Fix OverlayNG Area Check heuristic for difference

modules/core/src/main/java/org/locationtech/jts/operation/overlayng/OverlayUtil.java

@@ -392,6 +392,8 @@ public static boolean isResultAreaConsistent(Geometry geom0, Geometry geom1, int
     if (geom0 == null || geom1 == null) 
       return true;
 
+    if (result.getDimension() < 2) return true;
+
     double areaResult = result.getArea();
     double areaA = geom0.getArea();
     double areaB = geom1.getArea();
@@ -403,8 +405,7 @@ public static boolean isResultAreaConsistent(Geometry geom0, Geometry geom1, int
                   && isLess(areaResult, areaB, AREA_HEURISTIC_TOLERANCE);
       break;
     case OverlayNG.DIFFERENCE:
-      isConsistent = isLess(areaResult, areaA, AREA_HEURISTIC_TOLERANCE)
-                  && isGreater(areaResult, areaA - areaB, AREA_HEURISTIC_TOLERANCE);
+      isConsistent = isDifferenceAreaConsistent(areaA, areaB, areaResult, AREA_HEURISTIC_TOLERANCE);
       break;
     case OverlayNG.SYMDIFFERENCE:
       isConsistent = isLess(areaResult, areaA + areaB, AREA_HEURISTIC_TOLERANCE);
@@ -417,6 +418,23 @@ && isLess(areaB, areaResult, AREA_HEURISTIC_TOLERANCE)
     }
     return isConsistent;
   }
+
+  /**
+   * Tests if the area of a difference is greater than the minimum possible difference area.
+   * This is a heuristic which will only detect gross overlay errors.
+   * @param areaA the area of A
+   * @param areaB the area of B
+   * @param areaResult the result area
+   * @param tolFrac the area tolerance fraction
+   * 
+   * @return true if the difference area is consistent.
+   */
+  private static boolean isDifferenceAreaConsistent(double areaA, double areaB, double areaResult, double tolFrac) {
+    if (! isLess(areaResult, areaA, tolFrac))
+      return false;
+    double areaDiffMin = areaA - areaB - tolFrac * areaA;
+    return areaResult > areaDiffMin;
+  }
 
   private static boolean isLess(double v1, double v2, double tol) {
     return v1 <= v2 * (1 + tol);

modules/core/src/test/java/org/locationtech/jts/operation/overlayng/OverlayNGSnappingNoderTest.java

@@ -11,6 +11,7 @@
  */
 package org.locationtech.jts.operation.overlayng;
 
+import static org.locationtech.jts.operation.overlayng.OverlayNG.DIFFERENCE;
 import static org.locationtech.jts.operation.overlayng.OverlayNG.INTERSECTION;
 import static org.locationtech.jts.operation.overlayng.OverlayNG.UNION;
 
@@ -70,11 +71,27 @@ public void testTrianglesBSegmentsDisplacedUnion() {
     checkEqual(expected, union(a, b, 0.1));
   }
 
+  /**
+   * Failing due to OverlayUtil#isResultAreaConsistent
+   * See https://github.com/locationtech/jts/issues/951
+   */
+  public void testRotatedVerticesDifference() {
+    Geometry a = read("POLYGON ((0.37676311 2.57570853, 7.28652472 0.00028375, 7.60034931 0.81686059, 0.50229292 3.4551325, 0.37676311 2.57570853))");
+    Geometry b = read("POLYGON ((0.50229292 3.4551325, 7.60034931 0.81686059, 7.28652472 0.00028375, 0.37676311 2.57570853, 0.50229292 3.4551325))");
+    Geometry expected = read("POLYGON EMPTY");
+    checkEqual(expected, difference(a, b, 0.00001));
+  }
+
   public static Geometry union(Geometry a, Geometry b, double tolerance) {
     Noder noder = getNoder(tolerance);
     return OverlayNG.overlay(a, b, UNION, null, noder );
   }
 
+  public static Geometry difference(Geometry a, Geometry b, double tolerance) {
+    Noder noder = getNoder(tolerance);
+    return OverlayNG.overlay(a, b, DIFFERENCE, null, noder );
+  }
+
   private static Noder getNoder(double tolerance) {
     SnappingNoder snapNoder = new SnappingNoder(tolerance);
     return new ValidatingNoder(snapNoder);