EXT_mesh_lod

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Contributors

Takahiro Aoyagi, Mozilla, @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

"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	✅ Yes

Properties of Mesh LOD Array entry

Property	Type	Description	Requires
`mesh`	`integer`	`mesh` index for this LOD level. Refer to `node.mesh` in the core spec for the details.	✅ Yes
`skin`	`integer`	`skin` index for this LOD level `mesh` skinning. Refer to `node.skin` in the core spec for the details.	No
`weights`	`number [1-*]`	`weights` for this LOD level `mesh` morph. Refer to `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.	✅ Yes

Schema

node.EXT_mesh_lod.schema.json

Reference

glTF MSFT_lod extension: 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.

takahirox / EXT_mesh_lod