ORB and FREAK

// ORB
var orb = ORB.Create(1000);
KeyPoint[] keypoints = orb.Detect(gray);

// FREAK
FREAK freak = FREAK.Create();
Mat freakDescriptors = new Mat();
freak.Compute(gray, ref keypoints, freakDescriptors);

if (keypoints != null)
{
    var color = new Scalar(0, 255, 0);
    foreach (KeyPoint kpt in keypoints)
    {
        float r = kpt.Size / 2;
        Cv2.Circle(dst, kpt.Pt, (int)r, color, 1, LineType.Link8, 0);
        Cv2.Line(dst, 
            new Point2f(kpt.Pt.X + r, kpt.Pt.Y + r), 
            new Point2f(kpt.Pt.X - r, kpt.Pt.Y - r), 
            color, 1, LineType.Link8, 0);
        Cv2.Line(dst, 
            new Point2f(kpt.Pt.X - r, kpt.Pt.Y + r), 
            new Point2f(kpt.Pt.X + r, kpt.Pt.Y - r), 
            color, 1, LineType.Link8, 0);
    }
}

using (new Window("FREAK features", dst))
{
    Cv.WaitKey();
}