Physics Rigid Bodies

Overview

This capability provides requirements for physical simulation of solid objects that move and collide with each other. Rigid bodies maintain their shape during simulation while responding to forces, collisions, and constraints.

Summary

• The rigid body schema must be applied to any xformable prim that represent a rigid physical object

• Colliding Gprims need to have the Collision API applied

• Each rigid body or it’s descendant colliders, should have the mass API applied.

Granularity

If joints are not present, each rigid body will be simulated as a free body. Only create multiple rigid bodies in an asset if they are either connected by joints or if they are intended to be simulated without being connected to each other.

Schema / OpenUSD Specification

Rigid Body Physics are applied using the following two schemas that are part of the core OpenUSD specification.

• USD Rigid Bodies

• USD Physics Collision Shapes

USDA Sample

(
    kilogramsPerUnit = 1
)

def Xform "RobotHead" (
    prepend apiSchemas = ["PhysicsRigidBodyAPI", "PhysicsMassAPI"]
    ) {

    physics:mass = 10.0

    def Mesh "Head" (
        prepend apiSchemas = ["PhysicsCollisionAPI"]
    ) {
        # ...
    }

    def Mesh "Jaw" (
        prepend apiSchemas = ["PhysicsCollisionAPI"]
    ) {
        # ...
    }
}

def Mesh "GroundPlane" (
    prepend apiSchemas = ["PhysicsCollisionAPI"]
) {}

Requirements

Tags	Summary	Compatibility	Validator
🔑	Assets must contain at least one rigid body	openusd
🔑	Colliding Gprims must apply the Collision API.	openusd
✅	Rigid bodies have to be UsdGeomXformable prims.	openusd	latest+
✅	Rigid bodies cannot be part of a scene graph instance.	openusd	latest+
✅	Rigid bodies can not be nested unless xformOp reset xform stack is used.	openusd	latest+
✅	Rigid bodies have to be UsdGeomXformable prims without skew matrix.	openusd	latest+
✅	If an asset is expected to be static (no movement happens), it can not contain a rigid body, only a Collider body.	openusd
✅	The Mesh Collision API can only be assigned to Mesh Prims.	openusd
✅	The collision shape scale must be uniform for the following geometries: Sphere, Capsule, Cylinder, Cone & Points.	openusd	latest+
💎	Rigid bodies or their descendent collision shapes should have a mass specification.	openusd
💎	Rigid bodies or their descendent collision shapes may have detailed mass properties including density, center of mass, and inertia tensor.	openusd