Requirements to be met by surface

Geometric system + Planar, flat surface

The form can be convex or concave; however, it must be a simple polygon.

Convex polygon

Concave polygon

This means that the edges must not intersect. This would be a complex polygon.

Simple polygon

Complex polygon

Furthermore, the surfaces must not have islands or holes, i.e. the edges must not overlap.

Island

Hole

The path and the surfaces of the facade must meet these requirements.