[BUGFIX] Fix dropped road segments at z4-5

src/main/java/org/openmaptiles/layers/Transportation.java

@@ -168,6 +168,8 @@ public class Transportation implements
     .put(6, 100)
     .put(5, 500)
     .put(4, 1_000);
+  private static final ZoomFunction.MeterToPixelThresholds TRUNK_UPGRADE_LENGTH = ZoomFunction.meterThresholds()
+    .put(5, 1_000);
   // ORDER BY network_type, network, LENGTH(ref), ref)
   private static final Comparator<RouteRelation> RELATION_ORDERING = Comparator
     .<RouteRelation>comparingInt(
@@ -285,6 +287,22 @@ private static boolean isTrunkForZ5(String highway, List<RouteRelation> routeRel
       .anyMatch(Z5_TRUNK_BY_NETWORK::contains);
   }
 
+  /**
+   * Checks if a trunk segment is small enough to be processed at z5 so it can merge with surrounding motorways.
+   * 
+   * @param element the highway linestring element to check
+   * @return true if the trunk segment length is less than the threshold, false otherwise
+   */
+  private boolean isTrunkZ5MergeableLength(Tables.OsmHighwayLinestring element) {
+    try {
+      return element.source().length() < TRUNK_UPGRADE_LENGTH.apply(5).doubleValue();
+    } catch (GeometryException e) {
+      e.log(stats, "omt_transportation_trunk_length",
+        "Unable to get feature length for trunk upgrade: " + element.source().id());
+      return false;
+    }
+  }
+
   private static boolean isMotorwayWithNetworkForZ4(List<RouteRelation> routeRelations) {
     // All roads in network included in osm_national_network except gb-trunk and us-highway
     return routeRelations.stream()
@@ -570,6 +588,12 @@ int getMinzoom(Tables.OsmHighwayLinestring element, String highwayClass) {
           (z13Paths || !nullOrEmpty(element.name()) || routeRank <= 2 || !nullOrEmpty(element.sacScale())) ? 13 : 14;
         case FieldValues.CLASS_TRUNK -> {
           boolean z5trunk = isTrunkForZ5(highway, routeRelations);
+
+          //Allow small trunk segments to be processed at z5 so they can merge with surrounding motorways
+          if (isTrunkZ5MergeableLength(element)) {
+            z5trunk = true;
+          }
+
           // and if it is good for Z5, it may be good also for Z4 (see CLASS_MOTORWAY bellow):
           String clazz = FieldValues.CLASS_TRUNK;
           if (z5trunk && isMotorwayWithNetworkForZ4(routeRelations)) {
@@ -702,6 +726,19 @@ public List<VectorTile.Feature> postProcess(int zoom, List<VectorTile.Feature> i
       }
     }
 
+    //Upgrade small trunk segments at z5 so they can merge with surrounding motorways
+    if (zoom == 5) {
+      for (var item : items) {
+        var highway = item.tags().get(Fields.CLASS);
+        if (highway instanceof String highwayStr && highwayStr.equals(FieldValues.CLASS_TRUNK)) {
+          item.tags().put(Fields.CLASS, FieldValues.CLASS_MOTORWAY);
+        }
+        if (highway instanceof String highwayStr && highwayStr.equals(FieldValues.CLASS_TRUNK_CONSTRUCTION)) {
+          item.tags().put(Fields.CLASS, FieldValues.CLASS_MOTORWAY_CONSTRUCTION);
+        }
+      }
+    }
+
     var merged = FeatureMerge.mergeLineStrings(items, minLength, tolerance, BUFFER_SIZE);
 
