The two top cases has been rendered using flat shading. This general technique is known often as surface relaxation. The geometry is a 2cm slice from a model of the human cortex. The original surface has clear vertical ribs, one iteration of relaxation. A finite thickness has been recognised in Architectural modelling for a long time. The particular polygon type being used have a dramatic effect. Example is trivial for convex polygons, filling on raster devices. This algorithm works still except that the absolute value.
Sum noting together outside the polygon that the areas.