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Fix MLT MultiPolygon ring grouping for PostGIS-clipped geometries #53

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Fix MLT MultiPolygon ring grouping for PostGIS-clipped geometries #53

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62fe615
Fix MLT MultiPolygon ring grouping for PostGIS-clipped geometries
trasch Aug 18, 2026
f186a22
Dependency updates
trasch Aug 18, 2026
7ef6b1c
Revert the change, add a comment
trasch Aug 18, 2026
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6 changes: 3 additions & 3 deletions Package.resolved
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2 changes: 1 addition & 1 deletion Package.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,7 @@ let package = Package(
.package(url: "https://github.com/Outdooractive/gis-tools-csv", from: "1.1.0"),
.package(url: "https://github.com/Outdooractive/gis-tools-geopackage", from: "1.0.2"),
.package(url: "https://github.com/Outdooractive/gis-tools-gpx", from: "1.0.4"),
.package(url: "https://github.com/Outdooractive/gis-tools-fit", from: "1.0.0"),
.package(url: "https://github.com/Outdooractive/gis-tools-fit", from: "1.1.4"),
.package(url: "https://github.com/Outdooractive/gis-tools-shapefile", from: "1.0.2"),
.package(url: "https://github.com/1024jp/GzipSwift.git", from: "7.0.0"),
.package(url: "https://github.com/apple/swift-argument-parser", from: "1.8.2"),
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9 changes: 9 additions & 0 deletions Sources/MVTTools/Coders/MLT/MLTEncoder.swift
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Expand Up @@ -162,6 +162,15 @@ enum MLTEncoder {

case .multiPolygon:
let mpoly = feature.geometry as! MultiPolygon
// Encode the MultiPolygon structure as-is: each element's
// rings (first = exterior, subsequent = holes) are written
// to the topology so the decoder reconstructs the exact
// grouping. Any hole/interior-ring association is the
// responsibility of the data source (e.g. PostGIS
// ST_MakeValid) and must not be re-derived here via a
// winding-order heuristic, which corrupts legitimate
// MultiPolygons that contain several separate same-winding
// components (e.g. many disjoint forest patches).
for poly in mpoly.coordinates {
polygonRingCounts.append(UInt32(poly.count))
for ring in poly { collectRing(ring) }
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214 changes: 214 additions & 0 deletions Tests/MVTToolsTests/MLT/MLTRoundtripGeometryTests.swift
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Expand Up @@ -182,4 +182,218 @@ struct MLTRoundtripGeometryTests {
#expect(pt.altitude == nil)
}

// MARK: - MultiPolygon ring grouping

/// A MultiPolygon with multiple polygons, some with holes, must preserve
/// the ring grouping through an MLT encode → decode round-trip.
///
/// This is a regression test for the triangular-artifact bug where
/// interior rings (holes) were being separated from their parent polygons
/// and turned into standalone filled polygons, causing overlapping
/// triangles in maplibre-gl-js.
@Test
func multiPolygonWithHolesPreservesRingGrouping() throws {
// Polygon 0: exterior + 1 hole
let poly0 = try #require(Polygon([[
Coordinate3D(x: 0.0, y: 0.0, projection: .noSRID),
Coordinate3D(x: 1000.0, y: 0.0, projection: .noSRID),
Coordinate3D(x: 1000.0, y: 1000.0, projection: .noSRID),
Coordinate3D(x: 0.0, y: 1000.0, projection: .noSRID),
Coordinate3D(x: 0.0, y: 0.0, projection: .noSRID),
], [
Coordinate3D(x: 200.0, y: 200.0, projection: .noSRID),
Coordinate3D(x: 400.0, y: 200.0, projection: .noSRID),
Coordinate3D(x: 400.0, y: 400.0, projection: .noSRID),
Coordinate3D(x: 200.0, y: 400.0, projection: .noSRID),
Coordinate3D(x: 200.0, y: 200.0, projection: .noSRID),
]]))
// Polygon 1: just exterior
let poly1 = try #require(Polygon([[
Coordinate3D(x: 2000.0, y: 2000.0, projection: .noSRID),
Coordinate3D(x: 3000.0, y: 2000.0, projection: .noSRID),
Coordinate3D(x: 3000.0, y: 3000.0, projection: .noSRID),
Coordinate3D(x: 2000.0, y: 3000.0, projection: .noSRID),
Coordinate3D(x: 2000.0, y: 2000.0, projection: .noSRID),
]]))
// Polygon 2: exterior + 2 holes
let poly2 = try #require(Polygon([[
Coordinate3D(x: 4000.0, y: 4000.0, projection: .noSRID),
Coordinate3D(x: 5000.0, y: 4000.0, projection: .noSRID),
Coordinate3D(x: 5000.0, y: 5000.0, projection: .noSRID),
Coordinate3D(x: 4000.0, y: 5000.0, projection: .noSRID),
Coordinate3D(x: 4000.0, y: 4000.0, projection: .noSRID),
], [
Coordinate3D(x: 4200.0, y: 4200.0, projection: .noSRID),
Coordinate3D(x: 4400.0, y: 4200.0, projection: .noSRID),
Coordinate3D(x: 4400.0, y: 4400.0, projection: .noSRID),
Coordinate3D(x: 4200.0, y: 4400.0, projection: .noSRID),
Coordinate3D(x: 4200.0, y: 4200.0, projection: .noSRID),
], [
Coordinate3D(x: 4600.0, y: 4600.0, projection: .noSRID),
Coordinate3D(x: 4800.0, y: 4600.0, projection: .noSRID),
Coordinate3D(x: 4800.0, y: 4800.0, projection: .noSRID),
Coordinate3D(x: 4600.0, y: 4800.0, projection: .noSRID),
Coordinate3D(x: 4600.0, y: 4600.0, projection: .noSRID),
]]))

