Create double parabolic surface in STL file

import math


def h(x, y, xmin, xmax, ymin, ymax, hmin, hmax):

    x1 = (x - xmin) / (xmax - xmin) * 2.0 - 1.0
    y1 = (y - ymin) / (ymax - ymin) * 2.0 - 1.0
    return ((x1 * x1 - y1 * y1) + 1)* (hmax - hmin) * 0.5 + hmin


out = "hall2.stl"

f = open(out, 'w')

n = 20
m = 20

xmin, xmax = -20.0, 20.0
ymin, ymax = -15.0, 15.0
hmin, hmax = 10.0, 20.0

dx = 1.0 * (xmax - xmin) / m
dy = 1.0 * (ymax - ymin) / n

xmin -= dx
xmax += dx
ymin -= dy
ymax += dy


f.write("solid Hall (Meshed)\n")
for i in range(0, m+2):
    for j in range(0, n+2):
        x = xmin + i * dx
        y = ymin + j * dy
        x1 = x + dx
        y1 = y + dy

        ha = h(x, y, xmin, xmax, ymin, ymax, hmin, hmax)
        hb = h(x1, y, xmin, xmax, ymin, ymax, hmin, hmax)
        hc = h(x1, y1, xmin, xmax, ymin, ymax, hmin, hmax)
        hd = h(x, y1, xmin, xmax, ymin, ymax, hmin, hmax)

        f.write("\tfacet normal %.6f %.6f %.6f\n" % (0.0, 0.0, 1.0))
        f.write("\t\touter loop\n")
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % (1000.0*x, 1000.0*y, 1000.0*ha))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % (1000.0*x1, 1000.0*y, 1000.0*hb))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % (1000.0*x1, 1000.0*y1, 1000.0*hc))
        f.write("\t\tendloop\n")
        f.write("\tendfacet\n")
        f.write("\tfacet normal %.6f %.6f %.6f\n" % (0.0, 0.0, 1.0))
        f.write("\t\touter loop\n")
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % (1000.0*x1, 1000.0*y1, 1000.0*hc))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % (1000.0*x, 1000.0*y1, 1000.0*hd))
        f.write("\t\t\tvertex %.6f %.6f %.6f\n" % (1000.0*x, 1000.0*y, 1000.0*ha))
        f.write("\t\tendloop\n")
        f.write("\tendfacet\n")

f.write("endsolid Mesh\n")

f.close()