  We calculate the minimal surface bounded by four-sided figures whose
projection on a plane is a rectangle, starting with the bilinear interpolation
and using, for smoothness, the Chebyshev polynomial expansion in our
discretized numerical algorithm to get closer to satisfying the zero mean
curvature condition. We report values for both the bilinear and improved areas,
suggesting a quantitative evaluation of the bilinear interpolation. An
analytical expression of the Schwarz minimal surface with polygonal boundaries
and its 3-dimensional plot is also given.
