How to check if VERTEX ARRAY is clockwise?

Question

Solved, used this code:

if ( !isClockwise(TempVectArray) ) { Collections.reverse(TempVectArray); }

...

private boolean isClockwise(ArrayList<Vec2> arl){
    Iterator<Vec2> it = arl.iterator();
    Vec2 pt1 = (Vec2)it.next();
    Vec2 firstPt = pt1;
    Vec2 lastPt = null;
    double area = 0.0;
    while(it.hasNext()){
        Vec2 pt2 = (Vec2) it.next();
        area += (((pt2.x - pt1.x) * (pt2.y + pt1.y)) / 2);
        pt1 = pt2;
        lastPt = pt1;
    }
    area += (((firstPt.x - lastPt.x) * (firstPt.y + lastPt.y)) / 2);
    return area < 0;
}

---

Suppose I get a vertex array from the user tapping on the screen, but need it to be clockwise.

Maybe you know of some standard methods to check if it is clockwise and if it's not, then make it clockwise?

Thanks!

Answer 1

One way to do it is to first calculate the average point, and then sort everything around it by angle. Should be something like this:

public static void sortPointsClockwise(ArrayList<PointF> points) {
    float averageX = 0;
    float averageY = 0;

    for (PointF point : points) {
        averageX += point.x;
        averageY += point.y;
    }

    final float finalAverageX = averageX / points.size();
    final float finalAverageY = averageY / points.size();

    Comparator<PointF> comparator = new Comparator<PointF>() {
        public int compare(PointF lhs, PointF rhs) {
            double lhsAngle = Math.atan2(lhs.y - finalAverageY, lhs.x - finalAverageX);
            double rhsAngle = Math.atan2(rhs.y - finalAverageY, rhs.x - finalAverageX);

            // Depending on the coordinate system, you might need to reverse these two conditions
            if (lhsAngle < rhsAngle) return -1;
            if (lhsAngle > rhsAngle) return 1;

            return 0;
        }
    };

    Collections.sort(points, comparator);
}

public static void sortPointsCounterClockwise(ArrayList<PointF> points) {
    sortPointsClockwise(points);
    Collections.reverse(points);
}