fix: synthesize T-junction sidewalk caps

scripts/build-osm2streets-qgis.js

@@ -530,8 +530,10 @@ function filteredSidewalkCorners(intersections, maxDimensionMeters) {
 
 function buildSidewalkCorners(intersections, plain, network, maxDimensionMeters) {
   const out = filteredSidewalkCorners(intersections, maxDimensionMeters);
-  const fallback = synthesizeMissingSidewalkCorners(out, plain, network, maxDimensionMeters);
-  out.features.push(...fallback);
+  const missing = synthesizeMissingSidewalkCorners(out, plain, network, maxDimensionMeters);
+  out.features.push(...missing);
+  const caps = synthesizeTJunctionSidewalkCaps(out, plain, network, maxDimensionMeters);
+  out.features.push(...caps);
   return out;
 }
 
@@ -554,13 +556,11 @@ function synthesizeMissingSidewalkCorners(existing, plain, network, maxDimension
     const edges = buildIntersectionEdges(intersection, roads, roadFeatures, intersectionFeature);
     if (!edges.length) continue;
     const qualifyingPairs = qualifyingCornerPairs(edges);
-    if (qualifyingPairs.length < 3) continue;
+    if (!qualifyingPairs.length) continue;
 
     const current = existingByIntersection.get(intersectionId) || [];
-    if (qualifyingPairs.length - current.length !== 1) continue;
     const currentCenters = current.map((entry) => entry.center);
-    const targetDimension = median(current.map((entry) => maxFeatureDimensionMeters(entry.feature)).filter(Number.isFinite));
-    const candidates = [];
+    const missing = [];
 
     for (const [one, two] of qualifyingPairs) {
       const candidate = synthesizeCornerFeature(one, two, intersectionFeature, maxDimensionMeters);
@@ -572,17 +572,67 @@ function synthesizeMissingSidewalkCorners(existing, plain, network, maxDimension
       if (dimension === null || dimension > maxDimensionMeters) continue;
       if (polygonAreaMeters2(candidate) < 0.4) continue;
       if (currentCenters.some((point) => pointDistance(point, candidateCenter) <= 0.6)) continue;
-      candidates.push({
-        feature: candidate,
-        center: candidateCenter,
-        dimension,
-        score: Math.abs(dimension - targetDimension),
-      });
+      missing.push(candidate);
     }
 
-    if (!candidates.length) continue;
-    candidates.sort((a, b) => a.score - b.score || a.dimension - b.dimension);
-    synthesized.push(candidates[0].feature);
+    if (missing.length !== 1) continue;
+    synthesized.push(missing[0]);
+  }
+
+  return synthesized;
+}
+
+function synthesizeTJunctionSidewalkCaps(existing, plain, network, maxDimensionMeters) {
+  const roadFeatures = new Map((plain.features || [])
+    .filter((feature) => feature.properties?.type === "road")
+    .map((feature) => [Number(feature.properties.id), feature]));
+  const intersectionFeatures = new Map((plain.features || [])
+    .filter((feature) => feature.properties?.type === "intersection")
+    .map((feature) => [Number(feature.properties.id), feature]));
+  const roads = new Map((network.roads || []).map(([id, road]) => [Number(id), road]));
+  const intersections = new Map((network.intersections || []).map(([id, intersection]) => [Number(id), intersection]));
+  const existingByIntersection = assignCornersToIntersections(existing.features || [], intersectionFeatures);
+  const synthesized = [];
+
+  for (const [intersectionId, intersection] of intersections.entries()) {
+    const intersectionFeature = intersectionFeatures.get(intersectionId);
+    if (!intersectionFeature) continue;
+    if (intersectionFeature.properties?.intersection_kind !== "Intersection") continue;
+    if (new Set(intersection.roads || []).size !== 3) continue;
+
+    const edges = buildIntersectionEdges(intersection, roads, roadFeatures, intersectionFeature);
+    if (!edges.length) continue;
+    const current = existingByIntersection.get(intersectionId) || [];
+    const smallCurrent = current.filter((entry) => {
+      const dimension = maxFeatureDimensionMeters(entry.feature);
+      return Number.isFinite(dimension) && dimension <= maxDimensionMeters;
+    });
+    if (smallCurrent.length !== 2) continue;
+
+    const candidates = [];
+    for (const [one, two] of qualifyingCornerPairs(edges)) {
+      const candidate = synthesizeCornerFeature(one, two, intersectionFeature, 100);
+      if (!candidate) continue;
+      const center = featureCenter(candidate);
+      const dimension = maxFeatureDimensionMeters(candidate);
+      const area = polygonAreaMeters2(candidate);
+      if (!center || !Number.isFinite(dimension) || !Number.isFinite(area)) continue;
+      candidates.push({ feature: candidate, center, dimension, area });
+    }
+
+    // For a true T-junction, the remaining large candidate is the sidewalk "cap"
+    // opposite the side street, not another curb-return corner.
+    const caps = candidates.filter(({ dimension, area, center }) => (
+      dimension > maxDimensionMeters &&
+      dimension <= 12 &&
+      area >= 6 &&
+      area <= 20 &&
+      pointInPolygon(center, intersectionFeature.geometry.coordinates) &&
+      !smallCurrent.some((entry) => pointDistance(entry.center, center) <= 1)
+    ));
+    if (caps.length !== 1) continue;
+    caps[0].feature.properties.source = "fallback_t_cap";
+    synthesized.push(caps[0].feature);
   }
 
   return synthesized;
@@ -905,12 +955,6 @@ function polygonAreaMeters2(feature) {
   return Math.abs(area) / 2;
 }
 
-function median(values) {
-  if (!values.length) return 0;
-  const sorted = [...values].sort((a, b) => a - b);
-  return sorted[Math.floor(sorted.length / 2)];
-}
-
 function maxFeatureDimensionMeters(feature) {
   const points = [];
   collectCoords(feature.geometry?.coordinates, points);