Concave Polygons

For concave polygons, the zeroArea[a,b,c] function is replaced with the zeroPerim[a,b,c] function.

The shape functions for a concave quadrilateral can easily be constructed. The interpolant contains a square root. The branch points of the square root terms add a discontinuity in the gradient of the interpolant. This discontinuity is necessary for the modeling of re-entrant corners.

Figure 5. Interpolants on concave polygons do not require tesselation.