KHR_implicit_shapes extension Draft Proposal

extensions/2.0/Khronos/KHR_implicit_shapes/README.md

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+# KHR_implicit_shapes
+
+## Contributors
+
+* Eoin Mcloughlin, Microsoft, <mailto:[email protected]>
+* Rory Mullane, Microsoft, <mailto:[email protected]>
+* George Tian, Microsoft, <mailto:[email protected]>
+* Aaron Franke, Godot Engine, <mailto:[email protected]>
+* Eric Griffith, Meta, <mailto:[email protected]>
+* Janine Liu, Cesium, <mailto:[email protected]>
+* Sean Lilley, Cesium, <mailto:[email protected]>
+
+## Status
+
+Release Candidate
+
+## Dependencies
+
+Written against glTF 2.0 spec.
+
+## Overview
+
+This extension adds data definitions for implicit shapes. This extension does not mandate any particular behaviour for these objects aside from a description of their geometry. These types are to be used in combination with other extensions which should specify the behavior of these types.
+
+## glTF Schema Updates
+
+### Shapes
+
+This extension provides a set of document-level objects, which can be referenced by objects in a glTF document.
+
+Shapes are defined within a dictionary property in the glTF scene description file, by adding an `extensions` property to the top-level glTF 2.0 object and defining a `KHR_implicit_shapes` property with a `shapes` array inside it.
+
+Each shape defines a mandatory `type` property which designates the type of shape, as well as an additional structure which provides parameterizations specific to that type. The following example defines a sphere.
+
+```javascript
+"extensions": {
+    "KHR_implicit_shapes" : {
+        "shapes": [
+            {
+                "sphere": { "radius": 0.1 },
+                "type": "sphere"
+            }
+        ]
+    }
+}
+```
+
+To describe the geometry which represents the object, shapes must define at most one of the following properties:
+
+| |Type|Description|
+|-|-|-|
+|**sphere**|`object`|A sphere centered at the origin in local space.|
+|**box**|`object`|An axis-aligned box centered at the origin in local space.|
+|**cylinder**|`object`|A cylinder centered at the origin and aligned along the Y axis in local space, with potentially different radii at each end. A cone is a special case of cylinder when one of the radii is zero.|
+|**capsule**|`object`|A capsule (cylinder with hemispherical ends) centered at the origin and defined by two "capping" spheres with potentially different radii, aligned along the Y axis in local space.|
+
+When the `type` of a shape refers to a shape type defined by this extension, the shape must not supply a value for a different shape type e.g. a shape whose `type` is "box" must not have a defined `sphere` property. Similarly, if `type` does not reference a shape type declared by `KHR_implicit_shapes`, an extension or other mechanism should supply an alternate shape definition.
+
+Degenerate shapes are prohibited. A sphere must have a positive, non-zero radius. A box shape must have positive non-zero values for each component of `size`. The cylinder and capsule shapes must have a positive, non-zero `height`, both `radiusTop` and `radiusBottom` must be non-negative, and at least one of `radiusTop` and `radiusBottom` must be non-zero.
+
+### JSON Schema
+
+* glTF Document Extension: [glTF.KHR_implicit_shapes.schema.json](schema/glTF.KHR_implicit_shapes.schema.json)
+* Shape: [glTF.KHR_implicit_shapes.shape.schema.json](schema/glTF.KHR_implicit_shapes.shape.schema.json)
+* Sphere: [glTF.KHR_implicit_shapes.shape.sphere.schema.json](schema/glTF.KHR_implicit_shapes.shape.sphere.schema.json)
+* Box: [glTF.KHR_implicit_shapes.shape.box.schema.json](schema/glTF.KHR_implicit_shapes.shape.box.schema.json)
+* Cylinder: [glTF.KHR_implicit_shapes.shape.cylinder.schema.json](schema/glTF.KHR_implicit_shapes.shape.cylinder.schema.json)
+* Capsule: [glTF.KHR_implicit_shapes.shape.capsule.schema.json](schema/glTF.KHR_implicit_shapes.shape.capsule.schema.json)
+
+### Object Model
+
+With consideration to the glTF 2.0 Asset Object Model Specification document, the following pointer templates represent mutable properties defined in this extension.
+
+| Pointer | Type|
+|-|-|
+| `/extensions/KHR_implicit_shapes/shapes/{}/box/size` | `float3`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/capsule/height` | `float`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/capsule/radiusBottom` | `float`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/capsule/radiusTop` | `float`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/cylinder/height` | `float`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/cylinder/radiusBottom` | `float`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/cylinder/radiusTop` | `float`|
+| `/extensions/KHR_implicit_shapes/shapes/{}/sphere/radius` | `float`|
+
+Additional read-only properties
+
+| Pointer | Type|
+|-|-|
+| `/extensions/KHR_implicit_shapes/shapes.length` | `int`|
+
+## Appendix: Full Khronos Copyright Statement
+
+Copyright 2021-2023 The Khronos Group Inc.
+
+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
+https://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 under the process defined by Khronos for this specification;
+see https://www.khronos.org/conformance/adopters/file-format-adopter-program.
+
+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
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+
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+for identification and belong to their respective owners.

