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`|
+| **specularColorFactor** | `number[3]` | The F0 color of the specular reflection (RGB). | No, default: `[1.0, 1.0, 1.0]`|
+| **specularTexture** | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | Either a 4-channel texture that defines the specular color in the RGB channels and the specular factor in the alpha channel, a 3-channel texture that defines only the specular color, or a grayscale texture that defines only the specular factor. Will be multiplied by specularFactor and/or specularColorFactor. | No |
+
+## 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, specular, specularColor),
+    microfacet_brdf(roughness^2) * specular,
+    ior = 1.5,
+    f0 = ((1-ior)/(1+ior))^2 * specularColor)
+```
+
+This makes `f0` wavelength-dependent, i.e., it is now a 3-channel RGB value. For the directional albedo `E` and average albedo `Eavg`, this is reduced to 1 channel by taking the maximum of the RGB channels, This avoids unexpected color shifts caused by the subtraction.
+
+```
+                1           (1 - E(VdotN, max(f0))) * (1 - E(LdotN, max(f0)))
+diffuse_brdf = -- * mix(1, ---------------------------------------------------, specular)
+               pi                                1 - Eavg
+```
+
+The specular color does not affect the index of refraction that may also be used in `KHR_materials_transmission` to refract light rays going through the surface.

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."
+        },
+        "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."
+        },
+        "specularTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "An RGBA texture that defines the F0 color of the specular reflection and the specular factor.",
+            "gltf_detailedDescription": "Either a 4-channel texture that defines the specular color in the RGB channels and the specular factor in the alpha channel, a 3-channel texture that defines only the specular color, or a grayscale texture that defines only the specular factor. Will be multiplied by specularFactor and/or specularColorFactor."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}