static Rect rectRelative(double x1, double y1, double x2, double y2, Rect r) {
  ret rectFromPoints(
    iround_blend(r.x, r.x2(), x1),
    iround_blend(r.y, r.y2(), y1),
    iround_blend(r.x, r.x2(), x2),
    iround_blend(r.y, r.y2(), y2));
}