double f1(const double x, const double y)
{ const double w = x/3, z = y/3;
  const double a = sqrt(w*w+z*z);
  const double b = sin(a)/pi;
  const double c = sqrt((w-5)*(w-5)+(z-2)*(z-2));
  const double d = cos(c)/pi;
  return b*d; }

// interesting range x, y = -60 to +60