有没有办法在JTS中将自相交多边形转换为多面？

Question

取无效多边形POLYGON((0 100, 100 100, 0 0, 100 0, 0 100)) - 带有未申报交点的蛋计时器形状

许多说明都说JTS可以使用buffer方法创建一个有效的版本：

Geometry input = new WKTReader().read("POLYGON((0 100, 100 100, 0 0, 100 0, 0 100))");
Geometry output = geom.buffer(0);
return output;

但是，这会产生输出POLYGON ((0 100, 100 100, 50 50, 0 100))，其中部分多边形丢失：

有没有办法让JTS验证多边形，以便为给定的输入生成输出MULTIPOLYGON(((0 100, 100 100, 50 50, 0 100)), ((0 0, 100 0, 50 50, 0 0)))？

这似乎应该内置到API中（也许这种行为是一个错误） - 我错过了什么？

谢谢。

Answer 1

JTS似乎提供了我需要的行为，尽管我必须在我自己的代码中做一些小工作。我写的validate函数将多边形/多边形分解为非自相交线串的集合，然后使用Polygonizer类从结果中构建多边形。我已经在以下（有限的）输入集上测试了它，它看起来像我需要的那样：

    POLYGON((0 100, 100 100, 0 0, 100 0, 0 100))
    POLYGON((0 0, 0 100, 100 100, 100 0, 0 0))
    MULTIPOLYGON(((0 0, 0 100, 100 100, 100 0, 0 0)),((50 50, 50 150, 150 150, 150 50, 50 50)))
    POLYGON((0 0, 50 50, 100 0, 150 0, 200 50, 250 0, 0 0))

代码：

/**
 * Get / create a valid version of the geometry given. If the geometry is a polygon or multi polygon, self intersections /
 * inconsistencies are fixed. Otherwise the geometry is returned.
 * 
 * @param geom
 * @return a geometry 
 */
public static Geometry validate(Geometry geom){
    if(geom instanceof Polygon){
        if(geom.isValid()){
            geom.normalize(); // validate does not pick up rings in the wrong order - this will fix that
            return geom; // If the polygon is valid just return it
        }
        Polygonizer polygonizer = new Polygonizer();
        addPolygon((Polygon)geom, polygonizer);
        return toPolygonGeometry(polygonizer.getPolygons(), geom.getFactory());
    }else if(geom instanceof MultiPolygon){
        if(geom.isValid()){
            geom.normalize(); // validate does not pick up rings in the wrong order - this will fix that
            return geom; // If the multipolygon is valid just return it
        }
        Polygonizer polygonizer = new Polygonizer();
        for(int n = geom.getNumGeometries(); n-- > 0;){
            addPolygon((Polygon)geom.getGeometryN(n), polygonizer);
        }
        return toPolygonGeometry(polygonizer.getPolygons(), geom.getFactory());
    }else{
        return geom; // In my case, I only care about polygon / multipolygon geometries
    }
}

/**
 * Add all line strings from the polygon given to the polygonizer given
 * 
 * @param polygon polygon from which to extract line strings
 * @param polygonizer polygonizer
 */
static void addPolygon(Polygon polygon, Polygonizer polygonizer){
    addLineString(polygon.getExteriorRing(), polygonizer);
    for(int n = polygon.getNumInteriorRing(); n-- > 0;){
        addLineString(polygon.getInteriorRingN(n), polygonizer);
    }
}

/**
 * Add the linestring given to the polygonizer
 * 
 * @param linestring line string
 * @param polygonizer polygonizer
 */
static void addLineString(LineString lineString, Polygonizer polygonizer){

    if(lineString instanceof LinearRing){ // LinearRings are treated differently to line strings : we need a LineString NOT a LinearRing
        lineString = lineString.getFactory().createLineString(lineString.getCoordinateSequence());
    }

    // unioning the linestring with the point makes any self intersections explicit.
    Point point = lineString.getFactory().createPoint(lineString.getCoordinateN(0));
    Geometry toAdd = lineString.union(point); 

    //Add result to polygonizer
    polygonizer.add(toAdd);
}

/**
 * Get a geometry from a collection of polygons.
 * 
 * @param polygons collection
 * @param factory factory to generate MultiPolygon if required
 * @return null if there were no polygons, the polygon if there was only one, or a MultiPolygon containing all polygons otherwise
 */
static Geometry toPolygonGeometry(Collection<Polygon> polygons, GeometryFactory factory){
    switch(polygons.size()){
        case 0:
            return null; // No valid polygons!
        case 1:
            return polygons.iterator().next(); // single polygon - no need to wrap
        default:
            //polygons may still overlap! Need to sym difference them
            Iterator<Polygon> iter = polygons.iterator();
            Geometry ret = iter.next();
            while(iter.hasNext()){
                ret = ret.symDifference(iter.next());
            }
            return ret;
    }
}