homography

The 3x3 projective transform matrix mapping points on one plane to their image on another.

A homography is the $3\times3$ projective transform matrix $H$ mapping every point on one plane (a physical surface, or one camera's image plane) to its corresponding point on another plane's image, up to the perspective divide. A homography has 8 degrees of freedom and can be solved from four or more point correspondences with no three collinear.

Examples

Four point correspondences between a photographed whiteboard's corners and its known rectangular shape are enough to solve for the homography that unwarps the photo flat.