怎么样判断两线段是否相交？

bool IsLineSegmentCross(POINT pFirst1, POINT pFirst2, POINT pSecond1, POINT pSecond2)  
{  
    //每个线段的两点都在另一个线段的左右不同侧，则能断定线段相交  
    //公式对于向量(x1,y1)->(x2,y2),判断点(x3,y3)在向量的左边,右边,还是线上.  
    //p=x1(y3-y2)+x2(y1-y3)+x3(y2-y1).p<0 左侧,    p=0 线上, p>0 右侧  
    long Linep1,Linep2;  
    //判断pSecond1和pSecond2是否在pFirst1->pFirst2两侧  
    Linep1 = pFirst1.x * (pSecond1.y - pFirst2.y) +  
        pFirst2.x * (pFirst1.y - pSecond1.y) +  
        pSecond1.x * (pFirst2.y - pFirst1.y);  
    Linep2 = pFirst1.x * (pSecond2.y - pFirst2.y) +  
        pFirst2.x * (pFirst1.y - pSecond2.y) +  
        pSecond2.x * (pFirst2.y - pFirst1.y);  
    if ( ((Linep1 ^ Linep2) >= 0 ) && !(Linep1==0 && Linep2==0))//符号位异或为0:pSecond1和pSecond2在pFirst1->pFirst2同侧  
    {  
        return false;  
    }  
    //判断pFirst1和pFirst2是否在pSecond1->pSecond2两侧  
    Linep1 = pSecond1.x * (pFirst1.y - pSecond2.y) +  
        pSecond2.x * (pSecond1.y - pFirst1.y) +  
        pFirst1.x * (pSecond2.y - pSecond1.y);  
    Linep2 = pSecond1.x * (pFirst2.y - pSecond2.y) +  
        pSecond2.x * (pSecond1.y - pFirst2.y) +  
        pFirst2.x * (pSecond2.y - pSecond1.y);  
    if ( ((Linep1 ^ Linep2) >= 0 ) && !(Linep1==0 && Linep2==0))//符号位异或为0:pFirst1和pFirst2在pSecond1->pSecond2同侧  
    {  
        return false;  
    }  
    //否则判为相交  
    return true;  
}