Improve MultiPolygon centroid robustness

Do this by allowing each polygon shell to set its own origin point,
instead of re-using the origin point from the first polygon in
the geometry.

Signed-off-by: Daniel Baston <[email protected]>

modules/core/src/main/java/org/locationtech/jts/algorithm/Centroid.java

@@ -153,7 +153,6 @@ else if (ptCount > 0){
 
   private void setBasePoint(Coordinate basePt)
   {
-    if (this.areaBasePt == null)
       this.areaBasePt = basePt;
   }
 

modules/core/src/test/java/org/locationtech/jts/algorithm/CentroidTest.java

@@ -0,0 +1,40 @@
+package org.locationtech.jts.algorithm;
+
+import junit.framework.TestCase;
+import org.locationtech.jts.geom.Coordinate;
+import org.locationtech.jts.geom.Geometry;
+import org.locationtech.jts.geom.GeometryFactory;
+import org.locationtech.jts.io.WKTReader;
+
+public class CentroidTest extends TestCase {
+
+
+    public CentroidTest(String name) {
+        super(name);
+    }
+
+    public void testCentroidMultiPolygon() throws Exception {
+        // Verify that the computed centroid of a MultiPolygon is equivalent to the
+        // area-weighted average of its components.
+        Geometry g = new WKTReader().read(
+                "MULTIPOLYGON ((( -92.661322 36.58994900000003, -92.66132199999993 36.58994900000005, -92.66132199999993 36.589949000000004, -92.661322 36.589949, -92.661322 36.58994900000003)), (( -92.65560500000008 36.58708800000005, -92.65560499999992 36.58708800000005, -92.65560499998745 36.587087999992576, -92.655605 36.587088, -92.65560500000008 36.58708800000005 )), (( -92.65512450000065 36.586800000000466, -92.65512449999994 36.58680000000004, -92.65512449998666 36.5867999999905, -92.65512450000065 36.586800000000466 )))"
+        );
+
+        double totalArea = g.getArea();
+        double cx = 0;
+        double cy = 0;
+
+        for(int i = 0; i < g.getNumGeometries(); i++) {
+            Geometry component = g.getGeometryN(i);
+            double areaFraction = component.getArea() / totalArea;
+
+            Coordinate componentCentroid = component.getCentroid().getCoordinate();
+
+            cx += areaFraction * componentCentroid.x;
+            cy += areaFraction * componentCentroid.y;
+        }
+
+        assertEquals(new Coordinate(cx, cy), g.getCentroid().getCoordinate());
+        System.out.println(new GeometryFactory().createPoint(new Coordinate(cx, cy)));
+    }
+}

modules/tests/src/test/resources/testxml/general/TestCentroid.xml

@@ -242,6 +242,30 @@
 <test><op name="getCentroid" arg1="A" >    POINT (56.52883333335 25.21033333335)  </op></test>
 </case>
 
-
+<case>
+  <desc>A - almost degenerate MultiPolygon</desc>
+  <a>
+    MULTIPOLYGON (((
+     -92.661322 36.58994900000003,
+     -92.66132199999993 36.58994900000005,
+     -92.66132199999993 36.589949000000004,
+     -92.661322 36.589949,
+     -92.661322 36.58994900000003)),
+    ((
+     -92.65560500000008 36.58708800000005,
+     -92.65560499999992 36.58708800000005,
+     -92.65560499998745 36.587087999992576,
+     -92.655605 36.587088,
+     -92.65560500000008 36.58708800000005
+    )),
+    ((
+     -92.65512450000065 36.586800000000466,
+      -92.65512449999994 36.58680000000004,
+     -92.65512449998666 36.5867999999905,
+      -92.65512450000065 36.586800000000466
+    )))
+  </a>
+  <test><op name="getCentroid" arg1="A" >POINT (-92.6553838608954 36.58695407733924)</op></test>
+</case>
 
 </run>