Как средствами GDI+ нарисовать тень для прозрачной картинки

Private dx As Integer = 3
Private dy As Integer = 3
Private bmp1 As Bitmap
Private Sub Button3_Click(sender As System.Object, e As System.EventArgs) Handles Button3.Click
    bmp1 = loadBitmap("tst2.png")
    Dim bmp2 As Bitmap = bmp1.Clone
    Dim w As Integer = bmp1.Width
    Dim h As Integer = bmp1.Height
    Dim cm As ColorMatrix = New ColorMatrix(New Single()() _
                   {New Single() {0, 0, 0, 0, 0}, _
                    New Single() {0, 0, 0, 0, 0}, _
                    New Single() {0, 0, 0, 0, 0}, _
                    New Single() {0, 0, 0, 0.25, 0}, _
                    New Single() {0, 0, 0, 0, 1}})
    Dim imgattr As New ImageAttributes()
    Dim rc As New Rectangle(dx, dy, w, h)
    Dim g As Graphics = PictureBox1.CreateGraphics
    imgattr.SetColorMatrix(cm)
    g.DrawImage(bmp2, rc, 0, 0, w, h, GraphicsUnit.Pixel, imgattr)
    Dim rc1 As New Rectangle(0, 0, w, h)
    g.DrawImage(bmp1, rc1, 0, 0, w, h, GraphicsUnit.Pixel)
    g.Dispose()
End Sub
Public Function loadBitmap(ByVal fileName As String) As Bitmap
    If Not IO.File.Exists(fileName) Then Return Nothing
    Using bm As Bitmap = New Bitmap(fileName)
        Return New Bitmap(bm)
    End Using
End Function

bmp2 = fnShade(bmp1, PictureBox1)
 
Public Function fnShade(ByVal bmp As Bitmap, ByVal pbx As PictureBox) As Bitmap
        fnShade = bmp 'по-умолчанию картинка не изменяется
        'задаём цвет тени 4-я строка, 4-й столбец
        Dim cm As ColorMatrix = New ColorMatrix(New Single()() _
                       {New Single() {0, 0, 0, 0, 0}, _
                        New Single() {0, 0, 0, 0, 0}, _
                        New Single() {0, 0, 0, 0, 0}, _
                        New Single() {0, 0, 0, 0.5, 0}, _
                        New Single() {0, 0, 0, 0, 1}})
        Dim imgattr As New ImageAttributes()
        imgattr.SetColorMatrix(cm)
        Dim rc As New Rectangle(0, 0, bmp.Width, bmp.Height)
        Dim g As Graphics = pbx.CreateGraphics
        g.DrawImage(bmp, rc, 0, 0, bmp.Width, bmp.Height, GraphicsUnit.Pixel, imgattr)
        g.Dispose()
End Function

Private Sub Button4_Click(sender As System.Object, e As System.EventArgs) Handles Button4.Click
    bmp1 = loadBitmap("tst2.png")
    Dim newBmp As Bitmap = fnShade(bmp1)
    PictureBox1.Image = newBmp
End Sub
Private Function fnShade(ByVal bmp As Bitmap) As Bitmap
    Dim w As Integer = bmp1.Width
    Dim h As Integer = bmp1.Height
    Dim myBmp As New Bitmap(w + dx, h + dy, Imaging.PixelFormat.Format32bppArgb)
    Dim g As Graphics = Graphics.FromImage(myBmp)
    Dim cm As ColorMatrix = New ColorMatrix(New Single()() _
           {New Single() {0, 0, 0, 0, 0}, _
            New Single() {0, 0, 0, 0, 0}, _
            New Single() {0, 0, 0, 0, 0}, _
            New Single() {0, 0, 0, 0.5, 0}, _
            New Single() {0, 0, 0, 0, 1}})
    Dim imgattr As New ImageAttributes()
    Dim rc As New Rectangle(dx, dy, w, h)
    imgattr.SetColorMatrix(cm)
    g.DrawImage(bmp.Clone, rc, 0, 0, w, h, GraphicsUnit.Pixel, imgattr)
    Dim rc1 As New Rectangle(0, 0, w, h)
    g.DrawImage(bmp1, rc1, 0, 0, w, h, GraphicsUnit.Pixel)
    g.Dispose()
    Return myBmp
End Function

Private Sub Button5_Click(sender As System.Object, e As System.EventArgs) Handles Button5.Click
    bmp1 = loadBitmap("tst2.png")
    Dim newColorBmp As Bitmap = toNewColor(bmp1.Clone, Color.Gray)
    PictureBox1.Image = newColorBmp
End Sub
Private Function toNewColor(ByVal bmp As Bitmap, ByVal clr As Color) As Bitmap
    If bmp.PixelFormat <> PixelFormat.Format32bppArgb Then Return Nothing
    Dim rect As New Rectangle(0, 0, bmp.Width, bmp.Height)
    Dim bmpData As BitmapData = bmp.LockBits(rect, Drawing.Imaging.ImageLockMode.ReadWrite, bmp.PixelFormat)
    Dim ptr As IntPtr = bmpData.Scan0
    Dim allbytes As Integer = bmpData.Stride * bmp.Height
    Dim rgbV(allbytes - 1) As Byte
    System.Runtime.InteropServices.Marshal.Copy(ptr, rgbV, 0, allbytes)
    For i = 0 To allbytes - 1 Step 4
        If rgbV(i + 3) > 0 Then
            rgbV(i) = clr.B
            rgbV(i + 1) = clr.G
            rgbV(i + 2) = clr.R
        End If
    Next
    System.Runtime.InteropServices.Marshal.Copy(rgbV, 0, ptr, allbytes)
    bmp.UnlockBits(bmpData)
    Return bmp
End Function