While shrinking facets form on the surface. The facet normals point in the directions in which the etching rate has a maximum. Here etching a hemisphere as an example. The left diagram is a polar plot of the normal directions at the finite difference nodes. Each red dot represents one node.
Triangles represent the directions, squares the directions and rectangles the directions.
The directions are the maxima in the etching rate. Notice the clustering of the red dots in the polar plots at the rectangles. Etching a concave body enlargens the body. In this process facets in the slowest etching directions are formed. Here the example of an etched hemispherical pit.