Java 判斷一個點是否在多邊形區域內【轉】

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

import java.util.ArrayList;

/** 
 * 判斷一個點，是否在一個多邊形區域內 
 */
public class Test
{

    public static void main ( String[] args )
    {

        double px = 113.705835;
        double py = 34.787479;
        ArrayList<Double> polygonXA = new ArrayList<Double>();
        ArrayList<Double> polygonYA = new ArrayList<Double>();
        polygonXA.add(113.700213);// 北京  
        polygonXA.add(113.706651);// 石家莊  
        polygonXA.add(113.706608);// 德州  
        polygonXA.add(113.707337);// 天津  
        polygonXA.add(113.701587);// 煙臺  
        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<Double> polygonXA , ArrayList<Double> 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;
    }
}