public void render(Color c, int stroke, int markType) {
    if (c == null) c = Color.RED;
    V.pushColor(c);
    if (defined()) {
      seg.render(c, stroke, markType);
    } else {

      for (int i = 0; i < 2; i++) {
        if (i == 1 && discA == discB) continue;

        EdObject obj = object(i);
        if (obj instanceof EdDisc) {
          EdDisc d = (EdDisc) obj;

          stroke = STRK_RUBBERBAND;
          V.pushStroke(stroke);
          double r = Math.max(d.getRadius() - 4, 2.0);
          V.drawCircle(d.getOrigin(), r);
          V.popStroke();
        } else {
          Tools.unimp("rendering undefined bitangents of polygons");
        }
      }
    }
    V.popColor();
  }
 public BiTangent(FPoint2 pt1, String lbl1, FPoint2 pt2, String lbl2) {
   EdDisc d1 = new EdDisc(pt1, 0);
   d1.setLabel(lbl1);
   EdDisc d2 = new EdDisc(pt2, 0);
   d2.setLabel(lbl2);
   construct(d1, d2);
 }
  private void construct(EdDisc a, EdDisc b) {
    this.discA = a;
    this.discB = b;

    if (EdDisc.partiallyDisjoint(a, b)) {

      // if (!UHullMain.oldBitanMethod())
      {
        final boolean db = false;

        if (a.getRadius() == b.getRadius()) {
          FPoint2 oa = a.getOrigin(), ob = b.getOrigin();

          FPoint2 n = new FPoint2(-(ob.y - oa.y), ob.x - oa.x);
          n.normalize();
          n.x *= a.getRadius();
          n.y *= a.getRadius();
          seg = new DirSeg(FPoint2.add(oa, n, null), FPoint2.add(ob, n, null));
          return;
        }

        boolean swap = a.getRadius() > b.getRadius();

        if (swap) {
          b = (EdDisc) discA;
          a = (EdDisc) discB;
        }

        if (db && T.update())
          T.msg(
              "BiTangent construct, arad="
                  + Tools.f(a.getRadius())
                  + " brad="
                  + Tools.f(b.getRadius())
                  + " swap="
                  + swap
                  + " origin.a="
                  + T.show(a.getOrigin()));

        FPoint2 oa = a.getOrigin();
        FPoint2 ob = b.getOrigin();

        double U = ob.x, V = ob.y;
        double A = oa.x - U, B = oa.y - V;
        double R1 = a.getRadius();
        double R2 = b.getRadius();
        double S = R2 - R1;

        double x1, y1, x2, y2;
        x1 = A;
        y1 = B;

        boolean secondRoot;
        boolean altSlope = Math.abs(B) < Math.abs(A);
        if (!altSlope) {

          double C1 = S * S / B, C2 = -A / B;
          double qA = 1 + C2 * C2, qB = 2 * C1 * C2, qC = C1 * C1 - S * S;
          double root = Math.sqrt(qB * qB - 4 * qA * qC);
          x2 = (-qB - root) / (2 * qA);
          y2 = C1 + C2 * x2;

          secondRoot = MyMath.sideOfLine(x2, y2, A, B, 0, 0) < 0;

          if (swap ^ secondRoot) {
            x2 = (-qB + root) / (2 * qA);
            y2 = C1 + C2 * x2;
          }
        } else {

          double C1 = S * S / A, C2 = -B / A;
          double qA = 1 + C2 * C2, qB = 2 * C1 * C2, qC = C1 * C1 - S * S;
          double root = Math.sqrt(qB * qB - 4 * qA * qC);
          y2 = (-qB - root) / (2 * qA);
          x2 = C1 + C2 * y2;

          secondRoot = MyMath.sideOfLine(x2, y2, A, B, 0, 0) < 0;

          if (swap ^ secondRoot) {
            y2 = (-qB + root) / (2 * qA);
            x2 = C1 + C2 * y2;
          }
        }
        // now grow both discs back to r1, r2

        double tx = U;
        double ty = V;

        //        if (S == 0) {
        //          FPoint2 unit = new FPoint2(-A, -B);
        //          if (swap) {
        //            unit.x = -unit.x;
        //            unit.y = -unit.y;
        //          }
        //          unit.normalize();
        //          tx += -unit.y * R1;
        //          ty += unit.x * R1;
        //        } else
        {
          double F = R1 / S;
          tx += x2 * F;
          ty += y2 * F;
        }

        if (db && T.update())
          T.msg("adding offset to both points: " + tx + ", " + ty + T.show(new FPoint2(tx, ty)));
        x1 += tx;
        y1 += ty;
        x2 += tx;
        y2 += ty;
        FPoint2 p1 = new FPoint2(x1, y1);
        FPoint2 p2 = new FPoint2(x2, y2);
        if (swap) {
          FPoint2 tmp = p1;
          p1 = p2;
          p2 = tmp;
        }

        seg = new DirSeg(p1, p2);

        if (db && T.update())
          T.msg(
              "swap="
                  + swap
                  + " altSlope="
                  + altSlope
                  + " secondRoot="
                  + secondRoot
                  + " dirseg="
                  + EdSegment.showDirected(p1, p2));
      }
      //      else {
      //
      //        double th = calcTheta(a, b);
      //        LineEqn eqn = new LineEqn(a.polarPoint(th + Math.PI / 2), th);
      //        double ta = eqn.parameterFor(a.getOrigin());
      //        double tb = eqn.parameterFor(b.getOrigin());
      //        seg = new DirSeg(eqn.pt(ta), eqn.pt(tb));
      //
      //      }
    }
  }