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这是点到线段的距离 不是点到直线的距离：

 public double PointLine_Disp(double xx, double yy, double x1, double y1, double x2, double y2)

        {             double a, b, c, ang1, ang2, ang, m;             double result = 0;             //分别计算三条边的长度             a = Math.Sqrt((x1 - xx) * (x1 - xx) + (y1 - yy) * (y1 - yy));             if (a == 0)                 return -1;             b = Math.Sqrt((x2 - xx) * (x2 - xx) + (y2 - yy) * (y2 - yy));             if (b == 0)                 return -1;             c = Math.Sqrt((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));             //如果线段是一个点则退出函数并返回距离             if (c == 0)             {                 result = a;                 return result;             }             //如果点(xx,yy到点x1,y1)这条边短             if (a < b)             {                 //如果直线段AB是水平线。得到直线段AB的弧度                 if (y1 == y2)                 {                     if (x1 < x2)                         ang1 = 0;                     else                         ang1 = Math.PI;                 }                 else                 {                     m = (x2 - x1) / c;                     if (m - 1 > 0.00001)                         m = 1;                     ang1 = Math.Acos(m);

                    if (y1 >y2)

                        ang1 = Math.PI*2  - ang1;//直线(x1,y1)-(x2,y2)与折X轴正向夹角的弧度                 }                 m = (xx - x1) / a;                 if (m - 1 > 0.00001)                     m = 1;                 ang2 = Math.Acos(m);

                if (y1 > yy)

                    ang2 = Math.PI * 2 - ang2;//直线(x1,y1)-(xx,yy)与折X轴正向夹角的弧度                                 ang = ang2 - ang1;                 if (ang < 0) ang = -ang;                                 if (ang > Math.PI) ang = Math.PI * 2 - ang;                 //如果是钝角则直接返回距离                 if (ang > Math.PI / 2)                     return a;                 else                     return a * Math.Sin(ang);             }             else//如果(xx,yy)到点(x2,y2)这条边较短             {                 //如果两个点的纵坐标相同，则直接得到直线斜率的弧度                 if (y1 == y2)                     if (x1 < x2)                         ang1 = Math.PI;                     else                         ang1 = 0;                 else                 {                     m = (x1 - x2) / c;                     if (m - 1 > 0.00001)                         m = 1;                     ang1 = Math.Acos(m);                     if (y2 > y1)                         ang1 = Math.PI * 2 - ang1;                 }                 m = (xx - x2) / b;                 if (m - 1 > 0.00001)                     m = 1;                 ang2 = Math.Acos(m);//直线(x2-x1)-(xx,yy)斜率的弧度

                if (y2 > yy)

                    ang2 = Math.PI * 2 - ang2;                 ang = ang2 - ang1;                 if (ang < 0) ang = -ang;                 if (ang > Math.PI) ang = Math.PI * 2 - ang;//交角的大小                 //如果是对角则直接返回距离                 if (ang > Math.PI / 2)                     return b;                 else                     return b * Math.Sin(ang);//如果是锐角，返回计算得到的距离             }

        }