VB.Net如何改变图像或位图的不透明度？

Question

我在VB.Net中有一个sub来执行图像的不透明度，但是，不能像我期望的那样工作：

Public Sub mc_SetOpacity(ByRef Image As Image, ByVal Opacity As Double)
        Dim vGraphics As Graphics = Graphics.FromImage(Image)
        Dim vColorMatrix As New ColorMatrix()
        Dim vImgAtt As New ImageAttributes()

        vColorMatrix.Matrix33 = Opacity

        vImgAtt.SetColorMatrix(vColorMatrix, ColorMatrixFlag.[Default], ColorAdjustType.Bitmap)
        vGraphics.DrawImage(Image, New Rectangle(0, 0, Image.Width, Image.Height), 0, 0, Image.Width, Image.Height, GraphicsUnit.Pixel, vImgAtt)

        vGraphics.Dispose() : vGraphics = Nothing
        vImgAtt.Dispose() : vImgAtt = Nothing
    End Sub

有人可以告诉我我做错了什么？