Why three pair matched keypoints in four cannot be colinear when computing homographies?

Question

Suppose extracting four keypoints from a real 3-d plane so that I can compute homograph matrix from 8 constraints. I cannot understand: why three pair keypoints in four cannot be collinear when computing homographies?

Answer 1

Intuitively, the homography maps a quadrilateral into another quadrilateral. If three points are collinear the quad is degenerate.