[vtkusers] Compute principle curvaturs

[Moti Freiman]

Hello!
I need to compute the principle curvatures at my software.
I saw that the vtkcurvatures class just compute the mean and Gaussian
curvatures. When I used the formula for extracting the principle
curvatures from mean and Gaussian,  I got undefind situation (negative
sqrt). Do someone know how I can get the principle curvatures with vtk? 
Thank you 
Moti freiman