Camera's often have non-linear radial image distortions. For example, OpenCV's camera calibration process computes these radial distortions along the way [1]. T

Camera's often have non-linear radial image distortions. For example, OpenCV's camera calibration process computes these radial distortions along the way [1]. They may not be very significant, but they exist. [1] https://docs.opencv.org/master/dc/dbb/tutorial_py_calibration.html https://docs.opencv.org/master/dc/dbb/tutorial_py_calibratio... Aligning points on a photo outside of more-or-less linear center region will certainly result in crossing lines. Which we see in the alignment attempt there in the article - the points we align are close to center and close to edge (max distortion). There is no mention of distortions in the entire article. But some other points are interesting to think about.