Merge update. (#2440)

* KHR_materials_ior: Minor typo: plural instead of singular (#2413)

* Update anisotropy extension spec (#2409)

* Clarify and simplify formula for attenuation. (#2414)

* Add TRACE prefix (#2417)

* Update clearcoat extension spec (#2415)

* Update README. (#2427)

* Add KHR_node_hoverability (#2428)

* Add KHR_node_hoverability
* Resort extension list

* EXT_mesh_gpu_instancing: Suggest extensionRequired usage (#2405)

* Add button/link to glTF Discord (#2429)

As there is a lot more traffic to the Khronos glTF Discord than there is to Slack, a button pointing users to Discord will be helpful.

---------

Co-authored-by: Andreas Atteneder <[email protected]>
Co-authored-by: Alexey Knyazev <[email protected]>
Co-authored-by: Ed Mackey <[email protected]>
Co-authored-by: Marco Hutter <[email protected]>
Co-authored-by: Khronos Group Web Services <[email protected]>
Co-authored-by: Arseny Kapoulkine <[email protected]>
Co-authored-by: James Riordon <[email protected]>

README.md

@@ -7,8 +7,9 @@ SPDX-License-Identifier: CC-BY-4.0
 <img src="specification/figures/glTF_RGB_June16.svg" width="340" height="170" />
 </p>
 
-[![Join the Slack group](https://img.shields.io/badge/chat-on%20slack-blue.svg)](https://www.khr.io/slack)
+[![Join glTF Discord](https://img.shields.io/badge/discuss-on%20discord-blue.svg)](https://khr.io/khrdiscord)
 [![Join the forums](https://img.shields.io/badge/discuss-in%20forums-blue.svg)](https://community.khronos.org/c/gltf-general)
+[![Join the Slack group](https://img.shields.io/badge/chat-on%20slack-blue.svg)](https://www.khr.io/slack)
 
 glTF™ (GL Transmission Format) is a royalty-free specification for the efficient transmission and loading of 3D scenes and models by applications. glTF minimizes both the size of 3D assets, and the runtime processing needed to unpack and use those assets. glTF defines an extensible, common publishing format for 3D content tools and services that streamlines authoring workflows and enables interoperable use of content across the industry.
 

extensions/2.0/Khronos/KHR_materials_anisotropy/README.md

@@ -64,18 +64,24 @@ Sample values:
 
 |                         | Type     | Description               | Required           |
 | ----------------------- | -------- | ------------------------- | ------------------ |
-| **anisotropyStrength**  | `number` | The anisotropy strength. When anisotropyTexture is present, this value is multiplied by the blue channel. | No, default: `0.0` |
-| **anisotropyRotation**  | `number` | The rotation of the anisotropy in tangent, bitangent space, measured in radians counter-clockwise from the tangent. When anisotropyTexture is present, anisotropyRotation provides additional rotation to the vectors in the texture. | No, default: `0.0` |
-| **anisotropyTexture**   | [`textureInfo`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-textureinfo) | The anisotropy texture. Red and green channels represent the anisotropy direction in [-1, 1] tangent, bitangent space, to be rotated by anisotropyRotation. The blue channel contains strength as [0, 1] to be multiplied by anisotropyStrength. | No |
+| **anisotropyStrength**  | `number` | The anisotropy strength. When the anisotropy texture is present, this value is multiplied by the texture's blue channel. | No, default: `0.0` |
+| **anisotropyRotation**  | `number` | The rotation of the anisotropy in tangent, bitangent space, measured in radians counter-clockwise from the tangent. When the anisotropy texture is present, this value provides additional rotation to the vectors in the texture. | No, default: `0.0` |
+| **anisotropyTexture**   | [`textureInfo`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-textureinfo) | The anisotropy texture. Red and green channels represent the anisotropy direction in $[-1, 1]$ tangent, bitangent space to be rotated by the anisotropy rotation. The blue channel contains strength as $[0, 1]$ to be multiplied by the anisotropy strength. | No |
 
 ## Anisotropy
 
 Two new material properties are introduced: a strength parameter and the direction in which the specular reflection elongates relative to the surface tangents.
-The strength parameter is a dimensionless number in the range `[0, 1]` and increases the roughness along a chosen direction. The default direction aligns with the tangent to the mesh as described in the glTF 2.0 specification, [Meshes section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes).
+The strength parameter is a dimensionless number in the range $[0, 1]$ and increases the roughness along a chosen direction. The default direction aligns with the tangent to the mesh as described in the glTF 2.0 specification, [Meshes section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes).
 
