public static void DrawRoundRectangle (Context cr, double x, double y, double r, double w, double h)
{
const double ARC_TO_BEZIER = 0.55228475;
double radius_x = r;
double radius_y = r / 4;
if (radius_x > w - radius_x)
radius_x = w / 2;
if (radius_y > h - radius_y)
radius_y = h / 2;
double c1 = ARC_TO_BEZIER * radius_x;
double c2 = ARC_TO_BEZIER * radius_y;
cr.NewPath ();
cr.MoveTo (x + radius_x, y);
cr.RelLineTo (w - 2 * radius_x, 0.0);
cr.RelCurveTo (c1, 0.0, radius_x, c2, radius_x, radius_y);
cr.RelLineTo (0, h - 2 * radius_y);
cr.RelCurveTo (0.0, c2, c1 - radius_x, radius_y, -radius_x, radius_y);
cr.RelLineTo (-w + 2 * radius_x, 0);
cr.RelCurveTo (-c1, 0, -radius_x, -c2, -radius_x, -radius_y);
cr.RelLineTo (0, -h + 2 * radius_y);
cr.RelCurveTo (0.0, -c2, radius_x - c1, -radius_y, radius_x, -radius_y);
cr.ClosePath ();
}