Homography app, wrote this as a testbed to remove/add perspective from/to images. Uses homography and svd to create matrix that can map one quad onto another. Written using MFC/C++/eigenlib/jpeglib, more details can be found here:

https://www.youtube.com/watch?v=w7J6oxk9tSA

Introduction to homography:

https://math.stackexchange.com/questions/494238/how-to-compute-homography-matrix-h-from-corresponding-points-2d-2d-planar-homog

Example of persepective removal: