Can you force an Immediate render update?

[MechWarrior99]

I have a 3D scene and I am rendering some UI over it with Avalonia. The UI is simple shapes drawn with DrawContext. The thing is that the position of theses shapes is tied to specific locations in my 3D scene (convert the 3D position to a 2D point and then drawing at that point). But when the camera moves quickly the shapes drawn with Avalonia doesn't update right away which results in a sort of 'floating' and 'ghost' type behavior as the UI lags behind the 3D position slightly.

So I was wondering if there was some way I could force a Avalonia to do a render update immediately.

And I don't be InvalidateVisual, as that just queues it for the next render update.

Let me know if anyone has any ideas. I don't minda bit of reflection if I need to.

[maxkatz6]

Rendering is done on a separated render thread, it will be delayed either way.
If your 3D scene uses custom render timer, you might want to use ICustomDrawOperation (to access render thread and draw from there) + window.RequestAnimationFrame() (to get notification when next frame starts) instead. You also can see our GPU interop samples, if you want to go deeper.

Let me know if anyone has any ideas. I don't minda bit of reflection if I need to.

With private API access, you can do at least this:

var renderRoot = (IRenderRoot)window;
renderRoot.Renderer.Paint(Bounds);

[MechWarrior99]

I see got it, thank you for the explanation. That gives me some ideas so I will see what I can come up with.

[kekekeks]

ICustomDrawOperation

Custom composition visual is recommended to be used instead since it's a stateful object on the render thread that can trigger repaints without touching the UI thread.

[MechWarrior99]

Custom composition visual

Interesting, I did a bit of googling and looking about, but wasn't super clear how to go about that. Is this just the case of needing to poke through the source code to figure it out? Or is there someplace I missed where I can read more about composition visuals and making a custom one.

[kekekeks]

Avalonia/samples/ControlCatalog/Pages/CompositionPage.axaml.cs

Lines 220 to 316 in 890df99

	class CustomVisualHandler : CompositionCustomVisualHandler
	{
	private TimeSpan _animationElapsed;
	private TimeSpan? _lastServerTime;
	private bool _running;
	private bool _preciseDirtyRects;
	public static readonly object StopMessage = new(),
	StartMessage = new(),
	UsePreciseDirtyRects = new(),
	UseNonPreciseDirtyRects = new();
	private List<(Point center, double size, ImmutableSolidColorBrush brush)> _ellipses = new();
	void UpdateRects()
	{
	if (_running)
	{
	if (_lastServerTime.HasValue) _animationElapsed += (CompositionNow - _lastServerTime.Value);
	_lastServerTime = CompositionNow;
	}
	_ellipses.Clear();
	const int cnt = 20;
	var maxPointSizeX = EffectiveSize.X / (cnt * 1.6);
	var maxPointSizeY = EffectiveSize.Y / 4;
	var pointSize = Math.Min(maxPointSizeX, maxPointSizeY);
	var animationLength = TimeSpan.FromSeconds(4);
	var animationStage = _animationElapsed.TotalSeconds / animationLength.TotalSeconds;
	var sinOffset = Math.Cos(_animationElapsed.TotalSeconds) * 1.5;
	for (var c = 0; c < cnt; c++)
	{
	var stage = (animationStage + (double)c / cnt) % 1;
	var colorStage =
	(animationStage + (Math.Sin(_animationElapsed.TotalSeconds * 2) + 1) / 2 + (double)c / cnt) % 1;
	var posX = (EffectiveSize.X + pointSize * 3) * stage - pointSize;
	var posY = (EffectiveSize.Y - pointSize) * (1 + Math.Sin(stage * 3.14 * 3 + sinOffset)) / 2 + pointSize / 2;
	var opacity = Math.Sin(stage * 3.14);
	_ellipses.Add((new Point(posX, posY), pointSize / 2, new ImmutableSolidColorBrush(Color.FromArgb(
	255,
	(byte)(255 - 255 * colorStage),
	(byte)(255 * Math.Abs(0.5 - colorStage) * 2),
	(byte)(255 * colorStage)
	), opacity)));
	}
	}
	public override void OnRender(ImmediateDrawingContext drawingContext)
	{
	if (_ellipses.Count == 0)
	UpdateRects();
	foreach(var e in _ellipses)
	drawingContext.DrawEllipse(e.brush, null, e.center, e.size, e.size);
	}
	public override void OnMessage(object message)
	{
	if (message == StartMessage)
	{
	_running = true;
	_lastServerTime = null;
	RegisterForNextAnimationFrameUpdate();
	}
	else if (message == StopMessage)
	_running = false;
	else if (message == UsePreciseDirtyRects)
	_preciseDirtyRects = true;
	else if (message == UseNonPreciseDirtyRects)
	_preciseDirtyRects = false;
	}
	void InvalidateCurrentEllipseRects()
	{
	foreach (var e in _ellipses)
	Invalidate(new Rect(e.center.X - e.size, e.center.Y - e.size, e.size * 2, e.size * 2));
	}
	public override void OnAnimationFrameUpdate()
	{
	if (_running)
	{
	if (_preciseDirtyRects)
	InvalidateCurrentEllipseRects();
	else
	Invalidate();
	UpdateRects();
	if(_preciseDirtyRects)
	InvalidateCurrentEllipseRects();
	RegisterForNextAnimationFrameUpdate();
	}
	}
	}