rigid-body-no-nesting#

Code

RB.006

Validator

latest+

Compatibility

openusd

Tags

✅

Summary#

Rigid bodies can not be nested unless xformOp reset xform stack is used.

Description#

Since UsdPhysicsRigidBodyAPI simulation ends up with changes to the world transformation, it has to be applied to the local transformation stack of the UsdGeomXformable. If rigid bodies are nested, it requires a defined order in which the schema is applied, and that is an unnecessary complication. To avoid this complication, if rigid bodies are nested, they must have their xformOp stack reset set.

Why is it required?#

  • Assets with UsdPhysicsRigidBodyAPI define rigid body for the simulator. Prims with this API will have their xformOp attributes updated after each simulation step hence there has to be a clear target where the transformation should end up.

Examples#

# Invalid: Nested rigid bodies
def Xform "Xform" (
   prepend apiSchemas = ["PhysicsRigidBodyAPI"]
)
{
   def Cube "cube" (
      prepend apiSchemas = ["PhysicsRigidBodyAPI", "PhysicsCollisionAPI"]
   ) {
   }
}

# Valid: Rigid bodies nested but with xformOp stack reset
def Xform "Xform" (
   prepend apiSchemas = ["PhysicsRigidBodyAPI"]
)
{
   def Cube "cube" (
      prepend apiSchemas = ["PhysicsRigidBodyAPI", "PhysicsCollisionAPI"]
   ) {
      quatd xformOp:orient = (1, 0, 0, 0)
      double3 xformOp:scale = (1, 1, 1)
      double3 xformOp:translate = (0, 0, 0)
      uniform token[] xformOpOrder = ["!resetXformStack!", "xformOp:translate", "xformOp:orient", "xformOp:scale"]
   }
}

How to comply#

The UsdPhysicsRigidBodyAPI cannot be nested unless xformOp resetXformStack is used.

For More Information#