Type of convexity of vtkHexahedron

Hello everyone,

How to choose for the vtkHexahedron whether the top surface is concave or convex? E.g. in the figure the top surface is convex and the chosen diagonal is between 0 and 2 (and 4 and 6).
Does anyone knows whether I can make vtk to triangulate it in the way that the top surface is concave (and the bottom one is convex)?