Figure 3. Gauss image of a vertex vi. The cycle of faces f1,...,f6 incident with vi defines a cycle g(vi) of unit normal vectors n1,..., n6 on the unit sphere which form the Gauss image g(v). The kink angle between faces fk, fk+1 coincides with the spherical edge length nk, nk+1. In the case shown here the Gauss image polygon g(v) has no self-intersections, so it is the boundary of two spherical domains — one of them contains unit vectors like nv which point to the outside of the primal mesh; it is called the interior of g(v). We can observe the sign of curvature (negative, from the fact that the two cycles have opposite orientations). Further, any interior point nv of the Gauss image polygon g(v) can be viewed as an auxiliary unit normal vector associated with the vertex vi. Right: The surface with point x and normal vector illustrates the smooth situation.