An active set strategy to generate quadrilateral grids

We consider the problem of the generation of quadrilateral grids on planar domains. This problem is numerically solved by a two phases method: an iterative procedure based on the well-known variational approach, and an active set procedure to obtain unfolded quadrilaterals. This second phase is performed only when it is really necessary, in fact the first phase alone gives satisfactory results on a large number of domains. This two phases approach provides a robust method with low computational cost. Numerical experiments show that this method is able to generate unfolded grids also on complex domains.