Merge pull request #1704 from tilezen/zerebubuth/fix-buffer-topology-problem

Fix buffer topology problem with building merging

Author: zerebubuth

test/test_transform.py

@@ -943,3 +943,25 @@ def test_half_left(self):
 
     def test_half_top(self):
         self._intersects((0, 0, 2, 5), (1, 1, 3, 4))
+
+
+class MergeBuildingTest(unittest.TestCase):
+
+    def test_merge_buildings(self):
+        from shapely.wkb import loads
+        from vectordatasource.transform import _merge_buildings
+
+        mp = loads(
+            '01060000000200000001030000000100000005000000295C8FC2F57A9040'
+            'E17A140E126B59410000000000A3904085EB51D8126B594148E17A14AEB9'
+            '9040C3F5285C0E6B5941666666666692904085EB51A80D6B5941295C8FC2'
+            'F57A9040E17A140E126B5941010300000001000000050000005C8FC2F528'
+            '439040713D0A070C6B5941666666666692904085EB51A80D6B5941333333'
+            '3333B99040EC51B84E066B594100000000006A9040B81E85AB046B59415C'
+            '8FC2F528439040713D0A070C6B5941'.decode('hex')
+        )
+        tolerance = 1.9109257071294063
+        result = _merge_buildings(mp, tolerance)
+
+        self.assertEquals(len(result), 1)
+        self.assertTrue(result[0].is_valid)

vectordatasource/transform.py

@@ -3190,6 +3190,11 @@ def _merge_buildings(polygon_shapes, tolerance):
     from shapely.geometry import JOIN_STYLE
 
     area_tolerance = tolerance * tolerance
+    # small factor, relative to tolerance. this is used so that we don't buffer
+    # polygons out by exactly the same amount as we buffer them inwards. using
+    # the exact same value ends up causing topology problems when two points on
+    # opposing sides of the polygon meet eachother exactly.
+    epsilon = tolerance * 1.0e-6
 
     result = _merge_polygons(polygon_shapes)
     if not result:
@@ -3202,14 +3207,15 @@ def _merge_buildings(polygon_shapes, tolerance):
     # keeps angles the same. to avoid spikes, we limit the mitre to a little
     # under 90 degrees.
     result = result.buffer(
-        tolerance, join_style=JOIN_STYLE.mitre, mitre_limit=1.5)
+        tolerance - epsilon, join_style=JOIN_STYLE.mitre, mitre_limit=1.5)
     result = result.simplify(tolerance)
     result = _drop_small_inners_multi(result, area_tolerance)
     result = result.buffer(
         -tolerance, join_style=JOIN_STYLE.mitre, mitre_limit=1.5)
     result = _drop_small_outers_multi(result, area_tolerance)
 
-    if result.is_empty:
+    # don't return invalid results!
+    if result.is_empty or not result.is_valid:
         return []
 
     return [result]