If the two level curves are tangent to each other that means they have the same tangent line. That means that the normal vectors should be parallel. Let me maybe draw a picture here.

如果两个等高线相切，说明它们的切线相同，也就是说他们的法向量应该平行，我再在这里画个图。

youdao

Take high level semantic information and low level picture visual characters as design patent characters, the two kinds of information cut down each other's shortage.

特征提取采用底层信息和高层语意相结合的方法，弥补了相互的不足；

youdao

One, two, three, four, so on each level of the picture I'm drawing, I've seem only to be looking at each number once.

，2，3，4，可见，在我画的,这幅图的每一层上，对每个数字,都只会用到一次。

哈佛公开课 - 计算机科学课程节选

These two top of the line TVs offers a revolutionary level of picture quality.

ENGADGET: Panasonic unveils Infinite Black Ultra Panel plasmas for 2012 (eyes-on)