Tckmap -contrast curvature

Hi MRtrix3 team,
I want to use tckmap command to acquire a track-weighted map based on the curvature of the streamlines. But I cannot find the detailed definition of curvature used in mrtrix3. I am curious about what method used in mrtrix3 to calculate the curvature value of the streamline. Is it based on differentiation or tangent method?

Thank you.