在曲面上做一些网格，你会看到一个网上的每一小块儿，都是小的平行四边形，我们要弄清楚它的面积和法向量是什么。
You will get some sort of mesh grid on your surface and you will look at a little parallelogram that is an elementary piece of that mesh and figure out what is its area and normal vector.
现在的问题是，曲面上这一小块儿的面积是什么，及其法向量是什么？
Well, the question we have now is what is the area of this little piece of surface and what is its normal vector?
平面上有一个三角形,用两个向量描述,记作A、B，怎样求三角形的面积？
Let's start with a triangle in the plane Well, then we need two vectors to describe it say A and B here. How do we find the area of a triangle?
应用推荐