computeBoundingSphere() {

		const geometry = this.geometry;

		if ( this.boundingSphere === null ) {

			this.boundingSphere = new Sphere();

		}

		this.boundingSphere.makeEmpty();

		const positionAttribute = geometry.getAttribute( 'position' );

		for ( let i = 0; i < positionAttribute.count; i ++ ) {

			this.getVertexPosition( i, _vertex );
			this.boundingSphere.expandByPoint( _vertex );

		}

	}
Computes the bounding sphere of the skinned mesh, and updates {@link SkinnedMesh#boundingSphere}.
The bounding sphere is automatically computed by the engine once when it is needed, e.g., for ray casting
and view frustum culling. If the skinned mesh is animated, the bounding sphere should be recomputed
per frame in order to reflect the current animation state.