Bad anti-aliasing when using WriteableBitmap in Silverlight

Question

Hi, My silverlight program draws thumbnails as shown below:

System.Windows.Controls.Image image = new System.Windows.Controls.Image();
// BitmapImage bitmapImage contains a big image
image.Source = bitmapImage ;
ScaleTransform transform = new ScaleTransform();
transform.ScaleX = 0.1;
transform.ScaleY = 0.1;
image.RenderTransform = transform;
canvas.Children.Add(image);

All works perfectly, anti-aliasing is excellent. But now I need to replace BitmapImage with WriteableBitmap:

// WriteableBitmap writeableBitmap contains the same big image
image.Source = writeableBitmap;

In this case the anti-aliasing has disappeared. Can anyone explain me a reason? How can I keep anti-aliasing for WriteableBitmap?

Thanks, Vlad2K