Help saving a visual to file with PixelShader effect - WPF - wpf

Question

I'm trying to save a wpf control to file, but I'm applying a PixelShader effect to it, and when I try to save, the saved image is entirely white, black or red... deppends on the parameters of the effect.
I'm using the code here: WPF - Programmatic Binding on a BitmapEffect
how can I properly save it?
thanks!
UPDATE:
the code I'm using is:
BitmapSource bitmap = preview.Source as BitmapImage;
Rectangle r = new Rectangle();
r.Fill = new ImageBrush(bitmap);
r.Effect = effect;
Size sz = new Size(bitmap.PixelWidth, bitmap.PixelHeight);
r.Measure(sz);
r.Arrange(new Rect(sz));
var rtb = new RenderTargetBitmap(bitmap.PixelWidth, bitmap.PixelHeight, bitmap.DpiX, bitmap.DpiY, PixelFormats.Pbgra32);
rtb.Render(r);
PngBitmapEncoder png = new PngBitmapEncoder();
png.Frames.Add(BitmapFrame.Create(rtb));
Stream stm = File.Create("new.png");
png.Save(stm);
stm.Close();

Answer 1

Try this piece of code:
/// <summary>
/// Creates a screenshot of the given visual at the desired dots/inch (DPI).
/// </summary>
/// <param name="target">Visual component of which to capture as a screenshot.</param>
/// <param name="dpiX">Resolution, in dots per inch, in the X axis. Typical value is 96.0</param>
/// <param name="dpiY">Resolution, in dots per inch, in the Y axis. Typical value is 96.0</param>
/// <returns>A BitmapSource of the given Visual at the requested DPI, or null if there was an error.</returns>
public static BitmapSource CaptureScreen(Visual target, double dpiX, double dpiY)
{
if (target == null)
{
return null;
}
RenderTargetBitmap rtb = null;
try
{
// Get the actual size
Rect bounds = VisualTreeHelper.GetDescendantBounds(target);
rtb = new RenderTargetBitmap((int)(bounds.Width * dpiX / 96.0),
(int)(bounds.Height * dpiY / 96.0),
dpiX,
dpiY,
PixelFormats.Pbgra32);
DrawingVisual dv = new DrawingVisual();
using (DrawingContext ctx = dv.RenderOpen())
{
VisualBrush vb = new VisualBrush(target);
ctx.DrawRectangle(vb, null, new Rect(new Point(), bounds.Size));
}
rtb.Render(dv);
}
catch (Exception ex)
{
Console.Error.WriteLine("Error capturing image: " + ex.Message);
return null;
}
return rtb;
}