题意：
给定一个凸多边形和一条直线
求这个多边形被切割后的面积
对于代码中的
“
d1=dblcmp(s1=cross( p.node[i],s,e ));//跨立
d2=dblcmp(s2=cross( p.node[i+1],s,e ));//跨立
”
不是很理解。。。求指教
View Code
1 #include<stdio.h> 2 #include<string.h> 3 #include<stdlib.h> 4 #include<math.h> 5 #include<algorithm> 6 using namespace std; 7 const double eps=1e-8; 8 const int maxn = 505; 9 struct point{ 10 double x,y; 11 }; 12 struct ploy{ 13 point node[ maxn ]; 14 int n; 15 }; 16 point left,right,s; 17 ploy p,p1,p2; 18 19 double cross( point a,point b,point c ){ 20 return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x); 21 } 22 23 double ploy_area( ploy res ){ 24 double ans=0; 25 res.node[ res.n ]=res.node[0]; 26 for( int i=0;i<res.n;i++ ){ 27 ans+=cross( s,res.node[i],res.node[i+1] ); 28 } 29 return ans; 30 } 31 32 int dblcmp(double a) {return a<-eps?-1:a>eps?1:0;} 33 34 ploy cut( ploy p,point s,point e ){ 35 point tmp; 36 ploy bb; 37 int cnt=0; 38 for( int i=0;i<p.n;i++ ){ 39 int d1,d2; 40 double s1,s2; 41 d1=dblcmp(s1=cross( p.node[i],s,e ));//跨立 42 d2=dblcmp(s2=cross( p.node[i+1],s,e ));//跨立 43 if( d1>=0 ){ 44 bb.node[ cnt ]=p.node[ i ]; 45 cnt++; 46 } 47 if( d1*d2<0 ){ 48 tmp.x=(s2*p.node[i].x-s1*p.node[i+1].x)/(s2-s1); 49 tmp.y=(s2*p.node[i].y-s1*p.node[i+1].y)/(s2-s1); 50 bb.node[ cnt ]=tmp; 51 cnt++; 52 } 53 } 54 bb.n=cnt; 55 bb.node[ cnt ]=bb.node[ 0 ]; 56 return bb; 57 } 58 59 int main(){ 60 while( scanf("%d",&p.n),p.n ){ 61 for( int i=0;i<p.n;i++ ){ 62 scanf("%lf%lf",&p.node[ i ].x,&p.node[ i ].y); 63 } 64 p.node[ p.n ]=p.node[ 0 ]; 65 scanf("%lf%lf%lf%lf",&left.x,&left.y,&right.x,&right.y); 66 s.x=s.y=0; 67 p1=cut( p,left,right ); 68 p2=cut( p,right,left ); 69 int res1,res2; 70 res1=int(fabs(ploy_area( p1 ))/2+0.5); 71 res2=int(fabs(ploy_area( p2 ))/2+0.5); 72 printf("%d %d\n",res1>res2?res1:res2,res1>res2?res2:res1); 73 } 74 return 0; 75 }