Микширование изображений

Imports System.Drawing.Imaging
Public Class Form2
    Private bt1, bt2, bt3, tbt2 As Bitmap
    Private isLoad As Boolean
    Private Sub Form2_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
        bt1 = New Bitmap("0_1.jpg")
        bt2 = New Bitmap("0_2.jpg")
        Panel1.BackgroundImage = bt1
        Panel1.BackgroundImageLayout = ImageLayout.Zoom
        Panel2.BackgroundImage = bt2
        Panel2.BackgroundImageLayout = ImageLayout.Zoom
    End Sub
    Private Sub Button3_Click(sender As System.Object, e As System.EventArgs) Handles Button3.Click
        Dim pFSize As Integer = Bitmap.GetPixelFormatSize(bt1.PixelFormat)
        If pFSize <> 32 OrElse bt1.PixelFormat = PixelFormat.Indexed Then FastPix.ConvertFormat(bt1)
        pFSize = Bitmap.GetPixelFormatSize(bt2.PixelFormat)
        If pFSize <> 32 OrElse bt2.PixelFormat = PixelFormat.Indexed Then FastPix.ConvertFormat(bt2)
        tbt2 = setTransparent(bt2, 40)
        bt3 = New Bitmap(bt1.Width, bt1.Height, Imaging.PixelFormat.Format32bppArgb)
        Dim g As Graphics = Graphics.FromImage(bt3)
        g.CompositingMode = Drawing2D.CompositingMode.SourceOver '!!!
        g.DrawImage(bt1, 0, 0)
        g.DrawImage(tbt2, 0, 0)
        Panel3.BackgroundImage = bt3
        Panel3.BackgroundImageLayout = ImageLayout.Zoom
    End Sub
    ''' <summary>
    ''' устанавливаем прозрачность
    ''' </summary>
    ''' <param name="bm">базовый растр</param>
    ''' <param name="tr">степень непрозрачности 0,...,100</param>
    ''' <returns>новый прозрачный Bitmap</returns>
    Private Function setTransparent(ByVal bm As Bitmap, ByVal tr As UInt16) As Bitmap
        Dim bmp As Bitmap = bm.Clone
        Using fp As New FastPix(bmp, True)
            Dim bytes As Byte() = fp.ColorByteArray
            Dim trb As UInt16 = tr * 255 / 100
            For i As Integer = 3 To bytes.Length - 1 Step 4
                bytes(i) = trb
            Next
        End Using
        Return bmp
    End Function
End Class