can i let the sphere follow a point(x.y. or.z) on plane?

what i wanna do is that sphere follows center of face everytime. and as you told me yes just on the plane.

i have face.x and face.y position information but it is just on plane limited.

here is part of code

 private void faceDetector()
 {
     currentFrameBgr = cvCapture.QueryFrame().ToImage<Bgr, byte>();

     Texture2D tex = new Texture2D(640, 480);

     if (currentFrameBgr != null)
     {
         Image<Gray, byte> grayFrame = currentFrameBgr.Convert<Gray, byte>();

         var faces = _cascadeClassifier.DetectMultiScale(grayFrame, 1.1, 10, new System.Drawing.Size(frameWidth/8, frameHeight/8));


         foreach (var face in faces)
         {
             currentFrameBgr.Draw(face, new Bgr(0, 255, 0), 3);
             FaceCenterPointX = face.X;
             FaceCenterPointY = face.Y;
 
         }

         //Convert this image into Bitmap, the pixel values are copied over to the Bitmap
         currentFrameBgr.ToBitmap();

         MemoryStream memstream = new MemoryStream();
         currentFrameBgr.Bitmap.Save(memstream, currentFrameBgr.Bitmap.RawFormat);

         tex.LoadImage(memstream.ToArray());
         mt.mainTexture = tex;
     }
 }