EXT_mesh_lod

extensions/2.0/Vendor/EXT_mesh_lod/README.md

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+# EXT\_mesh\_lod
+
+## Contributors
+
+* Takahiro Aoyagi, Mozilla, [@takahirox](https://github.com/takahirox)
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+This `EXT_mesh_lod` extension adds the ability to specify Levels of Detail
+(LOD) to glTF `mesh`. An implementation of this extension can use the
+LODs for various scenarios like rendering a different LOD based upon the
+distance of the object or progressively loading the glTF asset starting with
+the lowest LOD.
+
+This extension allows an LOD to be defined in a `node`. If the extension
+is defined in a `node`, `mesh` **MUST** also be defined in the `node`.
+The LODs can specify different meshes across various levels.
+
+The extension is added to the highest LOD level. The extension defines an
+`mesh_lod` property which is an array containing the objects including the
+`mesh` property that points to the index of the `mesehes` node as LOD mesh
+and the optional `skin` and `weights` property for skinning and morph for
+the LOD mesh, and the screen coverage number as a hint to determine the
+switching threshold for the LOD.
+
+Each `mesh` index in the array points to a LOD level that is lower in quality
+than the previous level.
+
+For example, if a `node` has a `mesh` and the extension, the `meshes` node
+referred by the `mesh` property in the `node` is the highest LOD level. Every
+`mesh` property specified in the array of the extension points to the index of
+`meshes` node which should be used as the lower LOD level.
+
+An implementation of this extension can parse through the array to create the
+list of LOD levels available for an asset. If an asset containing this extension
+is loaded in a client that doesn't implement the extension, then the highest
+LOD level will be loaded and all lower LODs will be ignored.
+
+## Examples
+
+```json
+"nodes": [{
+    "mesh": 0,
+    "extensions": {
+        "EXT_mesh_lod": {
+            "mesh_lod": [
+                {"mesh": 1, "coverage": 0.5},
+                {"mesh": 2, "coverage": 0.2}
+            ]
+        }
+    }
+}]
+ ```
+
+In this node, *mesh[0]* is the highest LOD. The first element in the `mesh_lod`
+array, *mesh[1]*, is the next lower LOD (or the Medium_LOD). The second element
+in the `mesh_lod` array, *mesh[2]*, is the lowest LOD.
+
+## Screen coverage
+
+The `coverage` property in the `mesh_lod` array specifies a screen coverage
+value that can be used as a hint to determine the switching threshold for the
+various LODs.
+
+Screen coverage is a ratio between `0.0` and `1.0` representing how big the
+mesh's rendered screen surface area is in the total screen surface area.
+
+The highest LOD `coverage` value is implied to `1.0`. The `coverage` value
+must be between `0.0` and `1.0`, and must be between the next higher LOD
+`coverage` value and the next lower LOD `coverage` value.
+
+The LOD is expected to be selected to be rendered if the current screen
+coverage is between the LOD's `coverage` value and the next lower LOD
+`coverage` value.
+
+In the example above the screen coverage values can be used as follows:
+
+* LOD0(high) Rendered from 1.0 to 0.5
+* LOD1(medium) Rendered from 0.5 to 0.2
+* LOD2(low) Rendered from 0.2 to 0
+
+## Mesh LOD Array property
+
+| Property | Type | Description | Requires |
+|:------|:------|:------|:------|
+| `mesh_lod` | `lod [1-*]` | Lower mesh LODs definition | :white_check_mark: Yes |
+
+## Properties of Mesh LOD Array entry
+
+| Property | Type | Description | Requires |
+|:------|:------|:------|:------|
+| `mesh` | `integer` | `mesh` index for this LOD level. Refer to [`node.mesh`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_node_mesh) in the core spec for the details. | :white_check_mark: Yes |
+| `skin` | `integer` | `skin` index for this LOD level `mesh` skinning. Refer to [`node.skin`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_node_skin) in the core spec for the details. | No |
+| `weights` | `number [1-*]` | `weights` for this LOD level `mesh` morph. Refer to [`node.weights`](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#_node_weights) in the core spec for the details. | No |
+| `coverage` | `number` | Screen coverage for switching for this LOD level. The value must be between `0.0` and `1.0`, and must be between the next higher LOD `coverage` value and the next lower LOD `coverage` value. | :white_check_mark: Yes |
+
+## Schema
+
+* [node.EXT_mesh_lod.schema.json](schema/node.EXT_mesh_lod.schema.json)
+
+## Reference
+
+* [glTF `MSFT_lod` extension](https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Vendor/MSFT_lod):
+A glTF LOD vendor extension proposed by Microsoft. It allows more flexible
+LOD configuration. The `EXT_mesh_lod` extension has simpler data structure
+than `MSFT_lod` so less flexible but easier to handle for the extension
+handlers.

extensions/2.0/Vendor/EXT_mesh_lod/schema/node.EXT_mesh_lod.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "EXT_mesh_lod glTF extension",
+    "type": "object",
+    "description": "glTF extension for specifying levels of detail (LOD) for mesh.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "mesh_lod": {
+            "type": "array",
+            "items": {
+                "type": "object",
+                "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+                "properties": {
+                    "mesh": {
+                        "allOf": [ { "$ref": "glTFid.schema.json" } ],
+                        "description": "LOD mesh index"
+                    },
+                    "skin": {
+                        "allOf": [ { "$ref": "glTFid.schema.json" } ],
+                        "description": "LOD skin index"
+                    },
+                    "weights": {
+                        "type": "array",
+                        "items": {
+                            "type": "number"
+                        },
+                        "allOf": [ { "$ref": "glTFid.schema.json" } ],
+                        "description": "Array containing weights values for morph",
+                        "minItems": 1
+                    },
+                    "coverage": {
+                        "type": "number",
+                        "minimum": 0.0,
+                        "maximum": 1.0,
+                        "description": "Screen coverage for switching"
+                    }
+                }
+                "required": [ "mesh", "coverage" ]
+            },
+            "description": "Array containing the objects including the mesh index as lower LOD and screen coverage as a hint to switch.",
+            "minItems": 1
+        },
+        "extensions": { },
+        "extras": { }
+    },
+    "required": [ "mesh_lod" ]
+}