使用带有参数capstyle flat的JTS BufferOP时点几何产生的Emty多边形结果

Question

I am using geotools and the jts class BufferOP to create a buffer around my geometries. During tesdting I came along a weard result with point geometries. If i set capstyle to flat, my result is always an emty polygon.

Lines and Polygons are working. Only points seems ti have this kind of issue.

If i change it to round or square parameter, I get the expected result.

I am using geotools snapshot 21 with maven and Jave 8. here is the maven pom file snipped I've been using and the code example

<properties>
 <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 <maven.compiler.source>1.7</maven.compiler.source>
 <maven.compiler.target>1.7</maven.compiler.target>
 <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
 <geotools.version>21-SNAPSHOT</geotools.version>
</properties>

<dependencies>
 <dependency>
  <groupId>junit</groupId>
  <artifactId>junit</artifactId>
  <version>4.11</version>
  <scope>test</scope>
 </dependency>
 <dependency>
  <groupId>org.geotools</groupId>
  <artifactId>gt-geometry</artifactId>
  <version>${geotools.version}</version>
 </dependency>
 <dependency>
  <groupId>org.geotools</groupId>
  <artifactId>gt-epsg-hsql</artifactId>
  <version>${geotools.version}</version>
 </dependency>
 <dependency>
  <groupId>org.geotools</groupId>
  <artifactId>gt-epsg-wkt </artifactId>
  <version>${geotools.version}</version>
 </dependency>

import org.geotools.geometry.jts.JTSFactoryFinder;
import org.geotools.referencing.CRS;
import org.geotools.util.factory.Hints;
import org.locationtech.jts.geom.*;
import org.locationtech.jts.io.ParseException;
import org.locationtech.jts.io.WKTReader;
import org.locationtech.jts.operation.buffer.BufferOp;
import org.locationtech.jts.operation.buffer.BufferParameters;
import org.opengis.referencing.FactoryException;
import org.opengis.referencing.crs.CoordinateReferenceSystem;
import org.opengis.referencing.operation.TransformException;
import java.io.IOException;

public class App 
{
 public static void main( String[] args ) throws ParseException, IOException, FactoryException, TransformException {


    Integer epsg= 32632;
    String wkt = "POINT (5293201.002716452 1208988.4067087262)";
    //setup geometry point in utm coordinates (meter)

    // create geometry
    CoordinateReferenceSystem crs = CRS.decode(("EPSG:"+ epsg.toString()));
    Hints hints = new Hints(Hints.CRS, crs);
    GeometryFactory geometryFactoryWKT = JTSFactoryFinder.getGeometryFactory(hints);
    WKTReader wktReader = new WKTReader(geometryFactoryWKT);
    Geometry geom =  wktReader.read(wkt);
    geom.setSRID(epsg);


    // creates BufferParameters
    BufferParameters bufferParam = new BufferParameters();

    bufferParam.setEndCapStyle(BufferParameters.CAP_FLAT);
    // if using any other parameter result is as expected
    // bufferParam.setEndCapStyle(BufferParameters.CAP_ROUND);
    bufferParam.setJoinStyle(BufferParameters.JOIN_BEVEL );
    bufferParam.setMitreLimit(5);
    bufferParam.setSimplifyFactor(0.01);
    bufferParam.setQuadrantSegments(8);


    // creates buffer geom on point with 10m distance and use set bufferParameters
    Geometry bufferGeom =  BufferOp.bufferOp(geom ,10, bufferParam);

    System.out.println(bufferGeom);
}

Does anyone know why?

Answer 1

Having had a look at the code it seems to come down to how OffsetCurveBuilder handles the end caps. It seems to (quite reasonably in my opinion) not calculate anything for flat end caps and since a point doesn't generate anything except ends you get nothing for a flat end cap.

  private void computePointCurve(Coordinate pt, OffsetSegmentGenerator segGen) {
    switch (bufParams.getEndCapStyle()) {
      case BufferParameters.CAP_ROUND:
        segGen.createCircle(pt);
        break;
      case BufferParameters.CAP_SQUARE:
        segGen.createSquare(pt);
        break;
      // otherwise curve is empty (e.g. for a butt cap);
    }
  }