MVT Writer Geometry shift

[megedsh]

Encountered an issue when writing an mvt where some of the geometries are shifted to a wrong location.
for the same geometry, higher and lower zoom display different behaviours.
In low zoom (4) the some of the polygons are not where they should be.
in high zoom (6) the issue is not reproduced.

Geometry used in test:
Italy.txt

`

    [Test]
    public void MvtWriterGeometryIssue()
    {
        writeTile(8, 6, 4);
        writeTile(34,24,6);
    }

    private static void writeTile(int x, int y, int z)
    {
        string italyTxt = getItalyText();
        Geometry geometry = new WKTReader().Read(italyTxt);
        Feature feature = new Feature(geometry, new AttributesTable(new Dictionary<string, object>()
        {
            { "id", 1 }
        }));
        Tile tileDefinition = new Tile(x, y, z);
        VectorTile vectorTile = new VectorTile { TileId = tileDefinition.Id };
        Layer layer = new Layer { Name = "layer1" };
        vectorTile.Layers.Add(layer);
        layer.Features.Add(feature);

        byte[] result;
        MemoryStream? ms;
        using (ms = new MemoryStream(1024 * 32))
        {
            vectorTile.Write(ms);
            result = ms.ToArray();
        }

        File.WriteAllBytes($"{x}_{y}_{z}.mvt", result);
    }

    private static string getItalyText() => File.ReadAllText("Italy.wkt");

Opend the tiles in QGIS, with an OSM layer in background to see the shift.

Original geometry:

Tile x=8,y=6,z=4

Tile x=34,y=24,z=6

[FObermaier]

The parts of the input polygon are not sorted by their area (descending). This might cause the MapBoxTileWriter to encode CoordinateSequences and later decide not to write them. At that point, the current x- and y- ordinates have already been updated so the next starting point of a sequence that is actually written is incorrect.

Could you verify that 8_6_4.zip is what you expect.

[megedsh]

I think it looks good..
I need to do some more testing..
Thanks !!

[megedsh]

so sorting the polygons by area will solve this without a fix ?
the source is a shp file that I also read with your lib, so I guess I can recreate the topology with sorting.

[FObermaier]

so sorting the polygons by area will solve this without a fix ?

I suppose so. Geometry.Normalize() should do.

[megedsh]

Sorry, Geometry.Normalize() dosent seem to work..

[FObermaier]

Sorry for the misleading suggestion. Geometry.Normalize does not sort items in a MultiPolygon by their area. Sorting does not help either, because the issue here arises from the interior ring that excludes San Marino.

I'll improve the fix.

[megedsh]

Thanks, Ill take the branch later and test it.
maybe ill create a better unit test by reading the tile and than compare the geometries

[megedsh]

Tested with the branch.. looks good on several zoom levels

I wrote a unit test that reads the features back from the tile, with the thought of asserting the original features
only to see that the tile writer distorts the the feature differently for every zoom level..
I dont know how to compare the similarity of the distorted polygon..
test is below, maybe you can alter it.

`

    [Test]
    public void MvtWriterGeometryIssue()
    {
        string italyTxt = getItalyText();
        MultiPolygon a = new WKTReader().Read(italyTxt) as MultiPolygon;
        //byte[] tile1 = writeTile(8, 6, 4, a);
        //IFeature[] features = readTile(8, 6, 4, tile1);

        byte[] tile1 = writeTile(4401, 3077, 13, a);
        IFeature[] features = readTile(4401, 3077, 13, tile1);

        Assert.AreEqual(1, features.Length);
        MultiPolygon b = features[0].Geometry as MultiPolygon;

        Assert.AreEqual(a.Count,b.Count);

        Geometry[] a_polygons = a.OrderBy(p => p.Area).ThenBy(p => p.Coordinates.Length).ToArray();
        Geometry[] b_polygons = b.OrderBy(p => p.Area).ThenBy(p => p.Coordinates.Length).ToArray();

        for (int i = 0; i < a_polygons.Length; i++)
        {
            Geometry pa = a_polygons[i].Normalized();
            Geometry pb = b_polygons[i].Normalized();
            

            //Geometries are distorted for different zoom levels, cant asser't similarity

            IntersectionMatrix intersectionMatrix = pa.Relate(pb);
            
