Vb.net Net如何将不透明度更改为图像或位图？

我在VB.Net中有一个sub来执行图像的不透明度，但是，它的工作方式与我预期的不同：

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

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有人能告诉我我做错了什么吗？

@Brad:使参数的图像更透明更改位图图像的不透明度