poj-1408-计算两线相交点坐标模板

4个点
(a1,b1) (a2,b2)组成一条线
(c1,d1) (c2,d2)组成一条线
x= (
(a2-a1)(c2-c1)(d2-b2)
+(b2-b1)*(c2-c1)*a2
-(d2-d1)*(a2-a1)*c2
)
/
(
(b2-b1)*(c2-c1)
-(d2-d1)*(a2-a1)
);
y= (b2-b1)/(a2-a1)*(x-a2)+b2;

#include<cstdio>#include<cmath>using namespace std;int n;struct left{  double x,y;}l[100];struct right{   double x,y;}r[100];struct up{   double x,y;}u[100];struct down{   double x,y;}d[100];struct coor{  double x,y;}co[100];void init(){     for(int i=1;i<=n;i++)    {      scanf("%lf",&d[i].x);      d[i].y = 0.0;    }       d[0].x = 0.0;       d[0].y = 0.0;       d[n+1].x = 1.0;       d[n+1].y = 0.0;    for(int i=1;i<=n;i++)    {      scanf("%lf",&u[i].x);      u[i].y = 1.0;    }       u[0].x = 0.0;       u[0].y = 1.0;       u[n+1].x = 1.0;       u[n+1].y = 1.0;      for(int i=1;i<=n;i++)    {      scanf("%lf",&l[i].y);      l[i].x = 0.0;    }       l[0].y = 0.0;       l[0].x = 0.0;       l[n+1].y = 1.0;       l[n+1].x = 0.0;      for(int i=1;i<=n;i++)    {      scanf("%lf",&r[i].y);      r[i].x = 1.0;    }       r[0].y = 0.0;       r[0].x = 1.0;       r[n+1].y = 1.0;       r[n+1].x = 1.0;}void toCalCoor(int sym,int a,int b){    co[sym].x = ((r[a].x-l[a].x)*(u[b].x-d[b].x)*(u[b].y-r[a].y)                 +(r[a].y-l[a].y)*(u[b].x-d[b].x)*r[a].x                 -(u[b].y-d[b].y)*(r[a].x-l[a].x)*u[b].x )                 /                 ((r[a].y-l[a].y)*(u[b].x-d[b].x)                  -(u[b].y-d[b].y)*(r[a].x-l[a].x));    co[sym].y = (r[a].y-l[a].y)/(r[a].x-l[a].x)*(co[sym].x-r[a].x)+r[a].y;    //计算相交点的坐标}double toCalArea(int i,int j){    double area = 0.0;    toCalCoor(0,i,j);    toCalCoor(1,i+1,j);    toCalCoor(2,i+1,j+1);    toCalCoor(3,i,j+1);    for(int k=0;k<4;k++)      area+=co[k].x*co[(k+1)%4].y-co[k].y*co[(k+1)%4].x;    return fabs(area*0.5);}int main(){    while(scanf("%d",&n)&&n)    {        double ans = -1.0;         init();         for(int i=0;i<=n;i++)           for(int j=0;j<=n;j++)         {             double s = toCalArea(i,j);             if(ans<s)ans = s;         }         printf("%f\n",ans);    }    return 0;}