   Robust method for Darboux Frame calculation
  Hello folks,I'm trying to fit a robust method to calculate the Darboux Frame. Currently i'm using a SVD based method, but it isn't good at handling cases where the the point matrix is near singular (like for planes). Do you know a better, more robust method (and fast)? Or should I try to find a robust SVD instead?Thanks for your admittance!   
  The Darboux frame is not defined for umbilical points, so if you are trying to compute it on points where the surface is nearly flat, it is possible that the problem is not well defined, so no robust solution will exist.   ;  How do I test if the SVD calculation for a certain matrix would be unstable? Determinant near zero? If yes, how near? Or are there more cases?   ;  Every time the two eigenvalues are very similar, you'll have instability.Why do you want to use the Darboux frame? What are you ultimately trying to do?;  I need to calculate the local surface curvature to perform curvature based sementation on a triangular mesh surface.   ;  You do not need the Darboux frame for this.  Just compute the curvature and metric tensors, which then are used in solving for the principal curvatures (from which you can compute mean curvature and/or Gaussian curvature).If you have a triangle mesh, you can estimate the curvatures.  For an example, see Conformal Mapping, which uses a class in Wild Magic for estimating principal curvatures and directions (files Wm4MeshCurvature.{h,cpp}).   ;  Interesting. I shall have a look at this. Thank you!   ;  This method seems to be much more reliable than my old one. Thank you!One more question:The method computes the normal derivative matrix and it's inverse. However, for planar points, the invert fails. Is there a good standard method to handle this case? Ideal would be some normal derivative matrix that yields (0,0,0) for min max direction and 0.0 for the curvatures.   ;  The DNormal matrix is never inverted, but the WWTrn matrix is.  When I tested this code with a planar mesh, the WWTrn matrix was invertible.  However, the DWTrn matrix was (effectively) zero.  I added code to detect this case.  When it happens, the min/max curvatures are set to zero and the tangent vectors are U and V (computed from the normal N).  The modified file has been posted to my web site.