The system (7-10) has two theoretical solutions (11-12), though there is only one that makes physical sense given a velocity map. Again, we used a geometrical argument to select the appropriate solution. We observed that even though a wavefront can make a sharp turn, the corresponding rays cannot (see the examples in the next section). We define a turn as ``sharp'' if it happens over a very small number of samples (say, three). Consequently, we decided to impose the condition that the correct solution is the one represented by the point farthest away from the preceding one on the same ray (Figure 3).