Introduction - If you have any usage issues, please Google them yourself
Corner: The most intuitive impression that the level of vertical change in both directions on both the larger point, that is, Ix, Iy are larger
Edge: only in the horizontal or vertical direction only in a greater change, that is, Ix and Iy, only one of the larger
Flat areas: the level of vertical changes in direction in both the smaller, that is, Ix, Iy are smaller