eigth slopes on the four corners determine the curvature of the surface through an interpolation of the one slope into another across the surface.
The curve is a quadratic spline based on a starting and an ending slope and a tension factor determining the influence of the slope on the curve.

NITROS INSTANCE: controls elaborated to surface. parametric changes cells , dimension and position

OUTSIDE NITROS: command: makesurface
editing the surface
moving the vertexes of the cornerslopes and to update command: update
to change number of cells in surface:
command: surfresolution