Integrating AvaloniaUI with D3D11

Integrating AvaloniaUI into a Direct3D 11 (D3D11) application involves bridging two different rendering systems. Since you have access to the swap chain and rendering device, you can render Avalonia UI elements onto a texture and then display that texture within your D3D11 scene. Below are detailed steps and considerations to achieve this integration.

1. Render Avalonia UI to an Off-Screen Texture

a. Initialize Avalonia in Off-Screen Mode

Since you don't need a traditional window for Avalonia, you can initialize it in a headless mode:

AppBuilder.Configure<App>()
    .UsePlatformDetect()
    .SetupWithoutStarting();

b. Create Your Avalonia UI Content

Create the Avalonia controls or user interface you want to render:

var myAvaloniaControl = new MyCustomControl
{
    Width = desiredWidth,
    Height = desiredHeight
};

c. Render to a RenderTargetBitmap

Use RenderTargetBitmap to render the Avalonia UI into a bitmap:

var pixelSize = new PixelSize((int)myAvaloniaControl.Width, (int)myAvaloniaControl.Height);
var dpi = new Vector(96, 96); // Adjust DPI as needed
var renderTarget = new RenderTargetBitmap(pixelSize, dpi);

await renderTarget.RenderAsync(myAvaloniaControl);

d. Extract Pixel Data

Get the pixel data from the rendered bitmap:

var pixelData = renderTarget.GetPixels();

2. Transfer Pixel Data to a D3D11 Texture

a. Create a D3D11 Texture

Create a D3D11 texture that matches the dimensions and format of the Avalonia-rendered bitmap:

Texture2DDescription textureDesc = new Texture2DDescription
{
    Width = (int)myAvaloniaControl.Width,
    Height = (int)myAvaloniaControl.Height,
    MipLevels = 1,
    ArraySize = 1,
    Format = Format.B8G8R8A8_UNorm,
    SampleDescription = new SampleDescription(1, 0),
    Usage = ResourceUsage.Default,
    BindFlags = BindFlags.ShaderResource,
    CpuAccessFlags = CpuAccessFlags.None,
    OptionFlags = ResourceOptionFlags.None
};

var texture = new Texture2D(device, textureDesc);

b. Update the Texture with Pixel Data

Copy the pixel data into the D3D11 texture:

DataBox dataBox = new DataBox(
    DataPointer: Marshal.UnsafeAddrOfPinnedArrayElement(pixelData.ToArray(), 0),
    RowPitch: pixelSize.Width * 4,
    SlicePitch: 0
);

deviceContext.UpdateSubresource(dataBox, texture, 0);

c. Create a Shader Resource View

Create a shader resource view to use the texture in shaders:

var shaderResourceView = new ShaderResourceView(device, texture);

3. Render the Texture in Your D3D11 Scene

a. Set Up Shaders

Ensure you have a pixel shader and vertex shader that can render a textured quad.

b. Render a Quad with the Avalonia Texture

Draw a screen-aligned quad and map the shader resource view to it:

deviceContext.PixelShader.SetShaderResource(0, shaderResourceView);
// Draw the quad

4. Handle Input Events

To make the UI interactive, you'll need to handle input events:

Mouse Input: Capture mouse events from your application and translate them into Avalonia input events.
Keyboard Input: Similarly, handle keyboard events.

Example:

void OnMouseMoved(int x, int y)
{
    var point = new Point(x, y);
    myAvaloniaControl.RaiseEvent(new PointerEventArgs
    {
        RoutedEvent = InputElement.PointerMovedEvent,
        Device = mouseDevice,
        Timestamp = stopwatch.ElapsedMilliseconds,
        Root = myAvaloniaControl,
        Position = point
    });
}

5. Synchronize Rendering Loops

Ensure that Avalonia's rendering loop is synchronized with your application's rendering loop:

Call the Avalonia render method each frame before transferring pixel data.
Manage threading appropriately; Avalonia may require rendering on the UI thread.

6. Performance Considerations

Optimize Rendering: Only update the Avalonia texture when the UI changes.
Use Dirty Rectangles: Update only portions of the texture that have changed.
Asynchronous Rendering: Offload Avalonia rendering to a separate thread if possible.

7. Alternative Approaches

a. Direct Rendering with Direct2D

Since Avalonia can use Direct2D for rendering, you might render directly onto a D3D11 texture:

Create a shared texture that both Direct2D and D3D11 can access.
Use Avalonia's Direct2D rendering backend.

b. Use Composition APIs (Windows Only)

Leverage Windows Composition APIs to overlay Avalonia UI elements over D3D content.

8. Sample Code Snippet

// Initialization
AppBuilder.Configure<App>()
    .UseDirect2D1()
    .SetupWithoutStarting();

var control = new MyControl();
var dpi = new Vector(96, 96);
var pixelSize = new PixelSize(width, height);

var renderTargetBitmap = new RenderTargetBitmap(pixelSize, dpi);
await renderTargetBitmap.RenderAsync(control);

var pixels = renderTargetBitmap.GetPixels();
IntPtr pixelPtr = Marshal.UnsafeAddrOfPinnedArrayElement(pixels.ToArray(), 0);

// Create D3D11 Texture
// ... (as shown earlier)

// Update Texture with Avalonia UI
deviceContext.UpdateSubresource(new DataBox(pixelPtr, rowPitch, 0), texture, 0);

// Render in D3D11
// ... (as shown earlier)

9. Potential Challenges

Resource Management: Ensure that GPU resources are properly managed to prevent memory leaks.
Threading Issues: Avalonia's UI thread must be handled carefully to avoid cross-thread operation exceptions.
Latency: Real-time synchronization between Avalonia rendering and D3D11 can introduce latency if not managed correctly.

10. Additional Tips

Consult Documentation: Review Avalonia's rendering documentation for custom rendering scenarios.
Community Support: Engage with the Avalonia community for shared experiences and solutions.
Prototype First: Build a small prototype to test the integration before applying it to the full application.

Conclusion

Integrating AvaloniaUI into a D3D11 application involves rendering the UI to a texture and displaying it within your 3D scene. While this process can be complex, especially regarding input handling and rendering synchronization, it provides a powerful way to overlay sophisticated UI elements onto your simulation. Be prepared to delve into both Avalonia's and Direct3D's rendering pipelines and manage the interaction between them carefully.

If you need further assistance or have questions about specific steps, feel free to ask!