Constrain visible roof grouping to compact clusters

backend/tests/test_building_label_normalization.py

@@ -179,3 +179,25 @@ def test_normalizer_merges_only_touching_visible_roof_instances(tmp_path: Path)
     assert sorted(item["properties"]["source_feature_count"] for item in normalized["features"]) == [1, 2]
     assert audit["accepted_source_feature_count"] == 3
     assert audit["accepted_feature_count"] == 2
+
+
+def test_visible_roof_merge_retains_large_touching_chains() -> None:
+    features = []
+    for index in range(13):
+        left = float(index)
+        features.append(
+            {
+                "type": "Feature",
+                "id": str(index),
+                "properties": {"source_feature_id": str(index)},
+                "geometry": {
+                    "type": "Polygon",
+                    "coordinates": [[[left, 0], [left + 1, 0], [left + 1, 1], [left, 1], [left, 0]]],
+                },
+            }
+        )
+    merged = module.merge_touching_roof_instances(features, "grb")
+    assert len(merged) == 13
+    assert {item["properties"]["label_semantics"] for item in merged} == {
+        "native_instance_complex_touch_group"
+    }

docs/BELGIUM_BUILDING_TRAINING_LOOP.md

@@ -102,3 +102,10 @@ and visible-instance counts. This mode is not the default: the Belgium v8
 experiment showed that unconditional touching-footprint dissolve can merge
 whole urban blocks and therefore must pass the same independent gates before
 it can replace native instances.
+
+The compact-roof variant therefore merges a connected group only when it has
+at most 12 source footprints and fills at least 55% of its axis-aligned
+envelope. Larger or irregular connected groups retain their native instances
+and are marked `native_instance_complex_touch_group`. These fixed criteria
+prevent administrative row-house chains from becoming one ambiguous detector
+box while keeping the experiment deterministic and auditable.

scripts/normalize_belgium_building_labels.py

@@ -91,7 +91,7 @@ def _polygonal(geometry: Any) -> Any | None:
 
 
 def merge_touching_roof_instances(features: list[dict[str, Any]], source_name: str) -> list[dict[str, Any]]:
-    """Dissolve only touching/overlapping footprints into imagery-visible roof instances."""
+    """Dissolve only compact touching groups into imagery-visible roof instances."""
     if not features:
         return []
     source_geometries = [(feature, shape(feature["geometry"])) for feature in features]
@@ -105,6 +105,17 @@ def merge_touching_roof_instances(features: list[dict[str, Any]], source_name: s
             if geometry.intersects(component)
         ]
         source_ids = sorted(str(feature["properties"]["source_feature_id"]) for feature in contributors)
+        envelope_area = component.envelope.area
+        fill_ratio = component.area / envelope_area if envelope_area else 0.0
+        # Large connected blocks and irregular chains are administratively
+        # adjacent but not one reliably box-shaped roof target. Preserve their
+        # native instances instead of creating a giant ambiguous detector box.
+        if len(contributors) > 12 or fill_ratio < 0.55:
+            for feature in contributors:
+                retained = json.loads(json.dumps(feature))
+                retained["properties"]["label_semantics"] = "native_instance_complex_touch_group"
+                merged.append(retained)
+            continue
         properties = dict(contributors[0]["properties"])
         properties.update(
             {
@@ -112,6 +123,7 @@ def merge_touching_roof_instances(features: list[dict[str, Any]], source_name: s
                 "source_feature_ids": source_ids,
                 "source_feature_count": len(source_ids),
                 "source_feature_id": source_ids[0],
+                "roof_group_fill_ratio": fill_ratio,
             }
         )
         digest = hashlib.sha256(component.normalize().wkb).hexdigest()[:24]