code: function isObjectViewable( object ) {
  
  		var geometry = object.geometry;
  
  		if ( geometry.boundingSphere === null )
  			geometry.computeBoundingSphere();
  
  		_sphere.copy( geometry.boundingSphere ).
  			applyMatrix4( object.matrixWorld );
  
  		return isSphereViewable( _sphere );
  
  	}
docs: TODO Duplicated code (Frustum)