Add EXT_implicit_geometry

extensions/2.0/Vendor/EXT_implicit_geometry/README.md

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+# EXT_implicit_geometry
+
+## Contributors
+- Sean Lilley, Cesium
+- Janine Liu, Cesium
+
+## Status
+Draft
+
+## Dependencies
+Written against the glTF 2.0 specification.
+
+## Overview
+
+This extension allows mesh primitives to represent renderable implicit surfaces. Typically, a glTF mesh primitive requires `attributes` that supply mesh data, and uses `mode` to indicate the mesh topology. With `EXT_implicit_geometry`, the primitive may omit these properties. The extension's properties will be used instead to infer the implicit 3D volume.
+
+The following example illustrates how a primitive with this extension may represent an implicit box.
+
+```
+{
+  "primitives": [
+    {
+      "extensions": {
+        "EXT_implicit_geometry": {
+          "box": {
+            "size": [2, 2, 2]
+          }
+        }
+      }
+    }
+  ]
+}
+```
+
+Primitives with this extension may still be affected by node transforms to position, orient, and scale the shape as needed.
+
+## Supported Geometry
+
+Currently, this extension supports the following implicit geometries:
+- [`box`](#box)
+- [`cylinder`](#cylinder)
+- [`sphere`](#sphere)
+- [`ellipsoid`](#ellipsoid)
+- [`region`](#region)
+
+Each shape is represented by its aptly named property in the extension. Only **one** shape may be defined at a time.
+
+By default, the implicit 3D volume is assumed to fill the entire shape specified in the extension. However, each shape allows an optional `bounds` property to define the subsection of the shape in which the 3D volume is actually rendered. For instance, while the extension may define an implicit sphere, its `bounds` can limit the volume to be only half of the full sphere.
+
+### Box
+
+The implicit **box** is axis-aligned and centered at the origin, spanning from `(-1, -1, -1)` to `(1, 1, 1)` in local space.
+
+![](figures/unit-box.png)
+
+The `size` property specifies the scale of the box in meters along the `x`, `y`, and `z` axes. This may be specified non-uniformly, as shown below.
+
+![](figures/non-uniform-box.png)
+
+The `bounds` indicate which fraction of the box is actually rendered along each axis. The range of the box is mapped between 0 and 1, where the minimum corner is represented by 0, and the maximum corner is represented by 1. Thus, the bounds on the volume are defined within this range.
+
+![](figures/unit-box-with-bounds.png)
+
+Like `size`, the `bounds` may also be non-uniform.
+
+![](figures/non-uniform-box-with-bounds.png)
+
+
+#### Full Example
+
+```
+{
+  "primitives": [
+    {
+      "extensions": {
+        "EXT_implicit_geometry": {
+          "box": {
+            "size": [2, 2, 2],
+            "bounds": {
+              "min": [0.25, 0, 0.25]
+              "max": [0.75, 0.5, 1]
+            }
+          }
+        }
+      }
+    }
+  ]
+}
+```
+
+### Cylinder
+
+The implicit **cylinder** is aligned with the `y`-axis and centered at the origin in local space. The unit cylinder has a radius of 1 and spans from `[-1, 1]` along the y-axis. The `radius` and `height` properties specify the scale of the cylinder in these respective parameters, in meters.
+
+**TODO** visual example
+
+```
+{
+  "primitives": [
+    {
+      "extensions": {
+        "EXT_implicit_geometry": {
+          "cylinder": {
+            "radius": 2,
+            "height": 2.5
+          }
+        }
+      }
+    }
+  ]
+}
+```
+
+### Sphere
+
+The implicit **sphere** is centered at the origin in local space. The unit sphere has a radius of 1. A `radius` is specified to scale the sphere in meters.
+
+**TODO** visual example
+
+```
+{
+  "primitives": [
+    {
+      "extensions": {
+        "EXT_implicit_geometry": {
+          "sphere": {
+            "radius": 2
+          }
+        }
+      }
+    }
+  ]
+}
+```
+
+### Ellipsoid
+
+The implicit **ellipsoid** is centered at the origin in local space. The unit ellipsoid has radii of `[1, 1, 1]` along the `x`, `y`, and `z` axes. The `radii` property is used to scale the ellipsoid along these axes in meters.
+
+## Optional vs. Required
+This extension is required, meaning it should be placed in both the `extensionsUsed` list and `extensionsRequired` list.

extensions/2.0/Vendor/EXT_implicit_geometry/schema/box.bounds.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "box.bounds.schema.json",
+    "title": "Box Bounds",
+    "type": "object",
+    "description": "A set of bounds for an implicit box surface. Defines the fraction of the box that the implicit volume actually occupies.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "min": {
+            "type": "array",
+            "description": "The minimum bounds of the box in three dimensions.",
+            "items": {
+                "type": "number",
+                "minimum": 0,
+                "maximum": 1,
+            },
+            "minItems": 3,
+            "maxItems": 3,
+            "default": [0, 0, 0]
+        },
+        "max": {
+            "type": "array",
+            "description": "The maximum bounds of the box in three dimensions.",
+            "items": {
+                "type": "number",
+                "minimum": 0,
+                "maximum": 1,
+            },
+            "minItems": 3,
+            "maxItems": 3,
+            "default": [1, 1, 1]
+        },
+    },
+}

