[RESOLVED] Problem sharpening semi-transparent image

[.paul.]

Here's the test project. I've tidied the code significantly, making it error free was a task and a half

Sharpen_Image_Test.zip

The relevant code...

Public Sub Convolution3x3(ByRef bmp As Bitmap)
    Dim Factor As Integer = Matrix.Factor

    If Factor = 0 Then
        Return
    End If

    Dim TopLeft As Integer = Matrix.TopLeft
    Dim TopMid As Integer = Matrix.TopMid
    Dim TopRight As Integer = Matrix.TopRight
    Dim MidLeft As Integer = Matrix.MidLeft
    Dim MidRight As Integer = Matrix.MidRight
    Dim BottomLeft As Integer = Matrix.BottomLeft
    Dim BottomMid As Integer = Matrix.BottomMid
    Dim BottomRight As Integer = Matrix.BottomRight
    Dim Pixel As Integer = Matrix.Pixel
    Dim Offset As Integer = Matrix.Offset

    Dim TempBmp As Bitmap = DirectCast(bmp.Clone(), Bitmap)

    Dim bmpData As BitmapData = bmp.LockBits(New Rectangle(0, 0, bmp.Width, bmp.Height), ImageLockMode.ReadWrite, PixelFormat.Format24bppRgb)
    Dim TempBmpData As BitmapData = TempBmp.LockBits(New Rectangle(0, 0, TempBmp.Width, TempBmp.Height), ImageLockMode.[ReadOnly], PixelFormat.Format24bppRgb)

    Dim ptr As Integer = bmpData.Scan0.ToInt32
    Dim TempPtr As Integer = TempBmpData.Scan0.ToInt32

    Dim Pix As Integer = 0
    Dim Stride As Integer = bmpData.Stride
    Dim DoubleStride As Integer = Stride * 2
    Dim Width As Integer = bmp.Width - 2
    Dim Height As Integer = bmp.Height - 2
    Dim stopAddress As Integer = ptr + bmpData.Stride * bmpData.Height

    For y As Integer = 0 To Height - 1
        For x As Integer = 0 To Width - 1
            Pix = (((Marshal.ReadByte(New IntPtr((TempPtr + 2))) * TopLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 5))) * TopMid + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 8))) * TopRight + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 2 + Stride))) * MidLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 5 + Stride))) * Pixel + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 8 + Stride))) * MidRight + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 2 + DoubleStride))) * BottomLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 5 + DoubleStride))) * BottomMid + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 8 + DoubleStride))) * BottomRight) \ _
                      Factor) + Offset)

            If Pix < 0 Then
                Pix = 0
            ElseIf Pix > 255 Then
                Pix = 255
            End If

            'Dim b As Byte = Marshal.ReadByte(New IntPtr(ptr + 5 + Stride))
            'If b Then ' how to detect if dest pixel is transparent???
            Marshal.WriteByte(New IntPtr(ptr + 5 + Stride), CByte(Pix))

            Pix = (((Marshal.ReadByte(New IntPtr((TempPtr + 1))) * TopLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 4))) * TopMid + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 7))) * TopRight + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 1 + Stride))) * MidLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 1 + Stride))) * Pixel + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 4 + Stride))) * MidRight + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 1 + DoubleStride))) * BottomLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 4 + DoubleStride))) * BottomMid + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 7 + DoubleStride))) * BottomRight) \ _
                      Factor) + Offset)

            If Pix < 0 Then
                Pix = 0
            ElseIf Pix > 255 Then
                Pix = 255
            End If

            'b = Marshal.ReadByte(New IntPtr(ptr + 4 + Stride))
            'If b Then ' how to detect if dest pixel is transparent???
            Marshal.WriteByte(New IntPtr(ptr + 4 + Stride), CByte(Pix))

            Pix = (((Marshal.ReadByte(New IntPtr((TempPtr))) * TopLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 3))) * TopMid + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 6))) * TopRight + _
                      Marshal.ReadByte(New IntPtr((TempPtr + Stride))) * MidLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 3 + Stride))) * Pixel + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 6 + Stride))) * MidRight + _
                      Marshal.ReadByte(New IntPtr((TempPtr + DoubleStride))) * BottomLeft + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 3 + DoubleStride))) * BottomMid + _
                      Marshal.ReadByte(New IntPtr((TempPtr + 6 + DoubleStride))) * BottomRight) \ _
                      Factor) + Offset)

            If Pix < 0 Then
                Pix = 0
            ElseIf Pix > 255 Then
                Pix = 255
            End If

            'b = Marshal.ReadByte(New IntPtr(ptr + 3 + Stride))
            'If b Then ' how to detect if dest pixel is transparent???
            Marshal.WriteByte(New IntPtr(ptr + 3 + Stride), CByte(Pix))

            ptr += 3
            TempPtr += 3
        Next
    Next

    bmp.UnlockBits(bmpData)
    TempBmp.UnlockBits(TempBmpData)
End Sub

[passel]

Looking at that code, I would expect the Alpha is being stripped off, and all the transparent pixels would end up being black.
It appears you're converting the pixels to 24bpp which ignores the Alpha, when putting the pixels into a byte array.

That also explains why the destination offsets of 3,4,5 didn't make sense to me. I would have expected 4,5,6 or 5,6,7 depending on which of the four bytes was the alpha byte.
i.e. the first pixel would be at offsets 0,1,2,3 and the second at 4,5,6,7 and third at 8,... if there were 32-bits per pixel.
But with 24-bpp the first pixel would be at offset 0,1,2 and the second at 3,4,5 and 3,4,5 offsets is what is in your code for the WriteBytes.

So, the sharpening code is expecting 24-bit rgb, not 32-bit argb.