Collecting pointer moved and only want points once the pointer has moved a certain amount.

gistfile1.cs

private void OnLoaded(object sender, RoutedEventArgs args)
{
    var pointerMoved = Observable.FromEventPattern<PointerEventHandler, PointerRoutedEventArgs>(h => PathRoot.PointerMoved += h, h => PathRoot.PointerMoved -= h).Select(e => e.EventArgs);
    var pointerInContact = pointerMoved.Where(p => p.Pointer.IsInContact);
    var points = pointerInContact.Select(p => p.GetCurrentPoint(PathRoot)).Select(p => new Vector2((float) p.Position.X, (float) p.Position.Y));

    var previousPoints = points.Skip(1);

    var deltas = points.Zip(previousPoints, (current, previous) => new Vector2Delta { Point = current, Delta = current - previous });
    var scannedDeltas = deltas.Scan(new Vector2Delta { Point = Vector2.Zero, Delta = Vector2.Zero }, ScanSignificantDeleta);
    var signficantDeltas = scannedDeltas.Where(p => p.Significant);

    signficantDeltas.Subscribe(p => AddEllipse(p.Point));
}

private Vector2Delta ScanSignificantDeleta(Vector2Delta scanned, Vector2Delta current)
{
    var delta = current.Delta + scanned.Delta;

    var significant = delta.Length() > 25;

    return new Vector2Delta
        {
            Point = current.Point,
            Delta = significant ? Vector2.Zero : delta,
            Significant = significant
        };
}