let mpoly = try #require(MultiPolygon([poly0, poly1, poly2]))
let feature = Feature(mpoly, id: .int(1))

guard let data = MLTEncoder.encode(
layers: ["test": VectorTile.LayerContainer(features: [feature], boundingBox: nil)],
x: 0, y: 0, z: 0, projection: .noSRID)
else { return }
let decoded = try MLTDecoder.decode(from: data, x: 0, y: 0, z: 0, projection: .noSRID)
let features = try #require(decoded["test"]?.features)
#expect(features.count == 1)

let decodedMpoly = try #require(features[0].geometry as? MultiPolygon)
// Must have 3 polygons
#expect(decodedMpoly.coordinates.count == 3)
// Polygon 0: 2 rings (exterior + 1 hole)
#expect(decodedMpoly.coordinates[0].count == 2)
// Polygon 1: 1 ring (exterior only)
#expect(decodedMpoly.coordinates[1].count == 1)
// Polygon 2: 3 rings (exterior + 2 holes)
#expect(decodedMpoly.coordinates[2].count == 3)
}

/// Multiple MultiPolygon features in the same layer, each with different
/// ring counts, must all preserve their ring grouping.
@Test
func multipleMultiPolygonsWithHolesInSameLayer() throws {
// Feature 1: MultiPolygon with 1 polygon that has 1 hole
let poly1 = try #require(Polygon([[
Coordinate3D(x: 0.0, y: 0.0, projection: .noSRID),
Coordinate3D(x: 1000.0, y: 0.0, projection: .noSRID),
Coordinate3D(x: 1000.0, y: 1000.0, projection: .noSRID),
Coordinate3D(x: 0.0, y: 1000.0, projection: .noSRID),
Coordinate3D(x: 0.0, y: 0.0, projection: .noSRID),
], [
Coordinate3D(x: 200.0, y: 200.0, projection: .noSRID),
Coordinate3D(x: 400.0, y: 200.0, projection: .noSRID),
Coordinate3D(x: 400.0, y: 400.0, projection: .noSRID),
Coordinate3D(x: 200.0, y: 400.0, projection: .noSRID),
Coordinate3D(x: 200.0, y: 200.0, projection: .noSRID),
]]))
let feat1 = Feature(try #require(MultiPolygon([poly1])), id: .int(1))

// Feature 2: MultiPolygon with 2 polygons, one with a hole, one without
let poly2a = try #require(Polygon([[
Coordinate3D(x: 2000.0, y: 2000.0, projection: .noSRID),
Coordinate3D(x: 3000.0, y: 2000.0, projection: .noSRID),
Coordinate3D(x: 3000.0, y: 3000.0, projection: .noSRID),
Coordinate3D(x: 2000.0, y: 3000.0, projection: .noSRID),
Coordinate3D(x: 2000.0, y: 2000.0, projection: .noSRID),
], [
Coordinate3D(x: 2200.0, y: 2200.0, projection: .noSRID),
Coordinate3D(x: 2400.0, y: 2200.0, projection: .noSRID),
Coordinate3D(x: 2400.0, y: 2400.0, projection: .noSRID),
Coordinate3D(x: 2200.0, y: 2400.0, projection: .noSRID),
Coordinate3D(x: 2200.0, y: 2200.0, projection: .noSRID),
]]))
let poly2b = try #require(Polygon([[
Coordinate3D(x: 4000.0, y: 4000.0, projection: .noSRID),
Coordinate3D(x: 4500.0, y: 4000.0, projection: .noSRID),
Coordinate3D(x: 4500.0, y: 4500.0, projection: .noSRID),
Coordinate3D(x: 4000.0, y: 4500.0, projection: .noSRID),
Coordinate3D(x: 4000.0, y: 4000.0, projection: .noSRID),
]]))
let feat2 = Feature(try #require(MultiPolygon([poly2a, poly2b])), id: .int(2))

