想象一下，你的面前有一个大圆柱，希望大家可以想象到，其法向量总是水平的。

If you imagine that you have this big cylindrical type in front of you, hopefully you can see that a normal vector is going to always be horizontal.

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我们是知道如何去求水平集的法向量的，也就是垂直于水平集的法向量。

And we know how to find a normal vector to the level set, namely the gradient vector is always perpendicular to the level set.

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但是在寻找法向量时，可以确定法向量与水平面垂直。

But, when it comes to looking for the normal vector, be rest assured that the normal vector to a horizontal plane is just vertical.

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