can anyone help me to Use watershed algorithm emgu.CV for blob detection (c#)

public void watershed(Bitmap bm)
{
    Image<Bgr, Byte> imWa = new Image<Bgr, byte>(bm);
    Image<Gray, Int32> imgr = new Image<Gray, int>(imWa.Width, imWa.Height);

    Rectangle rec = imWa.ROI;
    CvInvoke.cvZero(imgr);
    imgr.Draw(new CircleF(new PointF(rec.Left + rec.Width / 2.0f, rec.Top + rec.Height / 2.0f), (float)(Math.Min(imWa.Width, imWa.Height) / 4.0f)), new Gray(255), 0);
    CvInvoke.cvWatershed(imWa, imgr);
    pictureBox1.Image = imWa.ToBitmap() ;
    MessageBox.Show("imWa");
    pictureBox1.Image = imgr.ToBitmap();
    MessageBox.Show("imgr");
    Bitmap bmF = new Bitmap(bm.Width, bm.Height);
    pictureBox1.Image = bmF;

    
}