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Added shape and rendering hints to base 3DGS spec
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‎extensions/2.0/Khronos/KHR_gaussian_splatting/README.md

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@@ -50,40 +50,88 @@ The extension must be listed in `extensionsUsed`:
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Other extensions that depend on this extension such as 3D Gaussian splatting compression extensions may require that this extension be included in `extensionsRequired`.
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### Geometry Type
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## Geometry Type
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The `mode` of the `primitive` must be `POINTS`.
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### Schema Example
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## Schema Example
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Example shown below. This extension only affects any `primitive` nodes containting 3D Gaussian splat data.
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Example shown below including optional properties. This extension only affects any `primitive` nodes containting 3D Gaussian splat data.
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```json
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"meshes": [{
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"primitives": [{
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"attributes": {
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"meshes": [{
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"primitives": [{
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"attributes": {
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"POSITION": 0,
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"COLOR_0": 1,
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"KHR_gaussian_splatting:SCALE": 2,
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"KHR_gaussian_splatting:ROTATION": 3,
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"KHR_gaussian_splatting:SH_DEGREE_1_COEF_0": 4,
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"KHR_gaussian_splatting:SH_DEGREE_1_COEF_1": 5,
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"KHR_gaussian_splatting:SH_DEGREE_1_COEF_2": 6
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},
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"mode": 0,
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"indices": 7,
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"extensions": {
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"KHR_gaussian_splatting": {}
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}
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}]
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}],
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"_SCALE": 2,
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"_ROTATION": 3,
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"_SH_DEGREE_1_COEF_0": 4,
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"_SH_DEGREE_1_COEF_1": 5,
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"_SH_DEGREE_1_COEF_2": 6
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},
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"mode": 0,
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"indices": 7,
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"extensions": {
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"KHR_gaussian_splatting": {
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"shape": "ellipsoid",
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"hints": {
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"sortingMethod": "cameraDistance",
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"projection": "perspective"
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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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### Extension Properties
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## Extension Properties
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The extension specifies no additional properties but must be included on a point primitive with an empty body to indicate that the primitive should be treated as a 3D Gaussian splatting field.
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### Shape
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#### attributes
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Gaussian splats can have a variety of shapes and this has the potential to change over time. The `shape` property is an optional property that provides an indication to the renderer what these shapes may be. Typically `ellipsoid` refers to the shape generally considered to be a "splat" and this is considered the default value.
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Renderers are free to ignore any values they do not recognize, but are encouraged to follow the non-normative list below.
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#### Known Shapes
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*This section is non-normative and not comprehensive. It may change over time.*
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- `ellipsoid` _(Default Value)_
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- `triangle`
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- `quad`
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### Rendering Hints
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This extension provides a `hints` property that contains sub-properties that may help renderers understand how best to render the Gaussians to the screen. This property and all of it's sub-properties are optional, and renderers can choose to ignore them.
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#### Projection
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The `projection` property is an optional hint that specifies how the Gaussians should be projected into the image. This is typically provided by the training process for the splats. This is a freeform string field to allow new projections to be specified as they become available. The default value is `perspective`.
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Renderers are free to ignore any values they do not recognize, but are encouraged to follow the non-normative list below.
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##### Known Projection Methods
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*This section is non-normative and not comprehensive. It may change over time.*
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- `perspective` _(Default Value)_: The typical 3D perspective projection based on scene depth.
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- `orthographic`: A orthogonal projection of splats into a scene to preserve shape and scale and reduce distortion.
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#### Sorting Method
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The `sortingMethod` property is an optional hint that specifies how the Gaussians should be sorted during the rendering process. This typically is provided by the training process for the splats. This is a freeform string field to allow new sorting methods to be specified as they become available. The default value is `cameraDistance`.
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Renderers are free to ignore any values they do not recognize, but are encouraged to follow the non-normative list below.
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##### Known Sorting Methods
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*This section is non-normative and not comprehensive. It may change over time.*
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- `cameraDistance` _(Default Value)_: The distance between the center of the splat and the position of the camera.
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- `zDepth`: The projected z-depth in the camera projection.
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## Attributes
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Each 3D Gaussian splat has the following attributes. At minimum the attributes must contain `POSITION`, `COLOR_0`, `KHR_gaussian_splatting:ROTATION`, and `KHR_gaussian_splatting:SCALE`. `KHR_gaussian_splatting:SH_DEGREE_ℓ_COEF_n` attributes hold the spherical harmonics data and are not required. `POSITION` and `COLOR_0` are defined by the base glTF specification. If higher degrees of Spherical Harmonics are used then lower degrees are required implicitly.
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‎extensions/2.0/Khronos/KHR_gaussian_splatting/schema/mesh.primitive.KHR_gaussian_splatting.schema.json

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{
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"$schema": "http://json-schema.org/draft-04/schema",
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"title": "KHR_gaussian_splatting glTF Mesh Primitive Extension",
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"type": "object",
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"description": "Data for a 3D Gaussian Splat primitive.",
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"allOf": [ { "$ref": "glTFProperty.schema.json" } ],
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"properties": {
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"extensions": { },
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"extras": { }
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"$schema": "http://json-schema.org/draft-04/schema",
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"title": "KHR_gaussian_splatting glTF Mesh Primitive Extension",
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"type": "object",
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"description": "Data for a 3D Gaussian Splat primitive.",
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"allOf": [
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{
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"$ref": "glTFProperty.schema.json"
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}
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],
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"properties": {
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"shape": {
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"type": "string",
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"description": "The shape of the Gaussian to render. Gaussians may be available in different shapes, so this is a freeform field that defaults to ellipsoid. See the extension specification for futher known types.",
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"default": "ellipsoid"
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},
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"hints": {
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"type": "object",
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"description": "Optional rendering hints for rendering the 3D Gaussian splats. Renderers are free to ignore any of these.",
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"properties": {
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"projection": {
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"type": "string",
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"description": "Provides a hint specifying how to project the Gaussians to achieve a perspective correct value. This is a freeform field that defaults to perspective. See the extension specification for further known types.",
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"default": "perspective"
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},
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"sortingMethod": {
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"type": "string",
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"description": "Provides a hint specifying how to sort the Gaussians during rendering. This is a freeform field defaulting to cameraDistance. See the extension specification for further known types.",
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"default": "cameraDistance"
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}
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}
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},
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"extensions": {},
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"extras": {}
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}
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}

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