// Feature 3: simple Polygon (no hole)
let poly3 = try #require(Polygon([[
Coordinate3D(x: 5000.0, y: 5000.0, projection: .noSRID),
Coordinate3D(x: 6000.0, y: 5000.0, projection: .noSRID),
Coordinate3D(x: 6000.0, y: 6000.0, projection: .noSRID),
Coordinate3D(x: 5000.0, y: 6000.0, projection: .noSRID),
Coordinate3D(x: 5000.0, y: 5000.0, projection: .noSRID),
]]))
let feat3 = Feature(poly3, id: .int(3))

guard let data = MLTEncoder.encode(
layers: ["test": VectorTile.LayerContainer(features: [feat1, feat2, feat3], boundingBox: nil)],
x: 0, y: 0, z: 0, projection: .noSRID)
else { return }
let decoded = try MLTDecoder.decode(from: data, x: 0, y: 0, z: 0, projection: .noSRID)
let features = try #require(decoded["test"]?.features)
#expect(features.count == 3)

// Feature 1: MultiPolygon with 1 polygon, 2 rings
let mp1 = try #require(features[0].geometry as? MultiPolygon)
#expect(mp1.coordinates.count == 1)
#expect(mp1.coordinates[0].count == 2)

// Feature 2: MultiPolygon with 2 polygons, first has 2 rings, second has 1
let mp2 = try #require(features[1].geometry as? MultiPolygon)
#expect(mp2.coordinates.count == 2)
#expect(mp2.coordinates[0].count == 2)
#expect(mp2.coordinates[1].count == 1)

// Feature 3: Polygon with 1 ring
let p3 = try #require(features[2].geometry as? Polygon)
#expect(p3.rings.count == 1)
}

/// Encode the same VectorTile to both MVT and MLT and verify that
/// MultiPolygon ring grouping is preserved in both.
/// This simulates what `MVTPostgis.data()` does: create one VectorTile
/// and encode it to both formats.
@Test
func dualMvtMltEncodingPreservesRingGrouping() throws {
// MultiPolygon with 2 polygons, first has a hole
// Use realistic EPSG:3857 coordinates within tile 7/65/47
// (lng [2.81, 5.62], lat [40.98, 43.07])
// EPSG:3857: x = lng * 111319, y = lat(log) * ...
let poly0 = try #require(Polygon([[
Coordinate3D(x: 320000.0, y: 5160000.0, projection: .epsg3857),
Coordinate3D(x: 420000.0, y: 5160000.0, projection: .epsg3857),
Coordinate3D(x: 420000.0, y: 5300000.0, projection: .epsg3857),
Coordinate3D(x: 320000.0, y: 5300000.0, projection: .epsg3857),
Coordinate3D(x: 320000.0, y: 5160000.0, projection: .epsg3857),
], [
Coordinate3D(x: 340000.0, y: 5200000.0, projection: .epsg3857),
Coordinate3D(x: 360000.0, y: 5200000.0, projection: .epsg3857),
Coordinate3D(x: 360000.0, y: 5220000.0, projection: .epsg3857),
Coordinate3D(x: 340000.0, y: 5220000.0, projection: .epsg3857),
Coordinate3D(x: 340000.0, y: 5200000.0, projection: .epsg3857),
]]))
let poly1 = try #require(Polygon([[
Coordinate3D(x: 500000.0, y: 5000000.0, projection: .epsg3857),
Coordinate3D(x: 550000.0, y: 5000000.0, projection: .epsg3857),
Coordinate3D(x: 550000.0, y: 5100000.0, projection: .epsg3857),
Coordinate3D(x: 500000.0, y: 5100000.0, projection: .epsg3857),
Coordinate3D(x: 500000.0, y: 5000000.0, projection: .epsg3857),
]]))
let mpoly = try #require(MultiPolygon([poly0, poly1]))
let feature = Feature(mpoly, id: .int(1))

// Create a VectorTile at z7 with EPSG:3857 (same as PostGIS)
var tile = try VectorTile(x: 65, y: 47, z: 7, projection: .epsg3857)
tile.appendFeatures([feature], to: "landuse_a")

let options = VectorTile.ExportOptions(
bufferSize: .pixel(32),
compression: .level(9),
simplifyFeatures: .no)

// Encode to both formats using the SAME tile
let mvtData = try #require(tile.mvtData(options: options))
let mltData = try #require(tile.mltData(options: options))

// Decode MVT and check (MVT flattens rings, decoder uses winding order)
let mvtTile = try VectorTile(mvtData: mvtData, x: 65, y: 47, z: 7, projection: .epsg4326)
let mvtFeatures = mvtTile.features(for: "landuse_a")
#expect(mvtFeatures.count == 1)

// Decode MLT and check — MLT must preserve the explicit polygon grouping
let mltLayers = try MLTDecoder.decode(from: mltData, x: 65, y: 47, z: 7, projection: .epsg4326)
let mltFeatures = try #require(mltLayers["landuse_a"]?.features)
#expect(mltFeatures.count == 1)
let mltMpoly = try #require(mltFeatures[0].geometry as? MultiPolygon)
#expect(mltMpoly.coordinates.count == 2)
#expect(mltMpoly.coordinates[0].count == 2) // exterior + hole
#expect(mltMpoly.coordinates[1].count == 1) // exterior only
}

}
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