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曲率单调的二次有理h-Bézier曲线

Quadratic Rational h-Bézier Curve with Monotonic Curvature

  • 摘要: h-Bézier 曲线是一类广义 Bézier 曲线, 增加正实数权因子后得到的有理h-Bézier曲线可精确表示圆锥曲线.为了获得具有单调曲率的圆锥曲线段, 针对标准型二次有理h-Bézier 曲线, 提出曲率单调的二次有理h-Bézier曲线的构造方法. 首先, 通过引入曲率极值圆, 讨论标准型二次有理h-Bézier 曲线曲率极值的存在性; 其次, 借助曲率单调临界圆, 得到标准型二次有理h-Bézier曲线曲率单调的充要条件, 即对于首末控制顶点确定的曲线, 为得到曲率单调的圆锥曲线段, 只需中间控制顶点在曲率单调临界圆上或圆内; 进而, 根据曲率单调的充要条件, 选择合适的中间控制顶点位置、 权因子 w 和形状参数 h, 构造出曲率单调的二次有理h-Bézier 曲线. 数值实例构造出曲率单调递减或递增的二次有理h-Bézier 曲线, 与二次h-Bézier 曲线和二次有理 Bézier 曲线曲率单调的条件相比, 所提方法确定的参数 h 的范围和中间控制顶点的可选范围更广, 曲线造型更具灵活性.

     

    Abstract: The h-Bézier curves are a family of generalized Bézier curve, by adding positive real weights, the obtained rational h-Bézier curve can represent conic sections accurately. In order to obtain conic sections with monotonic curvature, a constructive method for monotonic curvature quadratic rational h-Bézier curves is proposed for standard type quadratic rational h-Bézier curves. Firstly, introducing the curvature extremum circles, the existence of the curvature extremum of the standard form quadratic rational h-Bézier curve is discussed. Secondly, using the curvature monotonic critical circles, a necessary and sufficient condition for the curvature monotonicity of standard type quadratic rational h-Bézier curves are obtained, for given initial and final control vertices, in order to obtain a conic section with monotonic curvature, just insure the intermediate control vertex on or within the curvature monotonic critical circle. Furthermore, based on the necessary and sufficient conditions for curvature monotonicity, location of the intermediate control vertex, the weight factor w, and the shape parameter h are properly selected to construct a curvature monotonic quad ratic rational h-Bézier curve. Numerical examples are constructed to obtain quadratic rational h-Bézier curves with monotonically decreasing or increasing curvature. Compared with the conditions for monotonic curvature of quadratic h-Bézier curves and quadratic rational Bézier curves, proposed method provides wider ranges of parameter h and intermediate control vertices, making the curve modeling more flexible.

     

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