How to edit stamp's opacity?

I made Rubber Stamp Annnot by an image file.
then, I'm trying to edit stamp's opacity.
I found getting stamp's appearance. and set opacity.
Show the below code.

// code
if (this.GetType().ToString() == "e_Stamp")
    Element element = null;
    ElementReader reader = new ElementReader();

    // "this" is object of current stamp annotation.
    pdftron.SDF.Obj stm = this.GetAppearance();
    if (stm != null)
        element = reader.Next();

then, How to adjust element at page?
I don't know.. How to do...