With the help of camera calibration, radial and tangential distortion of the images can be corrected or greatly reduced.
The radial distortion in images can be recognized when straight lines are curved (convex or concave) in the image. An extreme example is fisheye lenses (fisheye lenses are not recommended for contour tracing). Although these allow a wide field of view, straight lines that do not pass through the center of the image are displayed in a curved shape and the image is strongly barrel-shaped.
The tangential distortion occurs because the camera lens is not aligned perfectly parallel to the image plane. Therefore some areas in the image may look closer than expected.