WPF: Bitmap doesn't show

Question

I want to create some kind of simulation. There will be numerous sprites floating around. Because I think that rendering every frame thousand times the same primitives which make up a sprite, will be slow, I want render them once into a bitmap and then show this sprite every frame.

But it doesn't seem to work, the screen stays white.

My WPF source is trivial:

<Window x:Class="WPFGraphicsTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="800" Width="1000">
    <Canvas>

    </Canvas>
</Window>

And this is my code:

    public partial class MainWindow : Window
    {
        Ellipse e;
        RenderTargetBitmap bmp2;

        public MainWindow()
        {
            InitializeComponent();

            e = new Ellipse();
            e.Width = 40;
            e.Height = 40;
            e.Fill = new SolidColorBrush(Color.FromRgb(0, 0, 200));

            ((Canvas)this.Content).Children.Add(e);
            ((Canvas)this.Content).Measure(new Size(1000, 800));
            ((Canvas)this.Content).Arrange(new Rect(new Size(1000, 800)));

            RenderTargetBitmap bmp2 = new RenderTargetBitmap(40, 40, 96, 96, PixelFormats.Pbgra32);

            bmp2.Render(e);
            ((Canvas)this.Content).Children.Remove(e);
}


        protected override void OnRender(DrawingContext drawingContext)
        {
            base.OnRender(drawingContext);

            drawingContext.DrawImage(bmp2, new Rect(100,100, 40, 40));

        }
}

Why doesn't this work?

Answer 1

You could put an Image object on the Canvas and then use the RenderTargetBitmap to update the image. For example

<Window x:Class="WPFGraphicsTest.MainWindow" 
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
        Title="MainWindow" Height="800" Width="1000"> 
    <Canvas> 
      <Image Name="frameImage" />
    </Canvas> 
</Window> 

Then you can update the image like this, for the example, I am just rendering a new ellipse every 100 ms. Of course you should manage the Pens and Brushes better than what I show here, this is just an example to clarify the suggestion.

public partial class MainWindow : Window
  {
    DispatcherTimer _timer = new DispatcherTimer();

    RenderTargetBitmap _renderSurface = 
      new RenderTargetBitmap(100, 100, 96, 96, PixelFormats.Pbgra32);

    Random _rnd = new Random();

    public MainWindow()
    {
      InitializeComponent();

      _timer = new DispatcherTimer();
      _timer.Interval = TimeSpan.FromMilliseconds(100);
      _timer.Tick += new EventHandler(_timer_Tick);
      _timer.Start();      
    }

    void _timer_Tick(object sender, EventArgs e)
    {
      DrawingVisual visual = new DrawingVisual();
      DrawingContext context = visual.RenderOpen();
      int value = _rnd.Next(40);
      context.DrawEllipse(
        new SolidColorBrush(Colors.Red), 
        new Pen(new SolidColorBrush(Colors.Black), 1), 
        new Point(value, value), value, value);
      context.Close();

      _renderSurface.Render(visual);
      frameImage.Source = _renderSurface;
    }    
  }