Creating outline from a collection of points following the concavity

Question

I am trying to create an outline from a collection of points. I found this solution for Convex Hull. I specifically picked this implementation: https://en.wikibooks.org/wiki/Algorithm_Implementation/Geometry/Convex_hull/Monotone_chain

Edit: I can't find an algorithm that will find the concavity of the region specified in green. The yellow line is the Hull from the algorithm amove

I have the point is defined as:

[Serializable]
public struct Point : IComparable<Point>, IEquatable<Point>
{
    public int X;
    public int Y;

    public static int Cross(Point o, Point a, Point b)
    {
         return (a.X - o.X) * (b.Y - o.Y) - (a.Y - o.Y) * (b.X - o.X);
    }
}

Here is the implementation of the algorithm:

        public static List<Point> ConvexHull(List<Point> points)
        {
            if (points == null || points.Count <= 3) {
                return points;
            }

            var size = points.Count;
            var k = 0;
            var hull = new Point[2 * size];

            points.Sort();

            //build lower hull
            for (int i = 0; i < size; ++i) {
                while (k >= 2 && Point.Cross(hull[k - 2], hull[k - 1], points[i]) <= 0) {
                    k--;
                }

                hull[k++] = points[i];
            }

            //build upper hull
            for (int i = size - 1, t = k + 1; i > 0; --i) {
                while (k >= t && Point.Cross(hull[k - 2], hull[k - 1], points[i - 1]) <= 0) {
                    k--;
                }

                hull[k++] = points[i - 1];
            }

            var result = new List<Point>(hull);

            if (k > 1) {
                result.Resize(k - 1);// remove non-hull vertices after k; remove k - 1 which is a duplicate
            }

            return result;
        }