A control polygon scheme for design of planar C2 PH quintic spline curves

  • Authors:
  • Francesca Pelosi;Maria Lucia Sampoli;Rida T. Farouki;Carla Manni

  • Affiliations:
  • Dipartimento di Scienze Matematiche ed Informatiche, Università di Siena, Pian dei Mantellini 44, 53100 Siena, Italy;Dipartimento di Scienze Matematiche ed Informatiche, Università di Siena, Pian dei Mantellini 44, 53100 Siena, Italy;Department of Mechanical and Aeronautical Engineering, University of California, Davis, CA 95616, USA;Dipartimento di Matematica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy

  • Venue:
  • Computer Aided Geometric Design
  • Year:
  • 2007

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Abstract

A scheme to specify planar C^2 Pythagorean-hodograph (PH) quintic spline curves by control polygons is proposed, in which the ''ordinary''C^2 cubic B-spline curve serves as a reference for the shape of the PH spline. The method facilitates intuitive and efficient constructions of open and closed PH spline curves, that typically agree closely with the corresponding cubic B-spline curves. The C^2 PH quintic spline curve associated with a given control polygon and knot sequence is defined to be the ''good'' interpolant to the nodal points of the C^2 cubic spline curve with the same B-spline control points, knot sequence, and end conditions-it may be computed to machine precision by just a few Newton-Raphson iterations from a close starting approximation. The relation between the PH spline and its control polygon is invariant under similarity transformations. Multiple knots may be inserted to reduce the order of continuity to C^1 or C^0 at specified points, and by means of double knots the PH splines offer a linear precision and local shape modification capability. Although the non-linear nature of PH splines precludes proofs for certain features of cubic B-splines, such as convex-hull confinement and the variation-diminishing property, this is of little practical significance in view of the close agreement of the two curves in most cases (in fact, the PH spline typically exhibits a somewhat better curvature distribution).