*par Constraints *The filter requires an image with at least two dimensions and a vector *length of at least The theory supports extension to scalar but *the implementation of the itk vector classes do not **The template parameter TRealType must be floating point(float or double) or *a user-defined "real" numerical type with arithmetic operations defined *sufficient to compute derivatives. **\par Performance *This filter will automatically multithread if run with *SetUsePrincipleComponents