title: Structural invariance of spatial Pythagorean hodographs
abstract: The structural invariance of the four-polynomial characterization for three-dimensional Pythagorean hodographs introduced by Dietz et al. (1993), under arbitrary spatial rotations, is demonstrated. The proof relies on a factored-quaternion representation for Pythagorean hodographs in three-dimensional Euclidean space-a particular instance of the "PH representation map" proposed by Choi et al. (2002)-and the unit quaternion description of spatial rotations. This approach furnishes a remarkably simple derivation for the polynomials u(t), upsilon (t), p(t), q(t) that specify the canonical form of a rotated Pythagorean hodograph, in terms of the original polynomials u(t), upsilon (t), p(t), q(t) and the angle theta and axis n of the spatial rotation. The preservation of the canonical form of PH space curves under arbitrary spatial rotations is essential to their incorporation into computer-aided design and manufacturing applications, such as the contour machining of free-form surfaces using a ball-end mill and realtime PH curve CNC interpolators
keyphrases: structural invariance spatial Pythagorean hodographs four-polynomial characterization arbitrary spatial rotations spatial rotations PH representation map unit quaternion description contour machining free-form surfaces ball-end mill 3D Pythagorean hodographs factored quaternion representation 3D Euclidean space CAD/CAM real-time PH curve CNC interpolators