OMI_physics_body

Contributors

• Aaron Franke, The Mirror Megaverse Inc.
• Mauve Signweaver, Mauve Software Inc.

Status

Open Metaverse Interoperability Group Stage 1 Proposal

Dependencies

Written against the glTF 2.0 spec.

Depends on the OMI_physics_shape spec to be useful.

Overview

This extension allows for specifying physics bodies in glTF scenes.

Nodes with the OMI_physics_body extension may define motion, collider, and trigger properties.

If a node with a collider shape does not have a motion property on itself or an ancestor node, it should be a static solid object that does not move.

Example:

This example defines a static body node which has a single box collider as a child:

{
    "asset": {
        "version": "2.0"
    },
    "extensions": {
        "OMI_physics_shape": {
            "shapes": [
                {
                    "type": "box",
                    "box": {
                        "size": [1, 2, 3]
                    }
                }
            ]
        }
    },
    "extensionsUsed": [
        "OMI_physics_body",
        "OMI_physics_shape"
    ],
    "nodes": [
        {
            "extensions": {
                "OMI_physics_body": {
                    "motion": {
                        "type": "dynamic"
                    },
                    "collider": {
                        "shape": 0
                    }
                }
            },
            "name": "DynamicBox"
        }
    ],
    "scene": 0,
    "scenes": [{ "nodes": [0] }]
}

The above example shows dynamic motion and collision shape specified on one node. A nearly identical example using 2 nodes can be found in examples/basic/dynamic_box.gltf.

More example assets can be found in the examples/ folder. All of these examples use both OMI_physics_shape and OMI_physics_body.

glTF Schema Updates

This extension consists of a new OMI_physics_body data structure which can be added to a glTF node.

The extension must also be added to the glTF's extensionsUsed array and because it is optional, it does not need to be added to the extensionsRequired array.

The extension is intended to be used together with OMI_physics_shape, which defines the shapes used by the "shape" properties inside of the "collider" and "trigger" sub-JSON properties.

Property Summary

	Type	Description	Default value
motion	JSON	If present, this node has its motion controlled by physics.	null
collider	JSON	If present, this node is solid and can be collided with.	null
trigger	JSON	If present, this node is non-solid and can act as a trigger.	null

Each of these properties are recommended to be defined on separate nodes. This results in a very clear, simple, and portable document structure, and ensures that each behavior has its own transform. However, they may also be all defined on the same node. Implementations must support all of these cases in order to be compliant.

Motion

If a node has the "motion" property defined, its transform is driven by the physics engine.

The list of motion properties and their details can be found in the README.motion.md file.

Collider

If a node has the "collider" property defined, it is a solid collider node that objects can collide with.

The list of collider properties and their details can be found in the README.collider.md file.

Trigger

If a node has the "trigger" property defined, it is a non-solid trigger that can detect when objects enter it.

The list of trigger properties and their details can be found in the README.trigger.md file.

JSON Schema

See node.OMI_physics_body.schema.json for the main node schema, and these for the sub-JSON property schemas: * Motion: node.OMI_physics_body.motion.schema.json * Collider: node.OMI_physics_body.collider.schema.json * Trigger: node.OMI_physics_body.trigger.schema.json

Known Implementations

• Godot Engine: https://github.com/godotengine/godot/pull/78967

Resources:

• Unity colliders: https://docs.unity3d.com/Manual/CollidersOverview.html
• Unreal Engine Physics: https://docs.unrealengine.com/4.27/en-US/InteractiveExperiences/Physics/Collision/Overview/
• Godot Physics Body: https://docs.godotengine.org/en/stable/classes/class_physicsbody.html
• Godot Area: https://docs.godotengine.org/en/stable/classes/class_area.html
• Godot RigidBody3D: https://docs.godotengine.org/en/latest/classes/class_rigidbody3d.html
• Wikipedia Moment of Inertia: https://en.wikipedia.org/wiki/Moment_of_inertia
• Wikipedia Rigid Body Dynamics: https://en.wikipedia.org/wiki/Rigid_body_dynamics