Calculate the undistorted points of the calibration pattern of a fisheye lense

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I have successfully calibrated and undistorted the image, which is distorted by a fisheye lense. For the measurement of the undistortion i want calculate the error of the distortion. For this measurement i need to calculate the calibration pattern points in the image coordinate space (pefect points) and the undistorted points done by the undistortImage() method. I have already calculated the "perfect points" and i wanted to calculate the undistorted points using the formula in (http://docs.opencv.org/master/db/d58/...). But i didn't get the result I've expected. The problem is the transformation from the camera to the image coordinate system. u=fx(x′+αy′)+cx v=fy *y *y+cy. Has anybody an idea where my I make a mistake?