Also consider that ICP, as the name implies, is an iterative optimization algorithm. That means it runs in a loop until a convergence criterion is met. The convergence is controlled by setting some parameters of the vtkIterativeClosestPointTransform object. Hence, these parameters often severely impact the result. As general rule, iterative optimization has a tradeoff between speed and accuracy. Maybe you could study the class’ API: VTK: vtkIterativeClosestPointTransform Class Reference and become acquainted with the parameters in case you haven’t done so already.