Add LabelCenterline function

sql/LabelCenterline.sql

@@ -0,0 +1,71 @@
+/******************************************************************************
+### LabelCenterline ###
+
+Given a polygon or multipolygon, calculates a linestring suitable for a label.
+
+__Parameters:__
+
+- `geometry` inGeometry - A polygon or multipolygon.
+
+__Returns:__ `geometry(multiline)`
+******************************************************************************/
+CREATE OR REPLACE FUNCTION CountDisconnectedEndpoints(polyline geometry, testline geometry)
+    RETURNS integer AS $$
+    BEGIN
+        RETURN ST_NPoints(ST_RemoveRepeatedPoints(ST_Points(polyline)))
+                    - ST_NPoints(ST_RemoveRepeatedPoints(ST_Points(ST_Difference(polyline, testline))))
+                    - ST_NPoints(testline) + 2;
+    END
+    $$ LANGUAGE plpgsql;
+CREATE OR REPLACE FUNCTION TrimmedCenterline(inPolyline geometry)
+    RETURNS geometry AS $$
+    DECLARE outPolyline geometry;
+    BEGIN
+        WITH tbla AS (
+            SELECT inPolyline as polyline, (ST_Dump(inPolyline)).geom as edge
+        ),
+        tblb AS (
+            SELECT polyline, edge as shortestBranchLine
+            FROM tbla
+            WHERE CountDisconnectedEndpoints(polyline, edge) > 0
+            ORDER BY ST_Length(edge) ASC
+            LIMIT 1
+        ),
+        tblc AS (
+            SELECT ST_LineMerge(ST_Difference(polyline, shortestBranchLine)) as polyline
+            FROM tblb
+        )
+        SELECT TrimmedCenterline(polyline) as polyline
+        FROM tblc
+        WHERE ST_NumGeometries(polyline) > 1
+        UNION ALL
+        SELECT polyline INTO outPolyline FROM tblc;
+        RETURN outPolyline;
+    END
+    $$ LANGUAGE plpgsql;
+CREATE OR REPLACE FUNCTION LabelCenterline(inGeometry geometry)
+    RETURNS geometry AS $$
+    DECLARE outPolyline geometry;
+    BEGIN
+        WITH tbla AS (
+            SELECT inGeometry as inPolygon WHERE ST_GeometryType(inGeometry) = 'ST_Polygon'
+            UNION ALL
+            SELECT ST_ConcaveHull(ST_Simplify(inGeometry, 25), 0.2) as inPolygon WHERE ST_GeometryType(inGeometry)='ST_MultiPolygon'
+        ),
+        tblb AS (
+            SELECT ST_MakePolygon(ST_ExteriorRing(inPolygon)) as shellPolygon
+            FROM tbla
+        ),
+        tblc AS (
+            SELECT shellPolygon, (ST_Dump(ST_VoronoiLines(ST_LineInterpolatePoints(ST_Boundary(shellPolygon), 0.0075)))).geom as voroniLines
+            FROM tblb
+        ),
+        tbld AS (
+            SELECT ST_LineMerge(ST_Collect(voroniLines)) as voroniPolyline
+            FROM tblc
+            WHERE ST_Contains(shellPolygon, voroniLines)
+        )
+        SELECT ST_ChaikinSmoothing(ST_SimplifyPreserveTopology(TrimmedCenterline(voroniPolyline), 80), 3, false) INTO outPolyline FROM tbld;
+        RETURN outPolyline;
+    END
+    $$ LANGUAGE plpgsql;