C#的箭头算法

/// <summary>
       /// 绘制箭头
         /// </summary>
          /// <param name="graphics"></param>
         /// <param name="startPoint">起始点</param>
         /// <param name="endPoint">终止点</param>
          private void DrawArrowHead(Graphics graphics, PointF startPoint, PointF endPoint)
        {
              double distance = Math.Abs(Math.Sqrt(
                 (startPoint.X - endPoint.X) * (startPoint.X - endPoint.X) +
                 (startPoint.Y - endPoint.Y) * (startPoint.Y - endPoint.Y)));
            if (distance == 0)
             {
                 return;
            }
 
             double xa = endPoint.X + ArrowLength * ((startPoint.X - endPoint.X)
                 + (startPoint.Y - endPoint.Y) / RelativeValue) / distance;
             double ya = endPoint.Y + ArrowLength * ((startPoint.Y - endPoint.Y)
                 - (startPoint.X - endPoint.X) / RelativeValue) / distance;
            double xb = endPoint.X + ArrowLength * ((startPoint.X - endPoint.X)
                - (startPoint.Y - endPoint.Y) / RelativeValue) / distance;
            double yb = endPoint.Y + ArrowLength * ((startPoint.Y - endPoint.Y)
                + (startPoint.X - endPoint.X) / RelativeValue) / distance;
 
             PointF[] polygonPoints ={ 
                 new PointF(endPoint.X , endPoint.Y), 
                 new PointF( (float)xa   ,  (float)ya),
                 new PointF( (float)xb   ,  (float)yb)};
            DrawArrowHead(graphics, polygonPoints);
         }
 

 

         private static void DrawArrowHead(Graphics graphics, PointF[] points)
         {
            graphics.DrawPolygon(Pens.Red, points);
             graphics.FillPolygon(new SolidBrush(Color.Red), points);
         }