When calling calibrateCamera I specify the CV_CALIB_FIX_PRINCIPAL_POINT flag. So I expect the cx and cy of the resulting cameraMatrix to reflect the middle of the screen. My camera is 1280 width and 1024 high. But I get a cx of 512 and ca y of 640. I expected the other way around because x is in the width direction. Do I misunderstand something?