  Three-dimensional frame fields computed on CAD models often contain singular
curves that are not compatible with hexahedral meshing. In this paper, we show
how CAD feature curves can induce non meshable 3-5 singular curves and we study
four different approaches that aims at correcting the frame field topology. All
approaches consist in modifying the frame field computation, the two first ones
consisting in applying internal constraints and the two last ones consisting in
modifying the boundary conditions. Approaches based on internal constraints are
shown not to be very reliable because of their interactions with other
singularities. On the other hand, boundary condition modifications are more
promising as their impact is very localized. We eventually recommend the 3-5
singular curve boundary snapping strategy, which is simple to implement and
allows to generate topologically correct frame fields.
