Li, Q., Wills, D., Phillips, R., Viant, W. J., Griffiths, J. G., & Ward, J. (2004). Implicit fitting using radial basis functions with ellipsoid constraint. Computer graphics forum : journal of the European Association for Computer Graphics, 23(1), 55-69. doi:10.1111/j.1467-8659.2004.00005.x