projective transform

A more general transform than affine that can converge parallel lines to a vanishing point.

A projective transform generalizes an affine transform by allowing the bottom row of its homogeneous matrix to be anything, not just $[0,\,0,\,1]$ -- the extra freedom that lets a projective transform converge originally parallel lines toward a single vanishing point, the way a real camera's perspective does and an affine map never can.

Examples

A photo of parallel railroad tracks converges them toward a single vanishing point near the horizon -- something no affine transform can ever produce.