  Bicubic four-sided patches are widely used in computer graphics, CAD/CAM
systems etc. Their flexibility is high and enables to compress a surface
description before final rendering. However, computer graphics hardware
supports only triangular meshes. Therefore, four-sided bicubic patches are
approximated by a triangular mesh. The border curves of a bicubic patch are of
degree 3, while diagonal and anti-diagonal curves are of degree 6. Therefore
the resulting shape and texturing depend on the actual mapping, i.e. how the
tessellation of a bicubic patch is made. The proposed new modification of the
Hermite bicubic patch, the HS-patch, is a result of additional restriction put
on the Hermite bicubic patch formulation - the diagonal and anti-diagonal
curves are of degree 3. This requirement leads to a new Hermite based bicubic
four-sided patch with 12 control points and another 4 control points, i.e.
twist vectors, are computed from those 12 control points.