-All meshes with materials that use anisotropy **SHOULD** supply `TANGENT` vectors as a mesh attribute.  If `TANGENT` vectors are not supplied for such a mesh, the mesh **MUST** supply a `normalTexture`, and tangents are computed according to rules in the [Meshes section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#meshes) of the glTF specification. Likewise when `TANGENT` vectors are not supplied for a mesh, the mesh **MUST NOT** supply different texture coordinates on the `normalTexture` and `anisotropyTexture`.
+A mesh primitive using an anisotropy material **MUST** have a defined tangent space, i.e., it **MUST** have `NORMAL` and `TANGENT` attributes or its base material **MUST** have a normal texture. When the mesh primitive does not have `NORMAL` or `TANGENT` vectors, they are computed as defined in the glTF 2.0 specification.
 
-The `anisotropyTexture`, when supplied, encodes XY components of the direction vector in tangent space as red and green values, and strength as blue values, all stored with linear transfer function. After dequantization, red and green texel values **MUST** be mapped as follows: red [0.0 .. 1.0] to X [-1 .. 1], green [0.0 .. 1.0] to Y [-1 .. 1].  Blue does not require remapping.  When `anisotropyTexture` is not supplied, the default value is red 1.0 (X 1.0), green 0.5 (Y 0.0), and blue 1.0 (strength 1.0).  The direction of this XY vector specifies the per-texel direction of increased anisotropy roughness in tangent, bitangent space, prior to being rotated by `anisotropyRotation`.  The blue channel contains strength as [0.0 .. 1.0] to be multiplied by `anisotropyStrength` to determine the per-texel anisotropy strength.
+Since the glTF 2.0 specification does not mandate any particular tangent space derivation algorithm, mesh primitives using anisotropy materials **SHOULD** always provide their `NORMAL` and `TANGENT` vectors.
+
+When the material has both `normalTexture` and `anisotropyTexture` properties defined, these textures **SHOULD** use the same texture coordinates because they operate in the same tangent space and their texel values are usually correlated to each other.
+
+The anisotropy texture, when supplied, encodes XY components of the anisotropy direction vector in tangent space as red and green values, and the anisotropy strength as blue values, all stored with linear transfer function. After dequantization, red and green texel values **MUST** be mapped as follows: red $[0, 1]$ to X $[-1, 1]$, green $[0, 1]$ to Y $[-1, 1]$.  Blue does not require remapping.  When the anisotropy texture is not supplied, the default dequantized texel value is $(1.0; 0.5; 1.0)$, which corresponds to the $(1; 0)$ direction vector (+X axis) and full strength.
+
+The direction of this XY vector specifies the per-texel direction of increased anisotropy roughness in tangent, bitangent space, prior to being rotated by the `anisotropyRotation` property value.  After dequantization, the blue channel contains strength as $[0, 1]$ to be multiplied by the `anisotropyStrength` property value to determine the per-texel anisotropy strength.
 
 > **Note:** The direction vector of the anisotropy is the direction in which highlights will be stretched. The direction of the micro-grooves in the material causing the anisotropy will run perpendicular.
 

extensions/2.0/Khronos/KHR_materials_anisotropy/schema/material.KHR_materials_anisotropy.schema.json

@@ -11,18 +11,18 @@
             "default": 0.0,
             "minimum": 0.0,
             "maximum": 1.0,
-            "gltf_detailedDescription": "The anisotropy strength. When anisotropyTexture is present, this value is multiplied by the blue channel."
+            "gltf_detailedDescription": "The anisotropy strength. When the anisotropy texture is present, this value is multiplied by the texture's blue channel."
         },
         "anisotropyRotation": {
             "type": "number",
             "description": "The rotation of the anisotropy.",
             "default": 0.0,
-            "gltf_detailedDescription": "The rotation of the anisotropy in tangent, bitangent space, measured in radians counter-clockwise from the tangent. When anisotropyTexture is present, anisotropyRotation provides additional rotation to the vectors in the texture."
+            "gltf_detailedDescription": "The rotation of the anisotropy in tangent, bitangent space, measured in radians counter-clockwise from the tangent. When the anisotropy texture is present, this value provides additional rotation to the vectors in the texture."
         },
         "anisotropyTexture": {
             "allOf": [ { "$ref": "textureInfo.schema.json" } ],
             "description": "The anisotropy texture.",
-            "gltf_detailedDescription": "The anisotropy texture. Red and green channels represent the anisotropy direction in [-1, 1] tangent, bitangent space, to be rotated by anisotropyRotation. The blue channel contains strength as [0, 1] to be multiplied by anisotropyStrength."
+            "gltf_detailedDescription": "The anisotropy texture. Red and green channels represent the anisotropy direction in $[-1, 1]$ tangent, bitangent space, to be rotated by the anisotropy rotation. The blue channel contains strength as $[0, 1]$ to be multiplied by the anisotropy strength."
         },
         "extensions": { },
         "extras": { }

extensions/2.0/Khronos/KHR_materials_clearcoat/README.md

@@ -77,6 +77,12 @@ clearcoatRoughness = clearcoatRoughnessFactor * clearcoatRoughnessTexture.g
 
 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.
 
