计算两线段平面交点

struct StdPoint
{
    double x, y, z;
};

#define IS_BETWEEN(a, x, b) (((a) <= (x) && (x) <= (b)) || ((b) <= (x) && (x) <= (a)))

BOOL Get_Crossing_FS(StdPoint& ptCross, StdPoint a1, StdPoint a2, StdPoint b1, StdPoint b2, BOOL bExtend)
{
    // 构建向量
    double a_dx    = a2.x - a1.x;
    double a_dy    = a2.y - a1.y;

    double b_dx    = b2.x - b1.x;
    double b_dy    = b2.y - b1.y;

    double div = a_dx * b_dy - b_dx * a_dy;

    // 判断平行(比较的参数值为0.01,DBL_EPSILON_FS值过小),经纬度精度不够
    if (-0.00001f <= div || div <=0.00001f)
        return FALSE;

   double lambda = ((b1.x - a1.x) * b_dy - b_dx * (b1.y - a1.y)) / div;

    ptCross.x    = a1.x + lambda * a_dx;
    ptCross.y    = a1.y + lambda * a_dy;
    

    if (bExtend == TRUE)
        return TRUE;

   
      //不延长情况判断点是否在a1_a2 b1_b2内部
      if (EqualDouble(a1.x,a2.x) == FALSE && IS_BETWEEN(a1.x,ptCross.x,a2.x) == FALSE)
            return FALSE;
        if (EqualDouble(a1.y,a2.y) == FALSE && IS_BETWEEN(a1.y,ptCross.y,a2.y) == FALSE)
            return FALSE;
        if (EqualDouble(b1.x,b2.x) == FALSE && IS_BETWEEN(b1.x,ptCross.x,b2.x) == FALSE)
            return FALSE;
        if (EqualDouble(b1.y,b2.y) == FALSE && IS_BETWEEN(b1.y,ptCross.y,b2.y) == FALSE)
            return FALSE;
        return TRUE;
  }