The distance approximation heuristic. (a) Given two patches A (in green) and B (in yellow), we consider their inner spheres (pivots are marked using a darker shade). (b) Then, we enumerate all possible matches of the inner spheres. (c) If the RMSD between the inner spheres is less than 2Å, it serves as a seed for mapping the full patches. Finally we pick the superposition that yields the minimal RMSD value (circled in red).