Imports System.IO
Imports System.Drawing.Imaging
Imports System.Runtime.InteropServices
 
Public Class MeineKlasse
 
Public Enum SupportedFormat
      PNG
      GIF
      JPEG
      TIFF
      BMP
   End Enum
 
   Public Shared Function ChangeAlphaToWhiteBackground(ByVal Bilddatei_in As _
     Stream, ByVal Format As SupportedFormat) As MemoryStream
 
      Dim output As New MemoryStream()
 
      Dim bmp_in As New Bitmap(Bilddatei_in)
      Dim bmp_rect As New Rectangle(0, 0, bmp_in.Width, bmp_in.Height)
      Dim bmp = bmp_in.Clone(bmp_rect, bmp_in.PixelFormat)
 
      If Format.ToString.ToUpper <> "GIF" Then
         If (bmp.PixelFormat = PixelFormat.Format8bppIndexed) OrElse ( _
           bmp_in.PixelFormat = PixelFormat.Format32bppArgb) OrElse ( _
           bmp_in.PixelFormat = PixelFormat.Format64bppArgb) Then
            Debug.Print(bmp_in.PixelFormat.ToString)
 
            Dim bpp As Integer = Image.GetPixelFormatSize(bmp_in.PixelFormat) / _
              8
            Dim Byte_Index As Integer = -bpp
            Dim bmp_data As BitmapData = bmp.LockBits(bmp_rect, _
                                                   ImageLockMode.ReadWrite, _
                                                   bmp_in.PixelFormat)
 
            Dim bmp_ptr As IntPtr = bmp_data.Scan0
            Dim bmp_bytes As Integer = bmp.Width * bmp.Height * bpp
            Dim bmp_array(bmp_bytes - 1) As Byte
            Runtime.InteropServices.Marshal.Copy(bmp_ptr, bmp_array, 0, _
              bmp_bytes)
 
            For y = 1 To bmp.Height
               For x = 1 To bmp.Width
 
                  Byte_Index += bpp
 
'True, falls ein transparentes Pixel vorliegt
                  If bmp_array(Byte_Index + bpp - 1) = 0 Then
                     For i = 0 To bpp - 2
'Ändert die RGB Werte des Pixels zu 255,255,255
                        bmp_array(Byte_Index + i) = 255
                     Next
                  End If
 
               Next x
            Next y
 
            Runtime.InteropServices.Marshal.Copy(bmp_array, 0, bmp_ptr, _
              bmp_bytes)
            bmp.UnlockBits(bmp_data)
 
         End If
 
         Dim EncoderParameters As New EncoderParameters(2)
         EncoderParameters.Param(0) = New EncoderParameter(Encoder.Quality, _
           CType(100, Int32))
         EncoderParameters.Param(1) = New EncoderParameter(Encoder.Compression, _
         Fix(EncoderValue.CompressionNone))
 
         bmp.Save(output, EncoderInfo(Format.ToString), EncoderParameters)
 
      Else '--------------------------GIF---------------------------------------
 
      End If
 
      bmp.Dispose() : bmp_in.Dispose()
 
      Return output
   End Function
 
   ' Formatdescriptoren: BMP JPEG GIF TIFF PNG
   Private Shared Function EncoderInfo(ByVal FormatDescriptor As String) As _
     ImageCodecInfo
 
      Dim encoders() As ImageCodecInfo = ImageCodecInfo.GetImageEncoders()
      FormatDescriptor = Trim(UCase(FormatDescriptor))
 
      Dim i As Integer = 0
 
      While i < encoders.Length
         If UCase(encoders(i).FormatDescription) = FormatDescriptor Then
            Return encoders(i)
         End If
         i += 1
      End While
      Return Nothing
   End Function
End Class