WPF: How to fill by ImageBrush drawn by OnRender?

Question

4.00/5 (1 vote)

See more:

.NET4.5

Goal:
1. Create Canvas.
2. Create MyImage : Image.
3. Draw MyImage by OnRender.
4. Set MyImage source to ImageBrush source.
5. Set ImageBrush to Canvas.Background.

Problem: The canvas gets drawn empty instead of what OnRender should draw.
Question: How to make it draw what is set to draw inside OnRender?
Notes:
1. It draws into Brush, if to add an image via a link.
2. It draws on Canvas, but not into Brush, if to add MyImage as child to Canvas.Children. You can do it by turning a comment off in the code.

Code 1 (using OnRender):

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        Canvas canvas = new Canvas();
        this.Content = canvas;
        canvas.Background = Brushes.Black;

        MyImage myimage = new MyImage();
        myimage.InvalidateVisual();

        ImageBrush brush = new ImageBrush();
        brush.ImageSource = myimage.Source;
        canvas.Background = brush;

        //canvas.Children.Add(myimage); // Draws
    }
}

class MyImage : Image
{
    protected override void OnRender(DrawingContext dc)
    {
        base.OnRender(dc);

        dc.DrawRectangle(Brushes.Red, null, new Rect(0, 0, 100, 100));
    }
}

The second option results the same.
Code 2 (using DrawingVisual):

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        Canvas canvas = new Canvas();
        this.Content = canvas;
        canvas.Background = Brushes.Black;

        MyImage myimage = new MyImage();
        myimage.InvalidateVisual();

        ImageBrush brush = new ImageBrush();
        brush.ImageSource = myimage.Source;
        canvas.Background = brush;

        //canvas.Children.Add(myimage); // Draws
    }
}

class MyImage : Image
{
    public MyImage()
    {
        visuals = new VisualCollection(this);
        visual = new DrawingVisual();
        visuals.Add(visual);

        Draw();
    }

    private void Draw()
    {
        using (DrawingContext dc = visual.RenderOpen())
            dc.DrawRectangle(Brushes.Red, null, new Rect(0, 0, 100, 100));
    }

    VisualCollection visuals;
    DrawingVisual visual;

    protected override Visual GetVisualChild(int index)
    {
        return visuals[index];
    }
    protected override int VisualChildrenCount
    {
        get { return visuals.Count; }
    }
}

Posted 25-Oct-15 1:47am

Ziya1995

Updated 26-Oct-15 7:47am

v9

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Ziya1995 · Accepted Answer · 2015-10-28T23:26:00

1. OnRender doesn't render in the code 1 because the visual MyImage is not added into a visual tree. But it renders when you add it into the visual tree of Canvas.
Source:
WPF: InvalidateVisual doesn't draw by OnRender.[^]

2. Even if it renders on the surface of Canvas, it will not render into ImageBrush because the brush is for raster based images, but not for vector based images. That is why it renders a raster image when you add it via a link.

3. You should use VisualBrush[^] as it is for vector based images.

4. VisualCollection/DrawingVisual in the code 2 renders even if you don't add MyImage into the visual tree of Canvas.

Code:

C#

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();

        Canvas canvas = new Canvas();
        this.Content = canvas;
        canvas.Background = Brushes.Black;

        MyImage myimage = new MyImage();

        VisualBrush visualbrush = new VisualBrush();
        visualbrush.Visual = myimage;
        canvas.Background = visualbrush;
    }
}

class MyImage : Image
{
    public MyImage()
    {
        visuals = new VisualCollection(this);
        visual = new DrawingVisual();
        visuals.Add(visual);

        Draw();
    }

    private void Draw()
    {
        using (DrawingContext dc = visual.RenderOpen())
            dc.DrawRectangle(Brushes.Red, null, new Rect(0, 0, 100, 100));
    }

    VisualCollection visuals;
    DrawingVisual visual;

    protected override Visual GetVisualChild(int index)
    {
        return visuals[index];
    }
    protected override int VisualChildrenCount
    {
        get { return visuals.Count; }
    }
}