java - (JTS) 线串是否正在进入或退出多边形

Question

如果我在 JTS(或一般某种开放折线)中有一个 linestring ，其方向由起点定义，是否有一些智能方法可以判断与闭合 的交叉点Polygon linestring 是否“进入”多边形或退出多边形:

• 在 JRS 中(我在文档中找不到方法)，只有直线和闭合形状与交集相交的坐标
• 一些通用的聪明方法。我目前已经通过测试一个非常小的距离的点来完成它，沿着polygon边缘两侧的linestring并测试哪个是“in”，哪个是“out” 。如果多边形有(不太可能)非常尖锐的内部边缘，则可以想象，这可能会返回错误的结果。

Answer 1

检查线段的起点是在多边形内部还是外部，以确定它是进入还是退出多边形。简单代码示例:

// some demo polygon + line
Polygon polygon = new GeometryFactory().createPolygon(new Coordinate[]{new Coordinate(1,1), new Coordinate(6,1), new Coordinate(6,6), new Coordinate(1,6), new Coordinate(1,1)});
LineString line = new GeometryFactory().createLineString(new Coordinate[]{new Coordinate(0, 0), new Coordinate(5,5), new Coordinate(10,5)});

// check for intersection in the first place
if(line.intersects(polygon)){
    System.out.println("line intersects polygon!");
    // iterate over all segments of the linestring and check for intersections
    for(int i = 1; i < line.getNumPoints(); i++){
        // create line for current segment
        LineString currentSegment = new GeometryFactory().createLineString(new Coordinate[]{line.getCoordinates()[i-1], line.getCoordinates()[i]});
        // check if line is intersecting with ring
        if(currentSegment.intersects(polygon)){
            // segment is entering the ring if startpoint is outside the ring
            if(!polygon.contains(currentSegment.getStartPoint())){
                System.out.println("This segment is entering the polygon -> ");
                System.out.println(currentSegment.toText());
            // startpoint is inside the ring
            }
            if (polygon.contains(currentSegment.getStartPoint())) {
                System.out.println("This segment is exiting the polygon -> ");
                System.out.println(currentSegment.toText());
            }
        }
    }
} else {
    System.out.println("line is not intersecting the polygon!");
}

此代码并未涵盖所有可能性。例如。如果单个线段多次与多边形相交(进入+退出)，则本例中不涉及这一点。在这种情况下，只需计算交叉点的数量并在交叉点之间创建相应数量的线串。