Mapping quads to quads
A common operation for texture mapping: find a transformation mapping a rectangular image to a quadrilateral.
Goal: solve a somewhat more general problem: find a linear transform mapping one quad (A,B,C,D) to another (a,b,c,d). Assume all points given in homogeneous coordinates.
Idea: to define a 3x3 linear transform (=matrix) M, it is sufficient to define its action on three vectors.
Choose suitable points for which we know what the images should be.