Public Function GetPixel24(ByVal x As Integer, ByVal y As Integer, ByVal _
      buff() As Byte, ByVal stride As Integer) As Color
        Dim loc As Integer = y * stride + x * 3
        Return Color.FromArgb(buff(loc + 2), buff(loc + 1), buff(loc))
    End Function
    Public Sub SetPixel24(ByVal x As Integer, ByVal y As Integer, ByVal buff() _
      As Byte, ByVal stride As Integer, ByVal col As Color)
        Dim loc As Integer = y * stride + x * 3
        buff(loc + 2) = col.R
        buff(loc + 1) = col.G
        buff(loc) = col.B
    End Sub

        Dim bmp As Bitmap = New Bitmap("C:\Test\firewks1.jpg")
        Dim width As Integer = bmp.Width
        Dim height As Integer = bmp.Height
        Dim rect As New Rectangle(0, 0, width, height)
        Dim bmpData As System.Drawing.Imaging.BitmapData = bmp.LockBits(rect, _
            Drawing.Imaging.ImageLockMode.ReadWrite, _
            bmp.PixelFormat)
        Dim ptr As IntPtr = bmpData.Scan0
 
        Dim bytes As Integer = bmpData.Height * bmpData.Stride
        Dim pixels(bytes - 1) As Byte
        Marshal.Copy(ptr, pixels, 0, bytes)
 
        Dim x, y As Integer
        For y = height / 4 To 3 * height / 4 - 1 Step 1
            For x = width / 4 To 3 * width / 4 - 1 Step 1
                SetPixel24(x, y, pixels, bmpData.Stride, Color.FromArgb(255, 0, _
                  0))
            Next
        Next
 
        Marshal.Copy(pixels, 0, ptr, bytes)
        bmp.UnlockBits(bmpData)
        bmp.Save("C:\Test\firewksx.jpg", _
          System.Drawing.Imaging.ImageFormat.Jpeg)