A transformation that maps lines to lines is a projective transformation. Here, parallel lines can be formed with an angle that may intersect at some point. For example, the lines between a - d and d - c on the left side of the following diagram are not parallel and could meet at some point in time. At the same time, the lines between a - d and b - c are parallel and will never meet. After applying the projective transform, none of the sides will have parallel lines:
Fig 4.54: Applying projective transformation
 You can learn more about projective transformation at https://en.wikipedia.org/wiki/Homography.
In the next section of the chapter, we will learn about image rotation using the OpenCV library.
