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这个网站可能已经停止运营了，我们稍后更新一下doc，把这个链接移除吧
曲线切线的这个方法应该是比较明确的，简单画了一个图：
13cb832d-e57a-4101-a2e5-d5786ceadf2a.png
就是求一条 cubicbeziercurve 上某一点的切线方向，参数 t 就是曲线长度归一化 [0,1] 上的某一点，具体的求解过程可以参考源码
