Multiple-knot and rational cubic beta-splines

  • Authors:
  • Barry Joe

  • Affiliations:
  • Univ. of Alberta, Edmonton, Canada

  • Venue:
  • ACM Transactions on Graphics (TOG)
  • Year:
  • 1989

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Abstract

Goodman (Properties of Beta-splines. J. Approx. Theory 44, 2 (June 1985), 132-153) gave an explicit formula for cubic Beta-splines on a uniform knot sequence with varying &bgr;1 and &bgr;2 values at the knots. We establish an alternative explicit formula for cubic Beta-splines on a nonuniform knot sequence with constant &bgr;1 = 1 and varying &bgr;2 values at the knots. This alternative formula can also be used if the knot sequence contains multiple knots, and is useful for knot insertion. We show how to efficiently evaluate a cubic Beta-spline curve at many values using this formula. We introduce rational cubic Beta-spline curves and surfaces that have extra weight parameters for shape control, and show that they satisfy the same geometric continuity conditions and properties as nonrational cubic Beta-spline curves and surfaces.