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

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "glTF.KHR_implicit_shapes.schema.json",
+    "title": "KHR_implicit_shapes glTF Document Extension",
+    "type": "object",
+    "description": "Top level implicit shapes.",
+    "allOf": [ { "$ref" : "glTFProperty.schema.json" } ],
+    "properties": {
+        "shapes": {
+            "type": "array",
+            "description": "An array of shape descriptions.",
+            "items": {
+                "type": "object",
+                "$ref": "glTF.KHR_implicit_shapes.shape.schema.json"
+            },
+            "minItems": 1
+        },
+        "extensions": { },
+        "extras": { }
+    },
+    "required": [
+        "shapes"
+    ]
+}

extensions/2.0/Khronos/KHR_implicit_shapes/schema/glTF.KHR_implicit_shapes.shape.box.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "glTF.KHR_implicit_shapes.shape.box.schema.json",
+    "title": "KHR_implicit_shapes Box Shape",
+    "type": "object",
+    "description": "Parameters describing a box shape.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "size": {
+            "type": "array",
+            "description": "The extents of the box in each axis in local space.",
+            "items": {
+                "type": "number",
+                "exclusiveMinimum": 0.0
+            },
+            "minItems": 3,
+            "maxItems": 3,
+            "default": [ 1, 1, 1 ]
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/2.0/Khronos/KHR_implicit_shapes/schema/glTF.KHR_implicit_shapes.shape.capsule.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "glTF.KHR_implicit_shapes.shape.capsule.schema.json",
+    "title": "KHR_implicit_shapes Capsule Shape",
+    "type": "object",
+    "description": "Parameters describing a capsule shape.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "height": {
+            "type": "number",
+            "description": "The distance between the centers of the two capping spheres of capsule.",
+            "exclusiveMinimum": 0.0,
+            "default": 0.5
+        },
+        "radiusBottom": {
+            "type": "number",
+            "description": "The radius of the sphere located at the bottom of the capsule (i.e. the sphere at the half-height along -Y)",
+            "minimum": 0.0,
+            "default": 0.25
+        },
+        "radiusTop": {
+            "type": "number",
+            "description": "The radius of the sphere located at the top of the capsule (i.e. the sphere at the half-height along +Y)",
+            "minimum": 0.0,
+            "default": 0.25
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/2.0/Khronos/KHR_implicit_shapes/schema/glTF.KHR_implicit_shapes.shape.cylinder.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "glTF.KHR_implicit_shapes.shape.cylinder.schema.json",
+    "title": "KHR_implicit_shapes Cylinder Shape",
+    "type": "object",
+    "description": "Parameters describing a cylinder shape.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "height": {
+            "type": "number",
+            "description": "The height of the cylinder, centered along the Y axis.",
+            "exclusiveMinimum": 0.0,
+            "default": 0.5
+        },
+        "radiusBottom": {
+            "type": "number",
+            "description": "The radius of the bottom of the cylinder (the disk located along -Y.)",
+            "minimum": 0.0,
+            "default": 0.25
+        },
+        "radiusTop": {
+            "type": "number",
+            "description": "The radius of the top of the cylinder (the disk located along +Y.)",
+            "minimum": 0.0,
+            "default": 0.25
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/2.0/Khronos/KHR_implicit_shapes/schema/glTF.KHR_implicit_shapes.shape.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "glTF.KHR_implicit_shapes.shape.schema.json",
+    "title": "KHR_implicit_shapes Shape Resource",
+    "type": "object",
+    "description": "Parameters describing an implicit shape.",
+    "allOf": [ { "$ref": "glTFChildOfRootProperty.schema.json" } ],
+    "properties": {
+        "type": {
+            "description": "Specifies the shape type.",
+            "anyOf": [
+                {
+                    "const": "sphere",
+                    "description": "A sphere with a specified radius, centered at the origin in local space."
+                },
+                {
+                    "const": "box",
+                    "description": "An axis-aligned box with a size per-axis, centered at the origin in local space"
+                },
+                {
+                    "const": "capsule",
+                    "description": "A capsule shape, centered at the origin in local space, equivalent to the convex hull of two spheres located along the Y axis (in local space) at a specified distance."
+                },
+                {
+                    "const": "cylinder",
+                    "description": "A cylinder shape, centered at the origin in local space, equivalent to the convex hull of two circles in the X/Z plane positioned along the Y axis at a specified distance."
+                },
+                {
+                    "type": "string"
+                }
+            ]
+        },
+        "sphere": {
+            "type": "object",
+            "description": "A set of parameter values that are used to define a sphere shape.",
+            "$ref": "glTF.KHR_implicit_shapes.shape.sphere.schema.json"
+        },
+        "box": {
+            "type": "object",
+            "description": "A set of parameter values that are used to define a box shape.",
+            "$ref": "glTF.KHR_implicit_shapes.shape.box.schema.json"
+        },
+        "capsule": {
+            "type": "object",
+            "description": "A set of parameter values that are used to define a capsule shape.",
+            "$ref": "glTF.KHR_implicit_shapes.shape.capsule.schema.json"
+        },
+        "cylinder": {
+            "type": "object",
+            "description": "A set of parameter values that are used to define a cylinder shape.",
+            "$ref": "glTF.KHR_implicit_shapes.shape.cylinder.schema.json"
+        },
+        "name": { },
+        "extensions": { },
+        "extras": { }
+    },
+    "required": [
+        "type"
+    ]
+}

extensions/2.0/Khronos/KHR_implicit_shapes/schema/glTF.KHR_implicit_shapes.shape.sphere.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "glTF.KHR_implicit_shapes.shape.sphere.schema.json",
+    "title": "KHR_implicit_shapes Sphere Shape",
+    "type": "object",
+    "description": "Parameters describing a sphere shape.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "radius": {
+            "type": "number",
+            "description": "The radius of the sphere.",
+            "exclusiveMinimum": 0.0,
+            "default": 0.5
+        },
+        "extensions": { },
+        "extras": { }
+    }
+}