Using RenderTargetBitmap and DrawingVisual

Here goes:

I want to set a background image on a Canvas, using ImageBrush. The trouble starts when I want to resize the Canvas and leave the background picture intact. Meaning it will stay the same size in the topleft corner.

This is what I tried:

Private Function GenerateImageBrush(ByVal image As ImageSource, Optional ByVal SizeNew As Boolean = False) As ImageBrush
       Dim result As New ImageBrush

       Dim drawingVisual As New DrawingVisual()
       Dim drawingContext As DrawingContext = drawingVisual.RenderOpen()
       drawingContext.DrawRectangle(Brushes.White, New Pen(Brushes.Black, 1), New Rect(New Size(Me.Height, Me.Width)))
       drawingContext.DrawImage(image, New Rect(0, 0, imgWidth, imgHeight))
       drawingContext.Close()

       Dim bmp As New RenderTargetBitmap(Me.Height, Me.Width, 96, 96, PixelFormats.Pbgra32)
       bmp.Render(drawingVisual)

       result.ImageSource = bmp
       Return result
   End Function

   Dim imgWidth, imgHeight as Double

   Public Sub AddPicture(ByVal pic As String)
       Dim imgConv = New ImageSourceConverter()
       Dim imageSource As ImageSource
       imageSource = DirectCast(imgConv.ConvertFromString(pic), ImageSource)

       imgWidth = imageSource.Width
       imgHeight = imageSource.Height

       Me.Width = imageSource.Width
       Me.Height = imageSource.Height

       Me.Background = GenerateImageBrush(imageSource)
   End Sub

The idea is to return an Image that has preserved its original size, and filled the rest of the space with white color.

But When I try to resize the canvas, the picture starts to deform. It seems to want to preserve the original ratio Length/Width and adusts the image accordingly. But I cant seem to find out how to do this, does anybody know how to get this to work?

Tags: Visual Basic, .NET (.NET4.5), WPF