Figure 3.

Obtuse triangles in distance computations. Arrows show the growing direction of the distance (i.e., it's negative gradient direction). The front would first reach point A, then C, and finally B. Therefore, C is only supported by A so we can not recover the actual direction of the front. As we restrict the update to come from within the triangle (that is, the gradient direction must lay within the angle

, a situation like this can only occur if

is obtuse.