A simple method for drawing a rational curve as two Bézier segments

  • Authors:
  • Jean Gallier

  • Affiliations:
  • Univ. of Pennsylvania, Philadelphia

  • Venue:
  • ACM Transactions on Graphics (TOG)
  • Year:
  • 1999

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Abstract

In this paper we give a simple method for drawing a closed rational curve specified in terms of control points as two Bézier segments. The main result is the following:For every affine frame (r,s) (where r), for every rational curve F(t) specified over [r,s] by some control polygon (&bgr;0, …, &bgr;m) (where the &bgr;zero are weighted control points or control vectors), the control points (&thgr;0,… ,&thgr;m (w.r.t.[r,s]) of the rational curve G(t) = F4t are given by qi=-1 ibi, where 4:RP1→RP1 is the projectivity mapping [r,s] onto RP1−]r,s]. Thus, in order to draw the entire trace of the curve F over -∞,+∞ , we simply draw the curve segmentsF[(r,s]) and G([r,s]).The correctness of the method is established using a simple geometric argument about ways of partitioning the real projective line into two disjoint segments. Other known methods for drawing rational curves can be justified using similar geometric arguments.