Paper has used rotated cubic arc for fiffing a curve and has showed the method ofcurve-fitting with fitting flexuose directionofcurve.
文用单拐曲线弧作曲线填充,给出一个能符合指定弯曲方向的填充方法。
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And when I am on the curve I am on the boundary of the surface, so there is a direction along the surface that is the curve and the other one is pointing into the surface.