//###################################################################################################################
        /// <summary>
        /// Ray hit test in space from 2D coordinate of a camera field of view.
        /// </summary>
        /// <param name="camera"> the camera where the hit test start, UrhoSharp CullingCamera recommanded </param>
        /// <param name="x"> ray position on x axis from left up corner of camera view, normalised value </param>
        /// <param name="y"> ray position on y axis from left up corner of camera view, normalised value </param>
        /// <returns> ray hit test information. If no collision detected is detected, return null </returns>
        private RayQueryResult?RayHit(Camera camera, float x, float y)
        {
            Ray cameraRay = CullingCamera.GetScreenRay(x, y); // ray coresponding to center of field of view
            // hit test using the previous ray
            var result = Scene.GetComponent <Octree>().RaycastSingle(cameraRay, RayQueryLevel.Triangle, 100, DrawableFlags.Geometry, 0x70000000);

            return(result);
        }
        //###################################################################################################################
        protected override void OnUpdate(float timeStep)
        {
            Ray cameraRay = CullingCamera.GetScreenRay(0.5f, 0.5f); // ray coresponding to center of field of view
            // hit test using the previous ray
            var result = Scene.GetComponent <Octree>().RaycastSingle(cameraRay, RayQueryLevel.Triangle, 100, DrawableFlags.Geometry, 0x70000000);

            if (result != null) // if the ray hit something
            {
                // display green sphere at ray hit position
                sphere.Node.Position = result.Value.Position;
                sphere.Color         = Color.Green;
            }
            else
            {
                // display red sphere at max range of ray hit test
                sphere.Node.Position = CullingCamera.Node.Position + CullingCamera.Node.Rotation * new Vector3(0, 0, 5);
                sphere.Color         = Color.Red;
            }
        }