fix elongation()

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 * Fixed invalid results given by `PGS_Morphology.rounding()`.
+* `PGS_ShapePredicates.elongation()` now correctly measures shape elongation (previously inverted, now returns 1 for highly elongated shapes).
 
 ### Removed
 

src/main/java/micycle/pgs/PGS_ShapePredicates.java

@@ -415,11 +415,14 @@ public static double sphericity(final PShape shape) {
 	}
 
 	/**
-	 * Measures the elongation of a shape; the ratio of a shape's bounding box
-	 * length to its width.
+	 * Measures the elongation of a shape as the ratio of the difference between the
+	 * bounding box's length and width to the maximum dimension. A value of 1
+	 * indicates a highly elongated shape, while a value of 0 indicates a square or
+	 * nearly square shape.
 	 * 
 	 * @param shape
-	 * @return a value in [0, 1]
+	 * @return a value in the range [0, 1], where 1 represents high elongation and 0
+	 *         represents no elongation
 	 */
 	public static double elongation(final PShape shape) {
 		Geometry obb = MinimumDiameter.getMinimumRectangle(fromPShape(shape));
@@ -429,11 +432,9 @@ public static double elongation(final PShape shape) {
 		Coordinate c2 = rect.getCoordinates()[2];
 		double l = c0.distance(c1);
 		double w = c1.distance(c2);
-		if (l >= w) {
-			return w / l;
-		} else {
-			return l / w;
-		}
+		double max = Math.max(l, w);
+		double min = Math.min(l, w);
+		return 1 - (min / max);
 	}
 
 	/**

src/test/java/micycle/pgs/PGS_ShapePredicatesTests.java

@@ -18,7 +18,7 @@ class PGS_ShapePredicatesTests {
 
 	private static final double EPSILON = 1E-4;
 
-	static PShape square, triangle;
+	static PShape square, triangle, rect;
 
 	@BeforeAll
 	static void initShapes() {
@@ -29,6 +29,14 @@ static void initShapes() {
 		square.vertex(10, 10);
 		square.vertex(0, 10);
 		square.endShape(PConstants.CLOSE); // close affects rendering only -- does not append another vertex
+
+		rect = new PShape(PShape.GEOMETRY); // 10x10 rect
+		rect.beginShape();
+		rect.vertex(0, 0);
+		rect.vertex(10, 0);
+		rect.vertex(10, 20);
+		rect.vertex(0, 20);
+		rect.endShape(PConstants.CLOSE); // close affects rendering only -- does not append another vertex
 
 		float[] centroid = new float[] { 0, 0 };
 		float side_length = 10;
@@ -87,5 +95,11 @@ void testIsClockwise() {
 		assertFalse(PGS_ShapePredicates.isClockwise(PGS_Conversion.fromPVector(ccw)));
 
 	}
+
+	@Test
+	void testElongation() {
+		assertEquals(0, PGS_ShapePredicates.elongation(square));
+		assertEquals(0.5, PGS_ShapePredicates.elongation(rect));
+	}
 
 }