        }
    }
    
    private static byte[] writeTile(int x, int y, int z, Geometry geometry)
    {
        Feature feature = new Feature(geometry, new AttributesTable(new Dictionary<string, object>
        {
            { "id", 1 }
        }));
        Tile tileDefinition = new Tile(x, y, z);
        VectorTile vectorTile = new VectorTile { TileId = tileDefinition.Id };
        Layer layer = new Layer { Name = "layer1" };
        vectorTile.Layers.Add(layer);
        layer.Features.Add(feature);

        byte[] result;
        MemoryStream? fs;
        using (fs = new MemoryStream(1024 * 32))
        {
            vectorTile.Write(fs);
            result = fs.ToArray();
        }

        return result;
    }

    private static IFeature[] readTile(int x, int y, int z, byte[] tile)
    {
        MapboxTileReader reader = new MapboxTileReader();

        Tile tileDefinition = new Tile(x, y, z);

        List<IFeature> res = new List<IFeature>();
        using (MemoryStream ms = new MemoryStream(tile))
        {
            VectorTile? vt = reader.Read(ms, tileDefinition, "id");

            //Loop through each layer.
            foreach (Layer l in vt.Layers)
            {
                IList<IFeature> features = l.Features;
                res.AddRange(features);
            }
        }

        return res.ToArray();
    }

`

[FObermaier]

I dont know how to compare the similarity of the distorted polygon..

Try one or combine any of NetTopologySuite.Algorithm.Match.[Hausdorff|Area|Frechet]SimilarityMeasure.
They implement ISimilarityMeasure an interface that describes functions that compute a value describing the similarity of two geometries.

[megedsh]

Like you said.
used HausdorffSimilarityMeasure, and allowed 0.05 deveiation
Develop branch fails (similarity ~0.85), Fix branch 0.96

`

    [Test]
    public void MvtWriterGeometryIssue()
    {
        string italyTxt = getItalyText();
        Geometry? geom = new WKTReader().Read(italyTxt);

        assertSimilarityAfterExportImportTile(geom, 8, 6, 4, 0.05);
    }

    private static void assertSimilarityAfterExportImportTile(Geometry g, int x, int y, int zoom, double threashold)
    {
        byte[] tile1 = writeTile(x, y, zoom, g);
        IFeature[] features = readTile(x, y, zoom, tile1);
        Assert.NotNull(features);
        Assert.AreEqual(1, features.Length);

        Geometry geomFromTile = features[0].Geometry;
        if (geomFromTile is GeometryCollection gca && g is GeometryCollection gcb)
        {
            Assert.AreEqual(gca.Count, gcb.Count);
        }

        double similarity = new HausdorffSimilarityMeasure().Measure(geomFromTile, g);
        if (similarity >= 1)
        {
            Assert.Pass();
        }
        else
        {
            Assert.AreEqual(1, similarity, threashold, $"Geometry from tile not similar to original. Threashold = ({threashold})" );
        }
    }

    private static byte[] writeTile(int x, int y, int z, Geometry geometry)
    {
        Feature feature = new Feature(geometry, new AttributesTable(new Dictionary<string, object>
        {
            { "id", 1 }
        }));
        Tile tileDefinition = new Tile(x, y, z);
        VectorTile vectorTile = new VectorTile { TileId = tileDefinition.Id };
        Layer layer = new Layer { Name = "layer1" };
        vectorTile.Layers.Add(layer);
        layer.Features.Add(feature);

        byte[] result;
        MemoryStream? fs;
        using (fs = new MemoryStream(1024 * 32))
        {
            vectorTile.Write(fs);
            result = fs.ToArray();
        }

        return result;
    }

    private static IFeature[] readTile(int x, int y, int z, byte[] tile)
    {
        MapboxTileReader reader = new MapboxTileReader();

        Tile tileDefinition = new Tile(x, y, z);

        List<IFeature> res = new List<IFeature>();
        using (MemoryStream ms = new MemoryStream(tile))
        {
            VectorTile? vt = reader.Read(ms, tileDefinition, "id");

            //Loop through each layer.
            foreach (Layer l in vt.Layers)
            {
                IList<IFeature> features = l.Features;
                res.AddRange(features);
            }
        }

        return res.ToArray();
    }

`