extensions/2.0/Vendor/EXT_implicit_geometry/schema/box.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "box.schema.json",
+    "title": "Box",
+    "type": "object",
+    "description": "An implicit box centered at the local space origin.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "size": {
+            "type": "array",
+            "description": "The size of the box in three dimensions.",
+            "items": {
+                "type": "number"
+            },
+            "minItems": 3,
+            "maxItems": 3
+        },
+        "bounds": {
+            "$ref": "box.bounds.schema.json",
+            "description": "The optional bounds of the box."
+        }
+    },
+    "required": [
+        "size"
+    ]
+}

extensions/2.0/Vendor/EXT_implicit_geometry/schema/cylinder.bounds.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "cylinder.bounds.schema.json",
+    "title": "Cylinder Bounds",
+    "type": "object",
+    "description": "A set of bounds for an implicit cylinder surface. Defines the subsection of the cylinder that the implicit volume actually occupies.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "minRadius": {
+            "type": "number",
+            "description": "The minimum radial bound of the cylinder.",
+            "minimum": 0,
+            "maximum": 1,
+            "default": 0
+        },
+        "maxRadius": {
+            "type": "number",
+            "description": "The maximum radial bound of the cylinder.",
+            "minimum": 0,
+            "maximum": 1,
+            "default": 1
+        },
+        "minHeight": {
+            "type": "number",
+            "description": "The minimum height bound of the cylinder.",
+            "minimum": 0,
+            "maximum": 1,
+            "default": 0
+        },
+        "maxHeight": {
+            "type": "number",
+            "description": "The maximum height bound of the cylinder.",
+            "minimum": 0,
+            "maximum": 1,
+            "default": 1
+        },
+        "minAngle": {
+            "type": "number",
+            "description": "The maximum angular bound of the cylinder in radians. Values must be in the range [-pi, pi].",
+            "minimum": -3.14159265359,
+            "maximum": 3.14159265359,
+            "default": -3.14159265359
+        },
+        "maxAngle": {
+            "type": "number",
+            "description": "The maximum angular bound of the cylinder in radians. Values must be in the range [-pi, pi].",
+            "minimum": -3.14159265359,
+            "maximum": 3.14159265359,
+            "default": 3.14159265359
+        }
+    }
+}

extensions/2.0/Vendor/EXT_implicit_geometry/schema/cylinder.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "cylinder.schema.json",
+    "title": "Cylinder",
+    "type": "object",
+    "description": "An implicit cylinder centered at the local space origin.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "radius": {
+            "type": "number",
+            "description": "The radius of the cylinder in local space.",
+            "minimum": 0
+        },
+        "height": {
+            "type": "number",
+            "description": "The height of the cylinder in local space.",
+            "minimum": 0
+        },
+        "bounds": {
+            "$ref": "cylinder.bounds.schema.json",
+            "description": "The optional bounds of the cylinder."
+        }
+    },
+    "required": [
+        "radius",
+        "height"
+    ]
+}

extensions/2.0/Vendor/EXT_implicit_geometry/schema/ellipsoid.bounds.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "ellipsoid.bounds.schema.json",
+    "title": "Ellipsoid Bounds",
+    "type": "object",
+    "description": "A set of bounds for an implicit ellipsoid surface. Defines the subsection of the ellipsoid that the implicit volume actually occupies.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "minRadius": {
+            "type": "number",
+            "description": "The minimum radial bound of the ellipsoid.",
+            "minimum": 0,
+            "maximum": 1,
+            "default": 0
+        },
+        "maxRadius": {
+            "type": "number",
+            "description": "The maximum radial bound of the ellipsoid.",
+            "minimum": 0,
+            "maximum": 1,
+            "default": 1
+        },
+        "minAngle": {
+            "type": "array",
+            "description": "The minimum angular bounds of the ellipsoid in radians. The first element corresponds to the azimuthal angle (a.k.a longitude) and must be in the range [-pi, pi]. The second element corresponds to the polar angle (a.k.a. latitude) and must be in the range [-pi/2, pi/2].", 
+            "items": {
+                "type": "number",
+                "minimum": -3.14159265359,
+                "maximum": 3.14159265359,
+            },
+            "minItems": 2,
+            "maxItems": 2,
+            "default": [-3.14159265359, -1.57079632679]
+        },
+        "minAngle": {
+            "type": "array",
+            "description": "The maximum angular bounds of the ellipsoid in radians. The first element corresponds to the azimuthal angle (a.k.a longitude) and must be in the range [-pi, pi]. The second element corresponds to the polar angle (a.k.a. latitude) and must be in the range [-pi/2, pi/2].", 
+            "items": {
+                "type": "number",
+                "minimum": -3.14159265359,
+                "maximum": 3.14159265359,
+            },
+            "minItems": 2,
+            "maxItems": 2,
+            "default": [3.14159265359, 1.57079632679]
+        },
+    }
+}

extensions/2.0/Vendor/EXT_implicit_geometry/schema/ellipsoid.schema.json

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+{
+    "$schema": "https://json-schema.org/draft/2020-12/schema",
+    "$id": "ellipsoid.schema.json",
+    "title": "Ellipsoid",
+    "type": "object",
+    "description": "An implicit ellipsoid centered at the local space origin.",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "radii": {
+            "type": "array",
+            "description": "The radii of the ellipsoid in meters along the X, Y, and Z axes in local space.",
+            "items": {
+                "type": "number",
+                "minimum": 0
+            },
+            "minItems": 3,
+            "maxItems": 3
+        },
+        "bounds": {
+            "$ref": "ellipsoid.bounds.schema.json",
+            "description": "The optional bounds of the ellipsoid."
+        }
+    },
+    "required": [
+        "radii"
+    ]
+}