takahirox / EXT_node_lod

EXT_node_lod

Contributors

• Takahiro Aoyagi, Mozilla, @takahirox

Status

Draft

Dependencies

Written against the glTF 2.0 spec.

Overview

This EXT_node_lod extension adds the ability to specify various Levels of Detail (LOD) to a glTF asset. 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 at node level. The LODs can specify different geometry and materials across various levels.

The extension is added to the highest LOD level. The extension defines an lod property which is an array containing the objects including the node property that points to the index of the nodes as LOD node and the screen coverage number as a hint to determine the switching threshold for the LOD.

Every node index in the array of the extension points to the index of another node object which should be used as the LOD that is lower level than the previous array element.

An implementation of this extension can parse through the lod array of the extension 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": [
    {
        "name": "High_LOD",
        "mesh": 0,
        "extensions": {
            "EXT_node_lod": {
                "lod": [
                    {"node": 1, "coverage": 0.5},
                    {"node": 2, "coverage": 0.2},
                ],
            },
        },
    },
    {
        "name": "Medium_LOD",
        "mesh": 1,
    },
    {
        "name": "Low_LOD",
        "mesh": 2,
    },
]

In this node, node[0] is the highest LOD. The first element in the lod array, node[1], is the next lower LOD (or the Medium_LOD). The second element in the lod array, node[2], is the lowest LOD.

Screen coverage

The coverage property in the 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 (or 0 if it is the lowest LOD).

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

LOD node restriction

For simplicity there are some restrictions for LOD nodes.

Nested LOD nodes are disallowed

A node referred by a node of a lod array element of the EXT_node_lod extension or its descendant nodes MUST NOT define the EXT_node_lod extension.

"nodes": [
    {
        "name": "High_LOD",
        "mesh": 0,
        "extensions": {
            "EXT_node_lod": {
                "lod": [
                    {"node": 1, "coverage": 0.5},
                ],
            },
        },
    },
    {
        "name": "Low_LOD",
        "mesh": 1,
        "extensions": {
            // Nested LOD is Invalid. 
            "EXT_node_lod": {
                "lod": [
                    {"node": 2, "coverage": 0.5},
                ],
            },
        },
    },
    ....
]

Child nodes are part of that LOD

A node that defines the EXT_node_lod extension acts as the root node for the highest LOD and its descendant nodes are part of that LOD. Similarly a node referred by a node of a lod array element of the EXT_node_lod extension is the root node for a lower LOD and its descendant nodes will be part of that LOD.

"nodes": [
    {
        "name": "High_LOD",
        "mesh": 0,
        // child node[2] is part of the High LOD
        "children": [2],
        "extensions": {
            "EXT_node_lod": {
                "lod": [
                    {"node": 1, "coverage": 0.5},
                ],
            },
        },
    },
    {
        "name": "Low_LOD",
        "mesh": 1,
        // child node[3] is part of the Low LOD
        "children": [3],
    },
    ....
]

LOD nodes are regarded as part of the scene node graph

Not only the highest LOD node and its descendant nodes but also the lower LOD nodes and their children nodes MUST be regarded as part of the scene node graph and the lower LOD nodes MUST be regarded as siblings of the highest LOD node (so the parent of the highest LOD node is also regarded as the parent of the lower LOD nodes) when cheking whether the scene graph follows the node hierarchy rule.

Refer to the Scenes - Nodes and Hierarchy section in the glTF core specification for the node hierarchy rule details.

"nodes": [
    {
        "name": "Parent",
        "children": [1],
    },
    {
        "name": "High_LOD",
        "mesh": 0,
        "extensions": {
            "EXT_node_lod": {
                // node[2] is regarded as a sibling of this node and
                // regarded as whose parent is node[0]
                "lod": [
                    {"node": 2, "coverage": 0.5},
                ],
            },
        },
    },
    {
        "name": "Low_LOD",
        "mesh": 1,
        // Invalid because of forming a cycle in the node graph
        "children": [0],
    },
    {
        // Invalid because node[2] has two parents node[0] and node[3]
        "children": [2],
    },
    ....
]

LOD Array property

Property	Type	Description	Requires
lod	lod [1-*]	Lower LODs definition	✅ Yes

Properties of LOD Array element

Property	Type	Description	Requires
node	integer	node index for this LOD level.	✅ Yes
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_node_lod.schema.json

Reference

• glTF MSFT_lod extension: A glTF LOD vendor extension proposed by Microsoft. It allows more flexible LOD configuration. The EXT_node_lod extension has simpler data structure than MSFT_lod so less flexible but easier to write an implementation.