Visual Basic .net中图像的透明度？

Dim image As Image = Image.FromFile("path/to/image.png")
Dim bitmap As Bitmap = DirectCast(image, Bitmap)

For x As Integer = 0 To bitmap.Width - 1
    For y As Integer = 0 To bitmap.Height - 1
        Dim pixelColor As Color = bitmap.GetPixel(x, y)
        Dim alpha As Integer = pixelColor.A
        bitmap.SetPixel(x, y, Color.FromArgb(alpha, pixelColor))
    Next
Next

Dim image As Image = Image.FromFile("path/to/image.png")
Dim bitmap As Bitmap = DirectCast(image, Bitmap)

For x As Integer = 0 To bitmap.Width - 1
    For y As Integer = 0 To bitmap.Height - 1
        Dim pixelColor As Color = bitmap.GetPixel(x, y)
        Dim newAlpha As Integer = CInt((pixelColor.A / 255.0) * alphaValue)
        bitmap.SetPixel(x, y, Color.FromArgb(newAlpha, pixelColor))
    Next
Next