两条线段的交点（知道两条线段的各两个点）

 1 struct Point  
 2 {   
 3     double x, y;  
 4 };   
 5   
 6 bool between(double a, double X0, double X1)  
 7 {  
 8     double temp1= a-X0;  
 9     double temp2= a-X1;  
10     if ( ( temp1<1e-8 && temp2>-1e-8 ) || ( temp2<1e-6 && temp1>-1e-8 ) )  
11     {  
12         return true;  
13     }  
14     else  
15     {  
16         return false;  
17     }  
18 }  
19   
20   
21 // 判断两条直线段是否有交点，有则计算交点的坐标  
22 // p1,p2是直线一的端点坐标  
23 // p3,p4是直线二的端点坐标  
24 bool detectIntersect(Point p1, Point p2, Point p3, Point p4)  
25 {  
26     double line_x,line_y; //交点  
27     if ( (fabs(p1.x-p2.x)<1e-6) && (fabs(p3.x-p4.x)<1e-6) )  
28     {  
29         return false;  
30     }  
31     else if ( (fabs(p1.x-p2.x)<1e-6) ) //如果直线段p1p2垂直与y轴  
32     {  
33         if (between(p1.x,p3.x,p4.x))  
34         {  
35             double k = (p4.y-p3.y)/(p4.x-p3.x);  
36             line_x = p1.x;  
37             line_y = k*(line_x-p3.x)+p3.y;  
38   
39             if (between(line_y,p1.y,p2.y))  
40             {  
41                 return true;  
42             }  
43             else  
44             {  
45                 return false;  
46             }  
47         }  
48         else   
49         {  
50             return false;  
51         }  
52     }  
53     else if ( (fabs(p3.x-p4.x)<1e-6) ) //如果直线段p3p4垂直与y轴  
54     {  
55         if (between(p3.x,p1.x,p2.x))  
56         {  
57             double k = (p2.y-p1.y)/(p2.x-p1.x);  
58             line_x = p3.x;  
59             line_y = k*(line_x-p2.x)+p2.y;  
60   
61             if (between(line_y,p3.y,p4.y))  
62             {  
63                 return true;  
64             }  
65             else  
66             {  
67                 return false;  
68             }  
69         }  
70         else   
71         {  
72             return false;  
73         }  
74     }  
75     else  
76     {  
77         double k1 = (p2.y-p1.y)/(p2.x-p1.x);   
78         double k2 = (p4.y-p3.y)/(p4.x-p3.x);  
79   
80         if (fabs(k1-k2)<1e-6)  
81         {  
82             return false;  
83         }  
84         else   
85         {  
86             line_x = ((p3.y - p1.y) - (k2*p3.x - k1*p1.x)) / (k1-k2);  
87             line_y = k1*(line_x-p1.x)+p1.y;  
88         }  
89   
90         if (between(line_x,p1.x,p2.x)&&between(line_x,p3.x,p4.x))  
91         {  
92             return true;  
93         }  
94         else   
95         {  
96             return false;  
97         }  
98     }  
99 }