diff --git a/tools/extract/overview.py b/tools/extract/overview.py
index 7f6e0d0..dc2e8cf 100644
--- a/tools/extract/overview.py
+++ b/tools/extract/overview.py
@@ -30,17 +30,32 @@ def _to_px(ring, ox, oy):
     return [((x + ox - SLICE["x0"]) * SCALE, (SLICE["y1"] - (oy - z)) * SCALE) for x, z in ring]
 
 
-def _draw(draw, rings, ox, oy, fill, outline=None):
-    """Außenring füllen, Löcher wieder mit Wasserfarbe ausstanzen."""
-    draw.polygon(_to_px(rings[0], ox, oy), fill=fill, outline=outline)
+def _draw(img, rings, ox, oy, fill, outline=None):
+    """Fläche über eine Maske einsetzen, damit durch die Löcher die Ebene darunter bleibt.
+
+    Ein Loch mit Wasserfarbe zu übermalen wäre falsch: der Innenhof eines Blocks liegt auf
+    Stadtfläche, nicht im Wasser.
+    """
+    outer = _to_px(rings[0], ox, oy)
+    xs = [p[0] for p in outer]
+    ys = [p[1] for p in outer]
+    x0, y0 = int(min(xs)) - 1, int(min(ys)) - 1
+    x1, y1 = int(max(xs)) + 2, int(max(ys)) + 2
+    if x1 <= x0 or y1 <= y0:
+        return
+    mask = Image.new("L", (x1 - x0, y1 - y0), 0)
+    md = ImageDraw.Draw(mask)
+    md.polygon([(x - x0, y - y0) for x, y in outer], fill=255, outline=255)
     for hole in rings[1:]:
-        draw.polygon(_to_px(hole, ox, oy), fill=WATER)
+        md.polygon([(x - x0, y - y0) for x, y in _to_px(hole, ox, oy)], fill=0)
+    img.paste(fill, (x0, y0), mask)
+    if outline:
+        ImageDraw.Draw(img).polygon(outer, fill=None, outline=outline)
 
 
 def render(out_dir: Path, dst: Path) -> dict:
     size = int((SLICE["x1"] - SLICE["x0"]) * SCALE)
     img = Image.new("RGB", (size, size), WATER)
-    draw = ImageDraw.Draw(img)
     chunks = [json.loads(f.read_text()) for f in sorted(out_dir.glob("chunk_*.json"))]
     stats = {"land": 0, "strassen": 0, "bruecken": 0, "gebaeude": 0, "palmen": 0}
 
@@ -48,27 +63,27 @@ def render(out_dir: Path, dst: Path) -> dict:
     for chunk in chunks:
         ox, oy = chunk["origin"][0], -chunk["origin"][1]
         for rings in chunk["land"]:
-            _draw(draw, rings, ox, oy, LAND)
+            _draw(img, rings, ox, oy, LAND)
             stats["land"] += 1
     for chunk in chunks:
         ox, oy = chunk["origin"][0], -chunk["origin"][1]
         for surface in chunk["surfaces"]:
-            _draw(draw, surface["poly"], ox, oy, SURFACE[surface["kind"]])
+            _draw(img, surface["poly"], ox, oy, SURFACE[surface["kind"]])
     for chunk in chunks:
         ox, oy = chunk["origin"][0], -chunk["origin"][1]
         for road in chunk["roads"]:
-            _draw(draw, road["poly"], ox, oy, BRIDGE if road["bridge"] else ROAD)
+            _draw(img, road["poly"], ox, oy, BRIDGE if road["bridge"] else ROAD)
             stats["bruecken" if road["bridge"] else "strassen"] += 1
     for chunk in chunks:
         ox, oy = chunk["origin"][0], -chunk["origin"][1]
         for building in chunk["buildings"]:
             t = min(1.0, building["height"] / TALL)
             shade = (int(70 + 170 * t), int(70 + 120 * t), int(80 + 60 * t))
-            _draw(draw, building["poly"], ox, oy, shade, outline=(30, 30, 30))
+            _draw(img, building["poly"], ox, oy, shade, outline=(30, 30, 30))
             stats["gebaeude"] += 1
         for x, z in chunk["palms"]:
             px, py = _to_px([(x, z)], ox, oy)[0]
-            draw.ellipse([px - 1.6, py - 1.6, px + 1.6, py + 1.6], fill=PALM)
+            ImageDraw.Draw(img).ellipse([px - 1.6, py - 1.6, px + 1.6, py + 1.6], fill=PALM)
             stats["palmen"] += 1
 
     dst.parent.mkdir(parents=True, exist_ok=True)