bisectrix
void bisectrix(const line_parameters& l1,
bisectrix(dv.curr, dv.next, &dv.xb2, &dv.yb2);
bisectrix(dv.curr, dv.next, &dv.xb2, &dv.yb2);
bisectrix(prev, dv.curr, &dv.xb1, &dv.yb1);
bisectrix(dv.curr, dv.next, &dv.xb2, &dv.yb2);
bisectrix(lp1, lp2, &dv.xb1, &dv.yb1);
bisectrix(prev, dv.curr, &dv.xb1, &dv.yb1);
bisectrix(dv.curr, dv.next, &dv.xb2, &dv.yb2);