KHR_materials_dispersion

extensions/2.0/Khronos/KHR_materials_dispersion/README.md

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+# KHR\_materials\_disperson
+
+## Contributors
+
+- Ben Houston, Threekit [@bhouston](https://github.com/bhouston)
+
+Copyright 2023 The Khronos Group Inc. All Rights Reserved. glTF is a trademark of The Khronos Group Inc.
+See [Appendix](#appendix-full-khronos-copyright-statement) for full Khronos Copyright Statement.
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Exclusions
+
+- This extension must not be used on a material that also uses `KHR_materials_pbrSpecularGlossiness`.
+- This extension must not be used on a material that also uses `KHR_materials_unlit`.
+
+## Overview
+
+This extension adds one parameters to the metallic-roughness material: `dispersion`.
+
+`dispersion` allows users to configure the strength of the angular separation of colors (chromatic aberration) transmitting through a relatively clear volume.  Dispersion is represented in terms of the Abbe number parameterization.  The dispersion effect is a result of the wavelength-dependent index of refraction of a material.  Dispersion is a widely adopted parameter in modern PBR models.  It is present in both OpenPBR and the Dassault Enterprise PBR Shading Model.
+
+The Abbe number \( V \) is computed from the index of refraction at three wavelengths of visible light: 486.1 nm (short wavelength blue, Ns), 587.6 nm (central yellow, Nc), and 656.3 nm (long wavelength red, Nl).  The Abbe number makes the simplifying assumption that the index of refraction variance is linear:
+
+\[
+V = \frac{N_c - 1}{N_l - N_s}
+\]
+
+To calculate the index of refraction at a specific wavelength \( \lambda \), given an Abbe number \( V \) and the central index of refraction (assumed to be at the color blue, \( N_d \), as specified by the KHR_materials_ior extension):
+
+\[
+B = \frac{N_c - 1}{V \times \left( \frac{1}{\lambda_s} - \frac{1}{\lambda_l} \right)}
+\]
+\[
+A = N_c - \frac{B}{\lambda_c}
+\]
+\[
+N(\lambda) = A + \frac{B}{\lambda^2}
+\]
+
+![Dispersion on a Gem](./figures/Dispersion.jpg)
+
+## Extending Materials
+
+The dispersion, defined in terms of Abbe number, is defined by adding the `KHR_materials_dispersion` extension to any glTF material.
+
+```json
+{
+    "materials": [
+        {
+            "extensions": {
+                "KHR_materials_dispersion": {
+                    "dispersion": 55
+                }
+            }
+        }
+    ]
+}
+```
+
+Factor and texture are combined by multiplication to describe a single value.
+
+| |Type|Description|Required|
+|-|----|-----------|--------|
+| **dispersion** | `number` | The strength of the dispersion effect, specified as an Abbe number. | No, default: `0`|
+
+The default value of 0 has a special meaning in that no dispersion should be used.  This is the default value for backwards compatibility.  Any value other than zero is considered to be a valid dispersion value.
+
+Here is a table of common material dispersion Abbe numbers:
+
+| Material | Abbe Number |
+| -------- | ----------- |
+| Polycarbonate | 32 |
+| Diamond | 55 |
+| Water | 55 |
+| Crown Glass | 59 |
+
+## Implementation
+
+*This section is non-normative.*
+
+One real-time method for rendering dispersion effects is to trace volume transmission separately for each of color channel accounting for the per channel ior as determined by the Abbe number.  The resulting composite image will show color separation between the channels as a result.
+
+## Interaction with other extensions
+
+
+
+## Schema
+
+- [material.KHR_materials_dispersion.schema.json](schema/material.KHR_materials_dispersion.schema.json)
+
+## Reference
+
+### Theory, Documentation and Implementations
+
+[Abbe Number - Wikipedia](https://en.wikipedia.org/wiki/Abbe_number)
+
+[Abbe Number - Wolfram Formula Repository](https://resources.wolframcloud.com/FormulaRepository/resources/Abbe-Number#:~:text=The%20Abbe%20number%2C%20also%20known,of%20V%20indicating%20low%20dispersion.)
+
+[OpenPBR Surface specification](https://academysoftwarefoundation.github.io/OpenPBR/)
+
+[Enterprise PBR Shading Model](https://dassaultsystemes-technology.github.io/EnterprisePBRShadingModel/spec-2022x.md.html)
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 2023 The Khronos Group Inc.
+
+Some parts of this Specification are purely informative and do not define requirements
+necessary for compliance and so are outside the Scope of this Specification. These
+parts of the Specification are marked as being non-normative, or identified as
+**Implementation Notes**.
+
+Where this Specification includes normative references to external documents, only the
+specifically identified sections and functionality of those external documents are in
+Scope. Requirements defined by external documents not created by Khronos may contain
+contributions from non-members of Khronos not covered by the Khronos Intellectual
+Property Rights Policy.
+
+This specification is protected by copyright laws and contains material proprietary
+to Khronos. Except as described by these terms, it or any components
+may not be reproduced, republished, distributed, transmitted, displayed, broadcast
+or otherwise exploited in any manner without the express prior written permission
+of Khronos.
+
+This specification has been created under the Khronos Intellectual Property Rights
+Policy, which is Attachment A of the Khronos Group Membership Agreement available at
+www.khronos.org/files/member_agreement.pdf. Khronos grants a conditional
+copyright license to use and reproduce the unmodified specification for any purpose,
+without fee or royalty, EXCEPT no licenses to any patent, trademark or other
+intellectual property rights are granted under these terms. Parties desiring to
+implement the specification and make use of Khronos trademarks in relation to that
+implementation, and receive reciprocal patent license protection under the Khronos
+IP Policy must become Adopters and confirm the implementation as conformant under
+the process defined by Khronos for this specification;
+see https://www.khronos.org/adopters.
+
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+merchantability, fitness for a particular purpose, non-infringement of any
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extensions/2.0/Khronos/KHR_materials_dispersion/schema/material.KHR_materials_dispersion.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_dispersion glTF Material Extension",
+    "type": "object",
+    "description": "glTF extension that defines the strength of dispersion.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "dispersion": {
+            "type": "number",
+            "description": "The strength of the dispersion.",
+            "default": 59.0,
+            "minimum": 1.0,
+            "maximum": 1000.0,
+            "gltf_detailedDescription": "This parameter defines dispersion in terms of the Abby number formulation."
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}