Render target bitmap quality issues

ok, it's a simple case, i rendered a viewport3d object using RenderTargetBitmap @ resolution 600 * 600 @ 96 dpi, the problem is there are a big deference in quality between the view port and the saved image:

here are some code:

'rendering viewport3D to image
Dim viewportPlate As New RenderTargetBitmap(600, 600, 96 , 96 , PixelFormats.Pbgra32)
viewportPlate.Render(viewport3d)

'path to save
Dim path As String = imgSave
Dim fs As FileStream = New FileStream(path, FileMode.Create)

'encoding to PNG and saving
Dim encoder As BitmapEncoder = New PngBitmapEncoder()
encoder.Frames.Add(BitmapFrame.Create(viewportPlate))
encoder.Save(fs)

now i tried to render to a larger resolution then scale it down and it got better, but a new problem arises of the big render time (because of the software rendering) and the out of memory issues!!!

i'm using .net 4.0, is there a clear way to achieve a better anti-aliased solutions and render times, as it was for GDI+ which i consider is a bless besides WPF?????

any tips will be welcome :)

Tags: Visual Basic, WPF