How do I make RenderTargetBitmap and VisualBrush play nice together?

Question

My requirements:

• a persistent UserControl that handles logic for a custom image, such as a map or drawing
• a set of containers to implement caching on the image during zoom or pan movements
• VisualBrush copies of the UserControl that I can add to the containers for use with Effects

I currently implement image caching with a RenderTargetBitmap, but that seems to have trouble with the VisualBrush-covered Rectangle objects I'm using.

My question: What can I add/change in this code to get the VisualBrush objects to render correctly after RenderTargetBitmap uses them? What strange thing is RenderTargetBitmap doing that makes the VisualBrush invisible?

This is a problem that I have been unable to reproduce without a decent amount of code.

In my xaml file I have:

<Window x:Class="ElementRender.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Height="350" Width="525">
  <Grid>
    <Grid Name="_contentContainer">
      <Rectangle Fill="White"/>
      <Grid Name="_content">
        <Grid Name="_back"/>
        <Grid Name="_body"/>
      </Grid>
    </Grid>
    <StackPanel VerticalAlignment="Bottom" Orientation="Horizontal">
      <Button Content="New" Name="New"/>
      <Button Content="Move" Name="Move"/>
      <Button Content="Update" Name="Update"/>
    </StackPanel>
  </Grid>
</Window>

and the .xaml.cs:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

public partial class MainWindow : Window
{

  private const int imageWidth = 150;
  private const int imageHeight = 150;
  private readonly UserControl Control;

  public MainWindow()
  {
     InitializeComponent();

     // User Control setup
     Control = new UserControl() {
        Width = imageWidth, Height = imageHeight,
        Content = BuildImage()
     };
     _body.Children.Add(SoftCopy(Control));

     // event setup
     Move.Click += (sender, e) => _content.RenderTransform = new TranslateTransform(50, 50);
     New.Click += (sender, e) => {
        HardCopy();
        _content.RenderTransform = null;
        Control.Content = BuildImage();
     };
  }

  private FrameworkElement BuildImage()
  {
     return new Rectangle{Fill=Brushes.Blue};
  }
  private void HardCopy()
  {
     int width = (int) _contentContainer.ActualWidth;
     int height = (int) _contentContainer.ActualHeight;

     // render the current image
     var rtb = new RenderTargetBitmap(width, height, 96, 96, PixelFormats.Pbgra32);
     DrawingVisual dv = new DrawingVisual();
     using (var context = dv.RenderOpen())
     {
        var brush = new VisualBrush(_contentContainer) { Opacity = .5 };
        context.DrawRectangle(brush, null, new Rect(0, 0, width, height));
     }
     rtb.Render(dv);
     var lastRender = new Image
     {
        Source = rtb,
        Stretch = Stretch.None,
        HorizontalAlignment = HorizontalAlignment.Center,
        VerticalAlignment = VerticalAlignment.Center,
        Width = width,
        Height = height
     };
     _back.Children.Clear();
     _back.Children.Add(lastRender);
  }
  private FrameworkElement SoftCopy(FrameworkElement element)
  {
     return new Rectangle{Fill= new VisualBrush(element), Width=element.Width, Height=element.Height};
  }
}

A few helping notes about the code:

• the xaml's _contentContainer works with HardCopy() to copy the current images into the image cache, _back.
• SoftCopy returns a FrameworkElement that looks exactly like the one past in, but without any transforms, effects, or visual parents. This is very important.
• BuildImage simulates building a new image to be pasted over the cache after the initial image has been transformed somehow.

If you build and run the application removing the SoftCopy() from the _body.Children.Add(SoftCopy(Control));, you see the effect that I want to get: the new element is pasted above the old element, and the old element seems to retain its transform.

Alternatively, if you cut out the line var rtb = new RenderTargetBitmap(width, height, 96, 96, PixelFormats.Pbgra32); from HardCopy, the caching function is broken, but the SoftCopy is displayed correctly.

However, if you run the application as-is, you notice that the new BlueRectangle (as rendered through a VisualBrush) doesn't display at all, until you hit the "New" button again, pushing the image to the cache, and still not showing you the new created image.

Answer 1

I'm going to be pompous enough to call this a bug in WPF. I eventually found out how to fix the strange behavior I was getting:

var visual = visualBrush.Visual;
visualBrush.Visual = null;
visualBrush.Visual = visual;

This should essentially be a null operation: by the end, the visual brush has the same visual as when it started. However, adding this code segment after rendering the VisualBrush into the RenderTargetBitmap fixed the issue I was having.