判断一个点在任意四边形内

通过面积法，判断点P是否在四边形(A,B,C,D)内。

如果在四边形内，则四边形的面积=面积(P,A,B)+面积(P,B,C)+面积(P,C,D)+面积(P,D,A);
反之不在四边形内。

Java代码

public class Quadrangle 
{ 
/** 
* 点是否在四边形内 
* @param a 
* @param b 
* @param c 
* @param d 
* @param p 
* @return 
*/ 
public static boolean pInQuadrangle(Point a, Point b, Point c,Point d,Point p) 
{ 
Log.d("Walk Game","Quadrangle:"+ 
"new Point("+a.x+","+a.y+"),"+ 
"new Point("+b.x+","+b.y+"),"+ 
"new Point("+c.x+","+c.y+"),"+ 
"new Point("+d.x+","+d.y+"),"+ 
"new Point("+p.x+","+p.y+")," 
); 
double dTriangle = triangleArea(a,b,p)+triangleArea(b,c,p) 
+triangleArea(c,d,p)+triangleArea(d,a,p); 
double dQuadrangle = triangleArea(a,b,c)+triangleArea(c,d,a); 
return dTriangle==dQuadrangle; 
} 
public static boolean pInQuadrangle(float[] px,float[] py,float x,float y) 
{ 
Point a = new Point((int)px[0],(int)py[0]); 
Point b = new Point((int)px[1],(int)py[1]); 
Point c = new Point((int)px[2],(int)py[2]); 
Point d = new Point((int)px[3],(int)py[3]); 
Point p = new Point((int)x,(int)y); 
Log.d("Walk Game","Quadrangle:"+ 
"new Point("+a.x+","+a.y+"),"+ 
"new Point("+b.x+","+b.y+"),"+ 
"new Point("+c.x+","+c.y+"),"+ 
"new Point("+d.x+","+d.y+"),"+ 
"new Point("+p.x+","+p.y+")," 
); 
double dTriangle = triangleArea(a,b,p)+triangleArea(b,c,p) 
+triangleArea(c,d,p)+triangleArea(d,a,p); 
double dQuadrangle = triangleArea(a,b,c)+triangleArea(c,d,a); 
return dTriangle==dQuadrangle; 
} 
// 返回三个点组成三角形的面积 
private static double triangleArea(Point a, Point b, Point c) 
{ 
double result = Math.abs((a.x * b.y + b.x * c.y + c.x * a.y - b.x * a.y 
- c.x * b.y - a.x * c.y) / 2.0D); 
return result; 
} 
}