Merge pull request #7 from ux3d/extensions/KHR_materials_thickness

KHR_materials_thickness

extensions/2.0/Khronos/KHR_materials_thickness/README.md

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+# KHR\_materials\_thickness
+
+## Contributors
+
+* Norbert Nopper, UX3D [@UX3DGpuSoftware](https://twitter.com/UX3DGpuSoftware)
+* Jim Eckerlein, UX3D [@UX3DGpuSoftware](https://twitter.com/UX3DGpuSoftware)
+
+Copyright (C) 2018-2020 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
+
+## Overview
+
+This extension defines a thickness texture, which assigns each surface point the shortest distance to another surface point lying under the point's geometry normal. The thickness texture by itself does not alter any visuals but serves as data for other extensions requiring knowledge over the mesh's volume.
+
+## Extending Materials
+
+The thickness texture can be defined per material by adding the `KHR_materials_thickness` extension to any glTF material.
+
+```json
+{
+    "materials": [
+        {
+            "extensions": {
+                "KHR_materials_thickness": {
+                    "thickness": 10.0,
+                    "thicknessTexture": {
+                        "index" : 0,
+                        "texCoord" : 0
+                    }
+                }
+            }
+        }
+    ]
+}
+```
+
+### Thickness
+
+<p align="left">
+<img width="512" src="figures/thickness.png"/>
+</p>
+
+|                                  | Type                                                                            | Description                 | Required             |
+|----------------------------------|---------------------------------------------------------------------------------|-----------------------------|----------------------|
+|**thicknessFactor**               | `number`                                                                        | The thickness factor.       | No, default: `1.0`   |
+|**thicknessTexture**              | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo)             | The thickness texture.      | No, default: `1.0`.  |
+
+The final thickness for a given surface point is the result of the scalar multiplication of the material-wide `thickness` and the texture fetch from `thicknessTexture`.
+
+The local thickness map describes the expected thickness at a surface point. A value close to 0.0 means the surface point is thin, and lets a lot of light through the medium, while a value close to 1.0 means the surface point is thick, and means most of the light will be transmitted inside the material.
+
+## Schema
+
+- [glTF.KHR_materials_thickness.schema.json](schema/glTF.KHR_materials_thickness.schema.json)
+
+## Reference
+
+### Theory, Documentation and Implementations
+
+There are different ways to compute local thickness, one involves shooting rays through the object, measuring the distance between entry and exit-points. Another approach is to invert surface normals and render ambient occlusion to approximate thickness.
+
+This technique works well for static convex objects, concavities however, are not taken into account when computing local thickness. Animated objects (skeletal or morphing) cannot be approximated with this technique as the thickness changes over time.
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 2018-2020 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.
+
+Khronos makes no, and expressly disclaims any, representations or warranties,
+express or implied, regarding this specification, including, without limitation:
+merchantability, fitness for a particular purpose, non-infringement of any
+intellectual property, correctness, accuracy, completeness, timeliness, and
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+respective owners.

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

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_thickness glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the geometry thickness.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+
+        "thicknessFactor": {
+            "type": "number",
+            "description": "Modulates the thickness.",
+            "default": 1.0,
+            "minimum": 0.0,
+            "gltf_detailedDescription": ""
+        },
+
+        "thicknessTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "Texture describing the local thickness of the material. 0 is thin and 1 is thick.",
+            "gltf_detailedDescription": "Should be generated offline by flipping the normals of the mesh and computing the ambient occlusion map. This texture is a grayscale, where darker areas imply the material is locally thinner there, while brighter areas are locally thicker. There are many free and paid tools for baking these thickness maps."
+        },
+
+        "extensions": { },
+        "extras": { }
+    }
+}