public override void Derivation2At(GeoPoint2D uv, out GeoPoint location, out GeoVector du, out GeoVector dv, out GeoVector duu, out GeoVector dvv, out GeoVector duv)
{
location = PointAt(uv);
du = UDirection(uv);
dv = VDirection(uv);
double pos = GetPos(uv.y);
GeoPoint2D p2d = basisCurve2D.PointAt(pos);
duu = new GeoVector(-Math.Cos(uv.x), 0.0, -Math.Sin(uv.x));
GeoVector2D deriv1, deriv2;
if (basisCurve2D.TryPointDeriv2At(pos, out p2d, out deriv1, out deriv2))
{
ModOp rot = ModOp.Rotate(1, (SweepAngle)uv.x);
dvv = toSurface * rot * deriv2;
duv = GeoVector.NullVector;
}
else
{
dvv = GeoVector.NullVector;
duv = GeoVector.NullVector;
}
}