Usage of gdal.Polygonize(,,,)

Question

I am trying to code gdal_polygonize source.png -f "GeoJSON" destination.geojson with the java-bindings of gdal. I don't know how to make gdal.Polygonize(...) create the "DN"-Fields one obtains when you use gdal_polygonize. Here is my code so far:

public static void gdalPolygonize(String[] args){

    gdal.AllRegister();
    ogr.RegisterAll();
    args = gdal.GeneralCmdLineProcessor(args);

    Dataset hDataset = gdal.Open(args[0], gdalconstConstants.GA_ReadOnly);
    Band rasterBand = hDataset.GetRasterBand(1);
    Driver driver = ogr.GetDriverByName("GeoJSON");
    DataSource dataSource =  driver.CreateDataSource("C:\\Users\\User\\Desktop\\destination.geojson");
    Layer outputLayer =dataSource.CreateLayer("destination");
    gdal.Polygonize(rasterBand, null, outputLayer, 0);

}

the created polygons in the geojsonfiles look like this:

{ "type": "Feature", "properties": { }, "geometry": { "type": "Polygon", "coordinates": [ [ [ 7858.0, 964.0 ], [ 7858.0, 965.0 ], [ 7859.0, 965.0 ], [ 7858.0, 964.0 ] ] ] } }

but they should look like this:

{ "type": "Feature", "properties": {"DN": 2}, "geometry": { "type": "Polygon", "coordinates": [ [ [ 7858.0, 964.0 ], [ 7858.0, 965.0 ], [ 7859.0, 965.0 ], [ 7858.0, 964.0 ] ] ] } }

So the difference it the "DN" in "properties".

Answer 1

I found the answer. I had to create a FieldDefn See code:

public static void gdalPolygonize(String[] args){

    gdal.AllRegister();
    ogr.RegisterAll();
    args = gdal.GeneralCmdLineProcessor(args);

    Dataset hDataset = gdal.Open(args[0], gdalconstConstants.GA_ReadOnly);
    Band rasterBand = hDataset.GetRasterBand(1);
    Driver driver = ogr.GetDriverByName("GeoJSON");
    DataSource dataSource =  driver.CreateDataSource("C:\\Users\\User\\Desktop\\destination.geojson");
    Layer outputLayer =dataSource.CreateLayer("destination");
    FieldDefn field_def = new FieldDefn("DN",4);
    outputLayer.CreateField(field_def);
    gdal.Polygonize(rasterBand, null, outputLayer, 0);

}