The value of Ui depends on {Uj }j with Uj Ui . Compute (U) i using an optimal ordering. Front propagation, O(N log(N)) operations. triangulation with no obtuse angles. Bad Good u = (U) i is the solution of max(u Ui 1,j, u Ui+1,j, 0)2+ max(u Ui,j 1, u Ui,j+1, 0)2 = h2W2 i,j (upwind derivatives) Isotropic H(x) = W(x)2Id, square grid. Surface (ﬁrst fundamental form) xi xj xk Good Bad xi+1,j xi,j+1 xi,j