An area of geometry that grew from the study of perspective, with Desargues and Poncelet being early developers of the field. From different observers’ perspectives, angles and lengths may change, but collinearity will remain invariant. In the real projective plane, ellipses, parabolae, and hyperbolae are equivalent, each being viewed as a conic that has 0, 1, or 2 points at infinity. See Bézout’s theorem, duality, homogeneous coordinates, projective space, projective transformation.