Sketch of the local streamline topologies in the RQ-plane for the three cases P = 0 (left), P  >  0 (middle), and P  <  0 (right). The thick line is the discriminant surface Δ. The red line marks the separation between the stretching and compressing regions, and the dotted line indicate the separation between stable and unstable topologies. Shaded areas indicate topologies associated with local volumetric contraction (yellow) and expansion (green) emerging in compressible cases only.