Distance to an Ellipsoid

[1]  Alan H. Barr,et al.  Global and local deformations of solid primitives , 1984, SIGGRAPH.

[2]  William E. Lorensen,et al.  Geometric clipping using Boolean textures , 1993, Proceedings Visualization '93.

[3]  Barr,et al.  Superquadrics and Angle-Preserving Transformations , 1981, IEEE Computer Graphics and Applications.