private void checkBoxPrune_CheckedChanged(object sender, EventArgs e)
        {
            if (_faceDetect != null)
            {
                _faceDetect.SetPrune(checkBoxPrune.Checked);
            }

            // recompute the detected rectangles
            if (listBoxDetectedRect.Items.Count > 0)
            {
                listBoxDetectedRect.SelectedIndex = 0;
            }
            else
            {
                _fImgDirty = true;
                RenderImage(CreateGraphics());
            }
        }