/// <summary>
 /// 獲得貝塞爾曲線
 /// </summary>
 /// <param name="currentPt">當前點</param>
 /// <param name="lastPt">上一個點</param>
 /// <param name="nextPt1">下一個點1</param>
 /// <param name="nextPt2">下一個點2</param>
 /// <returns></returns>
 private BezierSegment GetBezierSegment(Point currentPt, Point lastPt, Point nextPt1, Point nextPt2)
 {
  //計算中點
  var lastC = GetCenterPoint(lastPt, currentPt);
  var nextC1 = GetCenterPoint(currentPt, nextPt1); //貝塞爾控制點
  var nextC2 = GetCenterPoint(nextPt1, nextPt2);
  //計算相鄰中點連線跟目的點的垂足
  //效果并不算太好，因為可能點在兩個線上或者線的延長線上，計算會有誤差
  //所以就直接使用中點平移方法。
  //var C1 = GetFootPoint(lastC, nextC1, currentPt);
  //var C2 = GetFootPoint(nextC1, nextC2, nextPt1);
  //計算“相鄰中點”的中點
  var c1 = GetCenterPoint(lastC, nextC1);
  var c2 = GetCenterPoint(nextC1, nextC2);
  //計算【"中點"的中點】需要的點位移
  var controlPtOffset1 = currentPt - c1;
  var controlPtOffset2 = nextPt1 - c2;
  //移動控制點
  var controlPt1 = nextC1 + controlPtOffset1;
  var controlPt2 = nextC1 + controlPtOffset2;
  //如果覺得曲線幅度太大，可以將控制點向當前點靠近一定的系數。
  controlPt1 = controlPt1 + 0 * (currentPt - controlPt1);
  controlPt2 = controlPt2 + 0 * (nextPt1 - controlPt2);
  var bzs = new BezierSegment(controlPt1, controlPt2, nextPt1, true);
  return bzs;
 }