贝塞尔曲线上的箭头

我在你的曲线上添加了锚点（动画中的红色多段线）：

事实是箭头正确显示：参考线最后一段的方向。箭头确实显示了极值点处的切线方向。但是你对中间点的计算产生了太多的扭结，所以曲线的表观方向与切线的方向相差太大。

尝试更改选择中间点的算法，我认为问题出在其中。（我不能提供我自己的算法，因为我不知道你的问题是什么。）

关于算法，这段代码表现得更好：

var startPoint = pathFigure.StartPoint;
//Кривая Безье
Vector vector = endPoint - startPoint;
Point point1 = new Point(startPoint.X + 3 * vector.X / 8, startPoint.Y + 1 * vector.Y / 8);
Point point2 = new Point(startPoint.X + 5 * vector.X / 8, startPoint.Y + 7 * vector.Y / 8);

为了以防万一，这里有一个完整的例子：

主窗口.xaml：

<Canvas Background="Transparent"
        MouseLeftButtonDown="OnMouseLeftButtonDown"
        MouseLeftButtonUp="OnMouseLeftButtonUp"
        MouseMove="OnMouseMove">
    <Path x:Name="pathMain" Stroke="Blue" StrokeThickness="2"/>
</Canvas>

主窗口.xaml.cs：

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

    public PolyLineSegment DrawArrow(Point a, Point b)
    {
        double HeadWidth = 10; // Ширина между ребрами стрелки
        double HeadHeight = 5; // Длина ребер стрелки

        double X1 = a.X;
        double Y1 = a.Y;

        double X2 = b.X;
        double Y2 = b.Y;

        double theta = Math.Atan2(Y1 - Y2, X1 - X2);
        double sint = Math.Sin(theta);
        double cost = Math.Cos(theta);

        Point pt3 = new Point(
            X2 + (HeadWidth * cost - HeadHeight * sint),
            Y2 + (HeadWidth * sint + HeadHeight * cost));

        Point pt4 = new Point(
            X2 + (HeadWidth * cost + HeadHeight * sint),
            Y2 - (HeadHeight * cost - HeadWidth * sint));

        PolyLineSegment arrow = new PolyLineSegment();
        arrow.Points.Add(b);
        arrow.Points.Add(pt3);
        arrow.Points.Add(pt4);
        arrow.Points.Add(b);

        return arrow;
    }

    private void DrawLine(MouseEventArgs e, UIElement host)
    {
        PathFigure pathFigure = ((PathGeometry)pathMain.Data).Figures[0];
        Point startPoint = pathFigure.StartPoint;
        Point endPoint = e.GetPosition(host);

        //Кривая Безье
        Vector vector = endPoint - startPoint;
        Point point1 = new Point(startPoint.X + 3 * vector.X / 8,
                                 startPoint.Y + 1 * vector.Y / 8);
        Point point2 = new Point(startPoint.X + 5 * vector.X / 8,
                                 startPoint.Y + 7 * vector.Y / 8);

        BezierSegment curve = new BezierSegment(point1, point2, endPoint, true);
        PolyLineSegment arrow  = DrawArrow(point2, endPoint);

        pathFigure.Segments.Clear();
        pathFigure.Segments.Add(curve);
        pathFigure.Segments.Add(arrow);
    }
    private UIElement currentHost;
    private void OnMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
        var host = (UIElement)sender;
        currentHost = host;
        Mouse.Capture(host);
        Point startPoint = e.GetPosition(host);
        PathFigure pathFigure = new PathFigure
        {
            StartPoint = startPoint,
            IsClosed = false
        };

        pathMain.Data = new PathGeometry() { Figures = { pathFigure } };
        DrawLine(e, currentHost);
    }

    private void OnMouseLeftButtonUp(object sender, MouseButtonEventArgs e)
    {
        Mouse.Capture(null);
        DrawLine(e, currentHost);
        currentHost = null;
    }

    private void OnMouseMove(object sender, MouseEventArgs e)
    {
        if (currentHost != null)
            DrawLine(e, currentHost);
    }
}