[RESOLVED] Problem sharpening semi-transparent image

I found this code for sharpening images online. I haven't done much testing with it yet, but i do know it's ruining the transparent parts of my bitmaps.
Can anyone give a solution, that retains the transparent pixels?

Code:

---

Imports System.Runtime.InteropServices
Imports System.Drawing.Imaging

Public Class Convolution
    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 = bmp.Clone()

        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 IntPtr = bmpData.Scan0
        Dim TempPtr As IntPtr = TempBmpData.Scan0

        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 = CInt(CInt(ptr)) + bmpData.Stride * bmpData.Height

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

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

                Marshal.WriteByte(CInt(ptr) + 5 + Stride, CByte(Pix))

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

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

                Marshal.WriteByte(CInt(ptr) + 4 + Stride, CByte(Pix))

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

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

                Marshal.WriteByte(CInt(ptr) + 3 + Stride, CByte(Pix))

                ptr = CInt(ptr) + 3
                TempPtr = CInt(TempPtr) + 3
            Next
        Next

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

    Public Property Matrix() As ConvolutionMatrix
        Get
            Return m_Matrix
        End Get
        Set(ByVal value As ConvolutionMatrix)
            m_Matrix = value
        End Set
    End Property
    Private m_Matrix As ConvolutionMatrix
End Class

---

Code:

---

Public Class ConvolutionMatrix
    Public Sub New()
        Pixel = 1
        Factor = 1
    End Sub

    Public Sub Apply(ByVal Val As Integer)
        TopLeft = Val
        TopMid = Val
        TopRight = Val
        MidLeft = Val
        MidRight = Val
        BottomLeft = Val
        BottomMid = Val
        BottomRight = Val
        Pixel = Val
    End Sub

    Public Property TopLeft() As Integer
        Get
            Return m_TopLeft
        End Get
        Set(ByVal value As Integer)
            m_TopLeft = value
        End Set
    End Property
    Private m_TopLeft As Integer

    Public Property TopMid() As Integer
        Get
            Return m_TopMid
        End Get
        Set(ByVal value As Integer)
            m_TopMid = value
        End Set
    End Property
    Private m_TopMid As Integer

    Public Property TopRight() As Integer
        Get
            Return m_TopRight
        End Get
        Set(ByVal value As Integer)
            m_TopRight = value
        End Set
    End Property
    Private m_TopRight As Integer

    Public Property MidLeft() As Integer
        Get
            Return m_MidLeft
        End Get
        Set(ByVal value As Integer)
            m_MidLeft = value
        End Set
    End Property
    Private m_MidLeft As Integer

    Public Property MidRight() As Integer
        Get
            Return m_MidRight
        End Get
        Set(ByVal value As Integer)
            m_MidRight = value
        End Set
    End Property
    Private m_MidRight As Integer

    Public Property BottomLeft() As Integer
        Get
            Return m_BottomLeft
        End Get
        Set(ByVal value As Integer)
            m_BottomLeft = value
        End Set
    End Property
    Private m_BottomLeft As Integer

    Public Property BottomMid() As Integer
        Get
            Return m_BottomMid
        End Get
        Set(ByVal value As Integer)
            m_BottomMid = value
        End Set
    End Property
    Private m_BottomMid As Integer

    Public Property BottomRight() As Integer
        Get
            Return m_BottomRight
        End Get
        Set(ByVal value As Integer)
            m_BottomRight = value
        End Set
    End Property
    Private m_BottomRight As Integer

    Public Property Pixel() As Integer
        Get
            Return m_Pixel
        End Get
        Set(ByVal value As Integer)
            m_Pixel = value
        End Set
    End Property
    Private m_Pixel As Integer

    Public Property Factor() As Integer
        Get
            Return m_Factor
        End Get
        Set(ByVal value As Integer)
            m_Factor = value
        End Set
    End Property
    Private m_Factor As Integer

    Public Property Offset() As Integer
        Get
            Return m_Offset
        End Get
        Set(ByVal value As Integer)
            m_Offset = value
        End Set
    End Property
    Private m_Offset As Integer
End Class

---

Code:

---

Public Shared Sub ApplySharpen(ByRef bmp As Bitmap, ByVal weight As Integer)
    Dim m As New ConvolutionMatrix()
    m.Apply(0)
    m.Pixel = weight
    m.TopMid = -2
    m.MidLeft = -2
    m.MidRight = -2
    m.BottomMid = -2
    m.Factor = weight - 8

    Dim C As New Convolution()
    C.Matrix = m
    C.Convolution3x3(bmp)
End Sub

---

I don't know, but I suppose you could try checking the Alpha value of the destination pixel you are working on, and if it is not 255, then skip all the processing so you don't modify that pixel, i.e. you won't take values from the surrounding pixels and modify the transparent/translucent pixel.

I think, looking at the code, I would have to watch it work to understand where the alpha value to be check would be in the loop.
A quick look at the code seems to suggest the loop is looking at a number of pixels around a reference pixel and then writing the result to a pixel above and to the right of the reference pixel being pointed at in the loop, so it would be the alpha value of the pixel the line above and to the right (thinking that the pixels are stored from the bottom up) that is being modified.

That was my initial thoughts about it too, but I also can’t see where to test the Alpha value of the destination pixel, or even how to test the alpha value of the destination pixel…

@passel… I can put together a quick test app, if you wouldn’t mind taking a look and want to see it running?

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

Attachment 187185

The relevant code...

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

---

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.

I'm a little confused by the code. I've seen similar before, loads of times, but i've never attempted to use it. Would it be very difficult to modify that to use 32bpp ?

Actually, I think i'll abandon this image sharpening. It was for an icon editing app, and there weren't any encouraging results before the transparency problem, so no big loss...