Kenneth Salisbury and Christopher Tarr.
Haptic rendering of surfaces defined by implicit functions.
In Proceedings of the ASME 6th Annual Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems, pp. 61--68, 1997.

Bibtex:

@InProceedings{  salisbury:1997:HRIS,
  author = 	 {Kenneth Salisbury and Christopher Tarr},
  title = 	 {Haptic rendering of surfaces defined by implicit functions},
  booktitle =    {Proceedings of the ASME 6th Annual Symposium on
                  Haptic Interfaces for Virtual Environment and
                  Teleoperator Systems},
  pages = 	 {61--68},
  year = 	 {1997},
}