三、地圖、java實現判斷點是否在多邊形內

一、有些時候，地圖的業務需要判斷一個點的經緯度坐標是否落在多邊形內

這是我找到的算法的java實現，誤差在1米內，可直接使用

代碼如下：

    /**
     * 判斷點是否在多邊形內，如果點位於多邊形的頂點或邊上，也算做點在多邊形內，直接返回true
     * @return      點在多邊形內返回true,否則返回false
     */
    public static boolean isPtInPoly(Point2D.Double point,List<Point2D.Double> pts){

        int N = pts.size();
        //如果點位於多邊形的頂點或邊上，也算做點在多邊形內，直接返回true
        boolean boundOrVertex = true;
        //cross points count of x
        int intersectCount = 0;
        //浮點類型計算時候與0比較時候的容差
        double precision = 2e-10;
        //neighbour bound vertices
        Point2D.Double p1, p2;
        Point2D.Double p = point;

        //當前點
        p1 = pts.get(0);
        //left vertex
        for(int i = 1; i <= N; ++i){
            //check all rays
            if(p.equals(p1)){
                //p is an vertex
                return boundOrVertex;

            }

            p2 = pts.get(i % N);
            //right vertex
            if(p.x < Math.min(p1.x, p2.x) || p.x > Math.max(p1.x, p2.x)){
                //ray is outside of our interests
                p1 = p2;
                continue;//next ray left point
            }

            if(p.x > Math.min(p1.x, p2.x) && p.x < Math.max(p1.x, p2.x)){
                //ray is crossing over by the algorithm (common part of)
                if(p.y <= Math.max(p1.y, p2.y)){
                    //x is before of ray
                    if(p1.x == p2.x && p.y >= Math.min(p1.y, p2.y)){
                        //overlies on a horizontal ray
                        return boundOrVertex;
                    }

                    if(p1.y == p2.y){
                        //ray is vertical
                        if(p1.y == p.y){
                            //overlies on a vertical ray
                            return boundOrVertex;
                        }else{
                            //before ray
                            ++intersectCount;
                        }
                    }else{//cross point on the left side
                        double xinters = (p.x - p1.x) * (p2.y - p1.y) / (p2.x - p1.x) + p1.y;
                        //cross point of y
                        if(Math.abs(p.y - xinters) < precision){
                            //overlies on a ray
                            return boundOrVertex;
                        }

                        if(p.y < xinters){
                            //before ray
                            ++intersectCount;
                        }
                    }
                }
            }else{
                //special case when ray is crossing through the vertex
                if(p.x == p2.x && p.y <= p2.y){
                    //p crossing over p2
                    Point2D.Double p3 = pts.get((i+1) % N);
                    //next vertex
                    if(p.x >= Math.min(p1.x, p3.x) && p.x <= Math.max(p1.x, p3.x)){
                        //p.x lies between p1.x & p3.x
                        ++intersectCount;
                    }else{
                        intersectCount += 2;
                    }
                }
            }
            p1 = p2;
            //next ray left point
        }

        if(intersectCount % 2 == 0){
            //偶數在多邊形外
            return false;
        } else {
            //奇數在多邊形內
            return true;
        }
    }