dx= s->end.x - s->control2.x*3 + s->control1.x*3 - s->start.x;
dy= s->end.y - s->control2.y*3 + s->control1.y*3 - s->start.y;
dx= s->end.x - s->control2.x*3 + s->control1.x*3 - s->start.x;
dy= s->end.y - s->control2.y*3 + s->control1.y*3 - s->start.y;