
+
+```
+dielectric_brdf =
+  fresnel_mix(
+    diffuse_brdf(baseColor),
+    microfacet_brdf(roughness^2),
+    f0 = 0.04 * specular * specularColor,
+    f90 = specular)
+```
+
+Based on [Appendix B](/specification/2.0/README.md#appendix-b-brdf-implementation), the modified `fresnel_mix` operation is defined as follows:
+
+```
+fresnel_mix(base, layer, ior) = base * (1 - fr(ior)) + layer * fr(ior)
+fr(ior) = f0 + (f90 - f0) * (1 - cos)^5
+f0 = 0.04 * specular * specularColor
+f90 = specular
+```
+
+If `KHR_materials_ior` is used in combination with `KHR_materials_specular`, the constant `0.04` is replaced by the value computed from the IOR:
+
+```
+f0 = ((ior - outside_ior) / (ior + outside_ior))^2 * specular * specularColor
+f90 = specular
+```
+
+The specular factor and specular color do not affect the index of refraction of the volume below the surface, defined in `KHR_materials_volume`. As a result, `specular` and `specularColor` do not take part in computing the outgoing direction for refraction.

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

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_specular glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the strength of the specular reflection.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "specularFactor": {
+            "type": "number",
+            "description": "The strength of the specular reflection.",
+            "default": 1.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "This parameter scales the amount of specular reflection on non-metallic surfaces. It has no effect on metals."
+        },
+        "specularTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "A 1-channel luminance texture that defines the strength of the specular reflection.",
+            "gltf_detailedDescription": "A 1-channel luminance texture that defines the strength of the specular reflection. Will be multiplied by specularFactor."
+        },
+        "specularColorFactor": {
+            "type": "array",
+            "items": {
+                "type": "number",
+                "minimum": 0.0,
+                "maximum": 1.0
+            },
+            "description": "The F0 RGB color of the specular reflection.",
+            "default": [ 1.0, 1.0, 1.0 ],
+            "minItems": 3,
+            "maxItems": 3,
+            "gltf_detailedDescription": "This is an additional RGB color parameter that tints the specular reflection of non-metallic surfaces. At grazing angles, the reflection still blends to white, and the parameter has not effect on metals. The value is linear."
+        },
+        "specularColorTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "An 1-channel luminance or 3-channel RGB texture that defines the F0 color of the specular reflection.",
+            "gltf_detailedDescription": "A luminance or RGB texture that defines the F0 color of the specular reflection. Will be multiplied by specularColorFactor."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}