OpenCVSharpにてMatを利用してピクセルのRGB値を変更する

setMatPixelColor.cs

Mat m = new Mat(@"C:\Users\Public\Pictures\Sample Pictures\tu.jpg");
Cv2.ImShow("before", m);

Mat m2 = new Mat(m.Height, m.Width, MatType.CV_8UC3);
for (int i = 0; i < m2.Height; i++){
  for (int j = 0; j < m2.Width; j++){
    byte by = 255;
    Vec3b pix = m2.At<Vec3b>(i, j);
    pix[0]=0;   //B
    pix[1] = by; //G
    pix[2]=0; //R
    m2.Set<Vec3b>(i, j, pix);
 }
}
Cv2.ImShow("after", m2);