+A mesh primitive using a clearcoat material with a clearcoat normal texture **MUST** have a defined tangent space, i.e., it **MUST** have `NORMAL` and `TANGENT` attributes or its base material **MUST** have a normal texture. When the mesh primitive does not have `NORMAL` or `TANGENT` vectors, they are computed as defined in the glTF 2.0 specification.
+
+Since the glTF 2.0 specification does not mandate any particular tangent space derivation algorithm, mesh primitives using clearcoat materials with clearcoat normal textures **SHOULD** always provide their `NORMAL` and `TANGENT` vectors.
+
+When the material has both `normalTexture` and `clearcoatNormalTexture` properties defined, these textures **SHOULD** use the same texture coordinates because they operate in the same tangent space and their texel values are usually correlated to each other.
+
 The clearcoat effect is modeled via a microfacet BRDF. The BRDF is layered on top of the glTF 2.0 Metallic-Roughness material, including emission and all extensions, using a new `fresnel_coat` function:
 
 ```

extensions/2.0/Khronos/KHR_materials_ior/README.md

@@ -47,7 +47,7 @@ Written against the glTF 2.0 spec.
 
 ## Overview
 
-The dielectric BRDF of the metallic-roughness material in glTF uses a fixed value of 1.5 for the index of refraction. This is a good fit for many plastics and glass, but not for other materials like water or asphalt, sapphire or diamond. This extensions allows users to set the index of refraction to a certain value.
+The dielectric BRDF of the metallic-roughness material in glTF uses a fixed value of 1.5 for the index of refraction. This is a good fit for many plastics and glass, but not for other materials like water or asphalt, sapphire or diamond. This extension allows users to set the index of refraction to a certain value.
 
 ## Extending Materials
 

extensions/2.0/Khronos/KHR_materials_volume/README.md

@@ -157,11 +157,15 @@ T(*x*) = e<sup>-σ<sub>t</sub>*x*</sup>
 
 where T is commonly referred to as *transmittance*.
 
-Substituting σ<sub>t</sub> in the previous equation by its definition via *attenuation color* and *attenuation distance*, as defined above, and setting *x* = *d* we get
+Substituting σ<sub>t</sub> in the previous equation by its definition via *attenuation color* and *attenuation distance*, as defined above, we get
 
-T(d<sub>a</sub>) = e<sup>(log(*c*) / *d*) * *d*</sup> = *c*
+T(x) = e<sup>(log(*c*) / *d*) * *x*</sup>
 
-So, after traveling distance *d* through the medium we get attenuation color *c*.
+The above formula can be simplified as:
+
+T(x) = *c*<sup> *x* / *d*</sup>
+
+So, after traveling distance *x* through a medium with attenuation color *c* and attenuation distance *d*, we get we get transmittance T(x).
 
 ## Base Color and Absorption
 

extensions/2.0/Vendor/EXT_mesh_gpu_instancing/README.md

@@ -57,6 +57,8 @@ All attribute accessors in a given node **must** have the same `count`.
 
 > **Implementation Note:** When instancing is used on the node, the non-instanced version of the mesh should not be rendered.
 
+Because the extension does not provide a way to specify a non-instanced fallback, files that use the extension **should** specify it in `extensionsRequired` array.
+
 ## Transformation Order
 
 When using instancing, the instance transform matrix is constructed by multiplying translation (if present), rotation (if present), and scale in the same order as they are for nodes:

extensions/Prefixes.md

@@ -82,6 +82,7 @@ To request a prefix, submit a [GitHub issue](https://github.com/KhronosGroup/glT
 | `SNAP` | Snap, Inc. | https://snap.com/ | [#2125](https://github.com/KhronosGroup/glTF/issues/2125) |
 | `SPECTRUM` | Spectrum | https://spectrumcustomizer.com/ | [#1804](https://github.com/KhronosGroup/glTF/issues/1804) |
 | `TENCENT` | Tencent, Inc. | https://www.tencent.com/ | [#2118](https://github.com/KhronosGroup/glTF/issues/2118) |
+| `TRACE` | Trace | https://www.trace3d.app/<br>`martin at trace3d.app` | [#2416](https://github.com/KhronosGroup/glTF/issues/2416) |
 | `TRYON` | TRYON Technology Ltd. | https://tryon.technology/ | [#1785](https://github.com/KhronosGroup/glTF/issues/1785) |
 | `UNITY` | Unity Software Inc. | https://unity.com/ | [#2185](https://github.com/KhronosGroup/glTF/issues/2185) |
 | `USSF` | US Space Force | https://www.spaceforce.mil/ | [#2177](https://github.com/KhronosGroup/glTF/issues/2177) |