将32位图像转换为黑白两色图像时出现问题

我有以下代码例程。。。

Public Shared Sub ConvertToBitonal(ByRef Source As System.Windows.Media.Imaging.WriteableBitmap)
    If Source.Format = System.Windows.Media.PixelFormats.BlackWhite Then
        Exit Sub
    End If

    Dim BytesPerPixel As Integer = (Source.Format.BitsPerPixel + 7) / 8
    Dim Stride As Integer = Source.PixelWidth * BytesPerPixel
    Dim NumBytes As Long = Source.PixelHeight * Stride
    Dim pixels As Byte() = New Byte(NumBytes - 1) {}

    Source.CopyPixels(pixels, Stride, 0)

    For cnt As Integer = 0 To pixels.Length - 1 Step BytesPerPixel
        Dim blue As Byte = pixels(cnt + 0)
        Dim green As Byte = pixels(cnt + 1)
        Dim red As Byte = pixels(cnt + 2)
        Dim intensity As Integer = CType(red, Integer) + CType(green, Integer) + CType(blue, Integer)
        Dim targetColor As Byte

        If intensity > 400 Then
            targetColor = 255
        Else
            targetColor = 0
        End If

        pixels(cnt + 0) = targetColor
        pixels(cnt + 1) = targetColor
        pixels(cnt + 2) = targetColor
        pixels(cnt + 3) = targetColor
    Next

    Source.WritePixels(New System.Windows.Int32Rect(0, 0, Source.PixelWidth, Source.PixelHeight), pixels, Stride, 0)
End Sub

当源图像是每像素24位时,输出完全符合我的要求,但当源图像为32位时,颜色就不稳定了,我得到了贯穿图像的垂直线。有人能告诉我如何修改例程,使32位图像像24位计数器部件一样显示出来吗?

这是我所说内容的屏幕截图。。。(显然我还没有足够的代表来发布图片,所以这里有一个链接