None.

[C#]

using (Bitmap bm = (Bitmap)Bitmap.FromFile(Server.MapPath("rez/transparent.png"))) {
  bm.PixelFormat = new PixelFormat(bm.PixelFormat, ColorChannel.Alpha, false);
  Effects.AdjustColors<object>(bm, null, (color, context) => {
    color.R = color.A;
    color.G = color.A;
    color.B = color.A;
    color.A = 0xFF;
    return color;
  });
  bm.Save(Server.MapPath("IG8_Effect_PixelFormat.jpg"));
}

The above reads a PNG with transparency and extracts the alpha channel as a gray intensity map. The input and output files are shown below.

transparent.png

IG8_Effect_PixelFormat.jpg