Java 判断一个点是否在多边形区域内【转】

原文地址：http://blog.163.com/kangle0925@126/blog/static/27758198201181484115912/

import java.util.ArrayList;

/** 
* 判断一个点，是否在一个多边形区域内 
*/
public class Test
{

 public static void main ( String[] args )
 {

 double px = 113.705835;
 double py = 34.787479;
 ArrayList polygonXA = new ArrayList();
 ArrayList polygonYA = new ArrayList();
 polygonXA.add(113.700213);// 北京 
 polygonXA.add(113.706651);// 石家庄 
 polygonXA.add(113.706608);// 德州 
 polygonXA.add(113.707337);// 天津 
 polygonXA.add(113.701587);// 烟台 
 polygonXA.add(113.700128);// 唐山jdal 

 polygonYA.add(34.791532);
 polygonYA.add(34.791568);
 polygonYA.add(34.789242);
 polygonYA.add(34.786633);
 polygonYA.add(34.786669);
 polygonYA.add(34.789242);

 Test test = new Test();
 System.out.println(test.isPointInPolygon(px, py, polygonXA, polygonYA));
 }

 public boolean isPointInPolygon ( double px , double py , ArrayList polygonXA , ArrayList polygonYA )
 {
 boolean isInside = false;
 double ESP = 1e-9;
 int count = 0;
 double linePoint1x;
 double linePoint1y;
 double linePoint2x = 180;
 double linePoint2y;

 linePoint1x = px;
 linePoint1y = py;
 linePoint2y = py;

 for (int i = 0; i < polygonXA.size() - 1; i++)
 {
 double cx1 = polygonXA.get(i);
 double cy1 = polygonYA.get(i);
 double cx2 = polygonXA.get(i + 1);
 double cy2 = polygonYA.get(i + 1);
 if ( isPointOnLine(px, py, cx1, cy1, cx2, cy2) )
 {
 return true;
 }
 if ( Math.abs(cy2 - cy1) < ESP )
 {
 continue;
 }

 if ( isPointOnLine(cx1, cy1, linePoint1x, linePoint1y, linePoint2x, linePoint2y) )
 {
 if ( cy1 > cy2 )
 count++;
 }
 else if ( isPointOnLine(cx2, cy2, linePoint1x, linePoint1y, linePoint2x, linePoint2y) )
 {
 if ( cy2 > cy1 )
 count++;
 }
 else if ( isIntersect(cx1, cy1, cx2, cy2, linePoint1x, linePoint1y, linePoint2x, linePoint2y) )
 {
 count++;
 }
 }
 System.out.println(count);
 if ( count % 2 == 1 )
 {
 isInside = true;
 }

 return isInside;
 }

 public double Multiply ( double px0 , double py0 , double px1 , double py1 , double px2 , double py2 )
 {
 return ((px1 - px0) * (py2 - py0) - (px2 - px0) * (py1 - py0));
 }

 public boolean isPointOnLine ( double px0 , double py0 , double px1 , double py1 , double px2 , double py2 )
 {
 boolean flag = false;
 double ESP = 1e-9;
 if ( (Math.abs(Multiply(px0, py0, px1, py1, px2, py2)) < ESP) && ((px0 - px1) * (px0 - px2) <= 0)
 && ((py0 - py1) * (py0 - py2) <= 0) )
 {
 flag = true;
 }
 return flag;
 }

 public boolean isIntersect ( double px1 , double py1 , double px2 , double py2 , double px3 , double py3 , double px4 ,
 double py4 )
 {
 boolean flag = false;
 double d = (px2 - px1) * (py4 - py3) - (py2 - py1) * (px4 - px3);
 if ( d != 0 )
 {
 double r = ((py1 - py3) * (px4 - px3) - (px1 - px3) * (py4 - py3)) / d;
 double s = ((py1 - py3) * (px2 - px1) - (px1 - px3) * (py2 - py1)) / d;
 if ( (r >= 0) && (r <= 1) && (s >= 0) && (s <= 1) )
 {
 flag = true;
 }
 }
 return flag;
 }
}