update method
Set the visibility of each level's object
based on distance from the camera.
Implementation
void update(Camera camera){
		final levels = this.levels;
		if ( levels.length > 1 ) {
			_v1.setFromMatrixPosition(camera.matrixWorld);
			_v2.setFromMatrixPosition(matrixWorld);
			final distance = _v1.distanceTo( _v2 ) / camera.zoom;
			levels[0].object?.visible = true;
			int i = 1;
    int l = levels.length;
			for(i = 1; i < l; i ++ ) {
				if ( distance >= levels[ i ].distance ) {
					levels[ i - 1 ].object?.visible = false;
					levels[ i ].object?.visible = true;
				}
      else {
					break;
				}
			}
			currentLevel = i - 1;
			for (;i < l;i++) {
				levels[i].object?.visible = false;
			}
		}
	}