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.

youdao

这样的2维宇宙模型就像是球体，不可能出现平行的测地线(测地线是曲面上的直线)——两直线必然在某点相交。

A two-dimensional model of such a universe would look like a sphere. It's impossible to have parallel geodesics (straight lines on a curved surface) — the two lines will cross at some point.

youdao

现在在上方的曲面上，有一个平行四边形。

Now, above that I will have a parallelogram on my surface.

youdao