判断质点在四边形之内

bool InQd(FXY a[4],FXY f1) //质点是否在四边形内
{
// char st[250];
CMyDlg *pDlg = (CMyDlg *)(AfxGetApp()->GetMainWnd());
int i,i1;
double sum,sum2;
FXY b[3];
sum2=0.;
b[0]=a[0];b[1]=a[1];b[2]=a[2];sum2+=area(b);
b[0]=a[0];b[1]=a[3];b[2]=a[2];sum2+=area(b);

sum=0.;
for(i=0;i<4;i++)
{
i1=i+1;i1%=4;
b[0]=a[i];b[1]=a[i1];b[2]=f1;
sum+=area(b);
}
// return sum-(sum2+0.00000000001);
if(sum>sum2+0.0000000001)
{return false;}//pDlg->m_lst.AddString("不在四边形内！");
else
{
// sprintf(st,"M4x= %9.3f %9.3f %9.3f %9.3f",a[0].x,a[1].x,a[2].x,a[3].x);pDlg->m_lst.AddString(st);
return true;
};//在里面pDlg->m_lst.AddString("在四边形内！");
}

double area(FXY b[3]) //S△=√[p(p-a)(p-b)(p-c)] [p=1/2(a+b+c)]
{