     for (var item : merged) {

src/test/java/org/openmaptiles/layers/TransportationTest.java

@@ -4,9 +4,11 @@
 import static com.onthegomap.planetiler.TestUtils.newPoint;
 import static com.onthegomap.planetiler.TestUtils.rectangle;
 import static java.util.Map.entry;
+import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 
 import com.onthegomap.planetiler.FeatureCollector;
+import com.onthegomap.planetiler.VectorTile;
 import com.onthegomap.planetiler.config.Arguments;
 import com.onthegomap.planetiler.config.PlanetilerConfig;
 import com.onthegomap.planetiler.geo.GeometryException;
@@ -16,6 +18,7 @@
 import com.onthegomap.planetiler.reader.osm.OsmRelationInfo;
 import com.onthegomap.planetiler.stats.Stats;
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.stream.Stream;
@@ -396,7 +399,7 @@ void testDuplicateRoute() {
       "_layer", "transportation",
       "class", "trunk",
       "network", "us-state",
-      "_minzoom", 6
+      "_minzoom", 5
     ), Map.of(
       "_layer", "transportation_name",
       "class", "trunk",
@@ -1217,7 +1220,7 @@ void testTransCanadaProvincialCaOnPrimaryRefOther() {
       "_layer", "transportation",
       "class", "trunk",
       "network", "ca-provincial",
-      "_minzoom", 6
+      "_minzoom", 5
     ), Map.of(
       "_layer", "transportation_name",
       "class", "trunk",
@@ -1271,7 +1274,7 @@ void testTransCanadaProvincialCaMbPthRefOther() {
       "_layer", "transportation",
       "class", "trunk",
       "network", "ca-provincial",
-      "_minzoom", 6
+      "_minzoom", 5
     ), Map.of(
       "_layer", "transportation_name",
       "class", "trunk",
@@ -1325,7 +1328,7 @@ void testTransCanadaProvincialCaAbPrimaryRefOther() {
       "_layer", "transportation",
       "class", "trunk",
       "network", "ca-provincial",
-      "_minzoom", 6
+      "_minzoom", 5
     ), Map.of(
       "_layer", "transportation_name",
       "class", "trunk",
@@ -1379,7 +1382,7 @@ void testTransCanadaProvincialCaBcRefOther() {
       "_layer", "transportation",
       "class", "trunk",
       "network", "ca-provincial",
-      "_minzoom", 6
+      "_minzoom", 5
     ), Map.of(
       "_layer", "transportation_name",
       "class", "trunk",
@@ -1404,7 +1407,7 @@ void testTransCanadaProvincialCaOther() {
     assertFeatures(13, List.of(Map.of(
       "_layer", "transportation",
       "class", "trunk",
-      "_minzoom", 6
+      "_minzoom", 5
     )), features);
     boolean caProvPresent = StreamSupport.stream(features.spliterator(), false)
       .flatMap(f -> f.getAttrsAtZoom(13).entrySet().stream())
@@ -2192,7 +2195,7 @@ void testARoad() {
     assertFeatures(13, List.of(Map.of(
       "_layer", "transportation",
       "class", "trunk",
-      "_minzoom", 6
+      "_minzoom", 5
     ), Map.of(
       "_layer", "transportation_name",
       "class", "trunk",
@@ -2227,4 +2230,55 @@ void testERoad() {
       "route_1_ref", "E 77"
     )), features);
   }
+
+  @Test
+  void testShortTrunkMerge() throws GeometryException {
+    var layer = Transportation.LAYER_NAME;
+
+    var motorwayZ6 = new VectorTile.Feature(
+      layer,
+      1,
+      VectorTile.encodeGeometry(newLineString(0, 0, 10, 10)),
+      new HashMap<>(Map.of("class", "motorway")),
+      0
+    );
+    var trunkZ6 = new VectorTile.Feature(
+      layer,
+      2,
+      VectorTile.encodeGeometry(newLineString(10, 10, 10, 11)),
+      new HashMap<>(Map.of("class", "trunk")),
+      0
+    );
+    var motorwayZ5 = new VectorTile.Feature(
+      layer,
+      1,
+      VectorTile.encodeGeometry(newLineString(0, 0, 10, 10)),
+      new HashMap<>(Map.of("class", "motorway")),
+      0
+    );
+    var trunkZ5 = new VectorTile.Feature(
+      layer,
+      2,
+      VectorTile.encodeGeometry(newLineString(10, 10, 10, 11)),
+      new HashMap<>(Map.of("class", "trunk")),
+      0
+    );
+
+    List<VectorTile.Feature> inputZ6 = List.<VectorTile.Feature>of(motorwayZ6, trunkZ6);
+    List<VectorTile.Feature> inputZ5 = List.<VectorTile.Feature>of(motorwayZ5, trunkZ5);
+
+    List<VectorTile.Feature> resultZ6 = profile.postProcessLayerFeatures(layer, 6, inputZ6);
+    List<VectorTile.Feature> resultZ5 = profile.postProcessLayerFeatures(layer, 5, inputZ5);
+
+    assertEquals(2, resultZ6.size(), "Should be separate features at zoom 6");
+    assertEquals(1, resultZ5.size(), "Should merge into a single feature at zoom 5");
+
+    VectorTile.Feature mergedFeatureZ5 = resultZ5.get(0);
+    assertEquals("motorway", mergedFeatureZ5.tags().get("class"), "Merged feature should be motorway class at zoom 5");
+
+    List<String> classesZ6 = resultZ6.stream()
+      .map(f -> (String) f.tags().get("class"))
+      .toList();
+    assertEquals(List.of("motorway", "trunk"), classesZ6, "At zoom 6, should have motorway and trunk classes");
+  }
 }