Created a visualizer for the ICP algorithm to figure out what's going wrong.

tests/ScanMatcherTest.java

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+import ScanGraph.ScanMatcher;
+import ScanGraph.ScanPoint;
+import org.junit.Before;
+import org.junit.jupiter.api.BeforeEach;
+import processing.core.PApplet;
+
+import org.junit.jupiter.api.Test;
+import Vector.Vector;
+
+import java.util.ArrayList;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static processing.core.PApplet.main;
+
+class ScanMatcherTest{
+    /**
+     * @brief Generate a scan point from a scan description
+     * @param offset The offset of the scan point from the origin
+     * @param scanDescription A vector which describes the length of the line and direction of the line
+     * @return A scan point with the given offset and scan description
+     */
+    public ScanPoint generateScanPoint(Vector offset, Vector scanDescription, int numPoints){
+        // generate a scan point with the given offset and scan description
+        ArrayList<Vector> scan = new ArrayList<>();
+
+        // divide the scan description by the number of points to allow us to scale it back up in the loop
+        Vector directionVector = scanDescription.div(numPoints-1);
+
+        for (int i = 0; i < numPoints; i++) {
+            scan.add(offset.add(directionVector.mul(i)));
+        }
+
+        return new ScanPoint(new Vector(0, 0), 0, scan);
+    }
+
+    /**
+     * @brief Append two scan points together
+     * @param scan1 The first scan point to append
+     * @param scan2 The second scan point to append
+     * @return A scan point that is the combination of the two scan points
+     */
+    public ScanPoint appendScanPoints(ScanPoint scan1, ScanPoint scan2){
+        ArrayList<Vector> points = new ArrayList<>();
+        points.addAll(scan1.getPoints());
+        points.addAll(scan2.getPoints());
+        return new ScanPoint(new Vector(0, 0), 0, points);
+    }
+
+    @Test
+    public void applyRotationAndTranslationMatrices() {
+        // generate one scan that is level and another that is rotated 45 degrees.
+        Vector scanDescription = new Vector(10, 0);
+        ScanPoint referenceScan = generateScanPoint(new Vector(0, 0), scanDescription, 10);
+        ScanPoint newScan = generateScanPoint(new Vector(0, 0), scanDescription.rotate2D((float) Math.PI / 4), 10);
+
+        // calculate the rotation and translation matrices between the two scans
+        ScanMatcher matcher = new ScanMatcher();
+        matcher.calculateRotationAndTranslationMatrices(referenceScan, newScan);
+        // apply the rotation and translation matrices to the new scan
+        ScanPoint newScanWithRotationAndTranslation = matcher.applyRotationAndTranslationMatrices(newScan);
+
+        // Get the first and last points of the new scan with rotation and translation and calculate the angle between them
+        ArrayList<Vector> points = newScanWithRotationAndTranslation.getPoints();
+        Vector firstPoint = points.get(0);
+        Vector lastPoint = points.get(points.size() - 1);
+        Vector rotatedDirection = lastPoint.sub(firstPoint);
+        float angle = scanDescription.angleDiff(rotatedDirection);
+
+        // The angle between the first and last points should be zero
+        assertEquals(0, angle);
+    }
+
+    @Test
+    public void getError() {
+        // generate two scans that are the same. The error should be zero.
+        ScanPoint scan1 = generateScanPoint(new Vector(0, 0), new Vector(10, 10), 12);
+        ScanPoint scan2 = generateScanPoint(new Vector(0, 0), new Vector(10, 10), 12);
+        ScanMatcher matcher = new ScanMatcher();
+        matcher.calculateRotationAndTranslationMatrices(scan1, scan2);
+        assertEquals(0, matcher.getError(scan1, scan2));
+
+        // generate two scans that are the same but one is offset by 10 in the y direction. The error should be 10.
+        scan1 = generateScanPoint(new Vector(0, 0), new Vector(10, 10), 12);
+        scan2 = generateScanPoint(new Vector(0, 10), new Vector(10, 10), 12);
+        matcher.calculateRotationAndTranslationMatrices(scan1, scan2);
+        assertEquals(10, matcher.getError(scan1, scan2));
+
+        // generate two scans that are the same but one is rotated by 45 degrees. The error should be near zero.
+        scan1 = generateScanPoint(new Vector(0, 0), new Vector(10, 10), 12);
+        scan2 = generateScanPoint(new Vector(0, 0), new Vector(10, 10).rotate2D((float) Math.PI / 4), 12);
+        matcher.calculateRotationAndTranslationMatrices(scan1, scan2);
+        assertEquals(0, matcher.getError(scan1, scan2), 0.1);
+    }
+
+    @Test
+    public void iterativeScanMatch() {
+        // generate two scans rotated by 45 degrees and append them together
+        ScanPoint scan1 = generateScanPoint(new Vector(0, 0), new Vector(10, 10), 12);
+        ScanPoint scan2 = generateScanPoint(new Vector(0, 0), new Vector(10, 10).rotate2D((float) Math.PI / 4), 12);
+        ScanPoint scan3 = appendScanPoints(scan1, scan2);
+
+
+        // generate two scans offset by some amount and rotated by 55 degrees and append them together
+        Vector rotated = (new Vector(10, 10)).rotate2D((float) Math.PI);
+        ScanPoint scan4 = generateScanPoint(new Vector(10, 10), rotated, 12);
+        ScanPoint scan5 = generateScanPoint(new Vector(10, 10), rotated.rotate2D((float) Math.PI / 4), 12);
+        ScanPoint scan6 = appendScanPoints(scan4, scan5);
+
+
+        // do a single scan match and calculate the error
+        ScanMatcher matcher = new ScanMatcher();
+        matcher.calculateRotationAndTranslationMatrices(scan3, scan6);
+        ScanPoint oneCalcMatch = matcher.applyRotationAndTranslationMatrices(scan6);
+        float singleScanMatchError = matcher.getError(scan3, oneCalcMatch);
+
+
+        // do an iterative scan match and calculate the error
+        ScanPoint matchedScan = matcher.iterativeScanMatch(scan1, scan2, 0.01f, 10);
+
+        // if it's null something has gone wrong with the algorithm because these scans can easily be matched.
+        assertNotNull(matchedScan);
+
+        float iterativeScanMatchError = matcher.getError(scan1, matchedScan);
+
+        // the iterative scan match should have a lower error than the single scan match
+        assertTrue(iterativeScanMatchError < singleScanMatchError);
+    }
+}