任何多边形都可定义为一组三角形的集合，因此三角形在3d实现中应用十分广泛。

Triangles are popular with 3d implementations because you can define any polygon as a set of triangles.

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求解轨迹线集合所形成的外包多边形和内部顺时针环，得到的多边形即为临界多边形。

Get the enclosing polygon and internal clockwise loops that was formed by the track line segments, finally the result polygons are the final NFPs.

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