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Improve MultiPolygon centroid robustness #118

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Aug 22, 2017
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Improve MultiPolygon centroid robustness #118

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36b8ae3
Improve MultiPolygon centroid robustness
dbaston Aug 19, 2017
ce19c0a
Refactor test case
dbaston Aug 22, 2017
317365b
Rename method for clarity
dbaston Aug 22, 2017
9e71b7d
Fix formatting of tolerance constant
dbaston Aug 22, 2017
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5 changes: 2 additions & 3 deletions modules/core/src/main/java/org/locationtech/jts/algorithm/Centroid.java
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Original file line number Diff line number Diff line change
Expand Up @@ -151,9 +151,8 @@ else if (ptCount > 0){
return cent;
}

private void setBasePoint(Coordinate basePt)
private void setAreaBasePoint(Coordinate basePt)
{
if (this.areaBasePt == null)
this.areaBasePt = basePt;
}

Expand All @@ -168,7 +167,7 @@ private void add(Polygon poly)
private void addShell(Coordinate[] pts)
{
if (pts.length > 0)
setBasePoint(pts[0]);
setAreaBasePoint(pts[0]);
boolean isPositiveArea = ! CGAlgorithms.isCCW(pts);
for (int i = 0; i < pts.length - 1; i++) {
addTriangle(areaBasePt, pts[i], pts[i+1], isPositiveArea);
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50 changes: 50 additions & 0 deletions modules/core/src/test/java/org/locationtech/jts/algorithm/CentroidTest.java
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Original file line number Diff line number Diff line change
@@ -0,0 +1,50 @@
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.io.WKTReader;

public class CentroidTest extends TestCase {

private static final double TOLERANCE = 1e-10;

public CentroidTest(String name) {
super(name);
}

/** Compute the centroid of a geometry as an area-weighted average of the centroids
* of its components.
*
* @param g a polygonal geometry
* @return Coordinate of the geometry's centroid
*/
private static Coordinate areaWeightedCentroid(Geometry g) {
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;
}

return new Coordinate(cx, cy);
}

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 )))"
);

assertTrue(areaWeightedCentroid(g).equals2D(g.getCentroid().getCoordinate(), TOLERANCE));
}

}
26 changes: 25 additions & 1 deletion modules/tests/src/test/resources/testxml/general/TestCentroid.xml
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Expand Up @@ -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>
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A similar comment here. This seems brittle WRT numeric precision. Not your problem, but it would be nice if the XML test syntax allowed defining a precision tolerance for expected results. (For centroid computation the tolerance can be quite fine, since the algorithm should produce very close results).

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Since the XML Test format doesn't provide a way of expressing a tolerance value, this is fine as it stands. If it breaks in the future, that will be motivation to add a tolerance!

</case>

</run>