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Add EXT_implicit_geometry
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‎extensions/2.0/Vendor/EXT_implicit_geometry/README.md

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# EXT_implicit_geometry
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## Contributors
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- Sean Lilley, Cesium
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- Janine Liu, Cesium
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## Status
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Draft
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## Dependencies
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Written against the glTF 2.0 specification.
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## Overview
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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 and instead use the extension to infer the implicit 3D volume.
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The following example illustrates how a primitive with this extension may represent an implicit box.
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```
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{
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"primitives": [
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{
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"extensions": {
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"EXT_implicit_geometry": {
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"box": {
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"size": [2, 2, 2]
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}
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}
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}
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}
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]
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}
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```
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Primitives with this extension may still be affected by node transforms to position, orient, and scale the shape as needed.
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## Supported Geometry
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Currently, this extension supports the following implicit geometries:
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- [`box`](#box)
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- [`cylinder`](#cylinder)
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- [`sphere`](#sphere)
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- [`ellipsoid`](#ellipsoid)
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- [`region`](#region)
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Only **one** shape may be defined at a time.
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By default, the implicit 3D volume is assumed to fill the entire shape specified in the extension. However, each shape allows an optional `slice` 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 `slice` can limit the volume to be only half of the full sphere.
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### Box
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The `box` property represents an axis-aligned box that is centered at the origin. The `size` property indicates its size in meters along the `x`, `y`, and `z` axes.
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<table>
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<tr>
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<th>
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Example
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</th>
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<th>
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Visual
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</th>
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</tr>
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<tr>
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<td><pre>
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"primitives": [
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{
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"extensions": {
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"EXT_implicit_geometry": {
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"box": {
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"size": [1, 0.5, 2],
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}
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}
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}
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}
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]
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</pre></td>
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<td>
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<img width="600px" src="figures/non-uniform-box.png"/>
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</td>
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</tr>
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</table>
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### Cylinder
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The `cylinder` property represents a cylinder that is aligned with the `y`-axis and centered at the origin. The `radius` and `height` properties indicate the dimensions of the cylinder in meters.
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<table>
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<tr>
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<th>
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Example
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</th>
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<th>
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Visual
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</th>
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</tr>
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<tr>
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<td><pre>
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"primitives": [
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{
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"extensions": {
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"EXT_implicit_geometry": {
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"cylinder": {
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"radius": 2,
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"height": 3
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}
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}
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}
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}
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]
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</pre></td>
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<td>
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**TODO** visual example
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</td>
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</tr>
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</table>
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### Sphere
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The `sphere` property represents a sphere that is centered at the origin. The `radius` of the sphere is specified in meters.
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<table>
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<tr>
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<th>
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Example
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</th>
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<th>
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Visual
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</th>
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</tr>
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<tr>
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<td><pre>
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"primitives": [
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{
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"extensions": {
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"EXT_implicit_geometry": {
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"sphere": {
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"radius": 2,
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}
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}
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}
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}
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]
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</pre></td>
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<td>
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**TODO** visual example
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</td>
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</tr>
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</table>
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### Ellipsoid
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The `ellipsoid` property represents an ellipsoid that is centered at the origin. The `radii` property indicates the radii of the ellipsoid in meters along the `x`, `y`, and `z` axes.
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**TODO** visual example
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<table>
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<tr>
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<th>
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Example
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</th>
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<th>
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Visual
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</th>
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</tr>
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<tr>
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<td><pre>
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"primitives": [
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{
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"extensions": {
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"EXT_implicit_geometry": {
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"ellpisoid": {
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"radii": [2, 3, 4]
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}
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}
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}
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}
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]
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</pre></td>
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<td>
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**TODO** visual example
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</td>
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</tr>
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</table>
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```
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### Region
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The `region` property represents a region above the surface of an ellipsoid. Though the ellipsoid itself is centered at the origin, the region may be distant from the origin due to its properties.
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The `semiMajorAxis` property indicates the radii of the ellipsoid in meters along the `x`, `y`, and `z` axes.
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## Optional vs. Required
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This extension is required, meaning it should be placed in both the `extensionsUsed` list and `extensionsRequired` list.

‎extensions/2.0/Vendor/EXT_implicit_geometry/figures/non-uniform-box.png

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

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

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

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

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

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

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

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{
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"$schema": "https://json-schema.org/draft/2020-12/schema",
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"$id": "ellipsoid.bounds.schema.json",
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"title": "Ellipsoid Bounds",
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"type": "object",
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"description": "A set of bounds for an implicit ellipsoid surface. Defines the subsection of the ellipsoid that the implicit volume actually occupies.",
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"allOf": [
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{
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"$ref": "glTFProperty.schema.json"
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}
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],
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"properties": {
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"minRadius": {
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"type": "number",
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"description": "The minimum radial bound of the ellipsoid.",
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"minimum": 0,
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"maximum": 1,
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"default": 0
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},
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"maxRadius": {
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"type": "number",
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"description": "The maximum radial bound of the ellipsoid.",
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"minimum": 0,
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"maximum": 1,
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"default": 1
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},
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"minAngle": {
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"type": "array",
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"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].",
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"items": {
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"type": "number",
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"minimum": -3.14159265359,
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"maximum": 3.14159265359,
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},
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"minItems": 2,
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"maxItems": 2,
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"default": [-3.14159265359, -1.57079632679]
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},
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"minAngle": {
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"type": "array",
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"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].",
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"items": {
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"type": "number",
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"minimum": -3.14159265359,
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"maximum": 3.14159265359,
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},
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"minItems": 2,
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"maxItems": 2,
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"default": [3.14159265359, 1.57079632679]
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},
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}
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}

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