WPF circle rotate via mousemove

//xaml
<Window x:Class="WpfApp184.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp184"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Ellipse
            x:Name="elp"
            Width="{Binding ElpWidth,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
            Height="{Binding ElpWidth,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
            VerticalAlignment="Center"
            HorizontalAlignment="Center"
            MouseDown="elp_MouseDown"
            MouseMove="elp_MouseMove"
            MouseUp="elp_MouseUp"
            RenderTransformOrigin="0.5,0.5">
            <Ellipse.Fill>
                <!--<LinearGradientBrush StartPoint="0,0" EndPoint="0,1">
                    <GradientStop Color="Orange" Offset="0"/>
                    <GradientStop Color="Orange" Offset="0.2"/>
                    <GradientStop Color="Yellow" Offset="0.2"/>
                    <GradientStop Color="Yellow" Offset="0.4"/>
                    <GradientStop Color="Green" Offset="0.4"/>
                    <GradientStop Color="Green" Offset="0.498"/>
                    <GradientStop Color="Red" Offset="0.498"/>
                    <GradientStop Color="Red" Offset="0.502"/>
                    <GradientStop Color="Green" Offset="0.502"/>
                    <GradientStop Color="Green" Offset="0.6"/>
                    <GradientStop Color="Blue" Offset="0.6"/>
                    <GradientStop Color="Blue" Offset="0.8"/>
                    <GradientStop Color="Cyan" Offset="0.8"/>
                    <GradientStop Color="Cyan" Offset="1.0"/>
                </LinearGradientBrush>-->
                <ImageBrush ImageSource="{Binding BgImgSource,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"
                            Stretch="Fill"/>
            </Ellipse.Fill>
            <Ellipse.RenderTransform>
                <RotateTransform x:Name="elpRotater"/>
            </Ellipse.RenderTransform>
        </Ellipse>
    </Grid>
</Window>



//xaml.cs
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.IO;
using static System.Net.Mime.MediaTypeNames;

namespace WpfApp184
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        public MainWindow()
        {
            InitializeComponent();
            this.DataContext = this;
            this.Loaded += MainWindow_Loaded;
            this.SizeChanged += MainWindow_SizeChanged;
        }

        private void MainWindow_SizeChanged(object sender, SizeChangedEventArgs e)
        {
            InitVariables();
        }

        private void MainWindow_Loaded(object sender, RoutedEventArgs e)
        {
            InitVariables();
        }

        private void InitVariables()
        {
            var fe = this.Content as FrameworkElement;
            if (fe != null)
            {
                ElpWidth = Math.Min(fe.ActualWidth, fe.ActualHeight) - 100;
                originPt = new Point(fe.ActualWidth / 2, fe.ActualHeight / 2);                
                string bgImgUrl = @"../../Images/2.jpg";
                BgImgSource=GetImgSourceViaUrl(bgImgUrl);
            }
        }

        private ImageSource GetImgSourceViaUrl(string imgUrl)
        {
            if(!File.Exists(imgUrl))
            {
                return null;
            }
            BitmapImage bmi = new BitmapImage();
            bmi.BeginInit();
            bmi.UriSource=new Uri(imgUrl,UriKind.RelativeOrAbsolute);
            bmi.EndInit();
            if(bmi.CanFreeze)
            {
                bmi.Freeze();
            }
            return bmi;
        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void OnPropertyChanged([CallerMemberName] string propName = "")
        {
            var handler = PropertyChanged;
            if (handler != null)
            {
                handler?.Invoke(this, new PropertyChangedEventArgs(propName));
            }
        }

        private double elpWidth;
        public double ElpWidth
        {
            get
            {
                return elpWidth;
            }
            set
            {
                if (value != elpWidth)
                {
                    elpWidth = value;
                    OnPropertyChanged(nameof(ElpWidth));
                }
            }
        }

        private ImageSource bgImgSource;
        public ImageSource BgImgSource
        {
            get
            {
                return bgImgSource;
            }
            set
            {
                if (value != bgImgSource)
                {
                    bgImgSource = value;
                    OnPropertyChanged(nameof(BgImgSource));
                }
            }
        }

        private Point prevPt { get; set; }
        private Point originPt { get; set; }
        private void elp_MouseDown(object sender, MouseButtonEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                prevPt = e.GetPosition(this);
                elp.CaptureMouse();
            }
        }

        private void elp_MouseMove(object sender, MouseEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Pressed)
            {
                Vector prevVec = Point.Subtract(prevPt, originPt);
                Point newPt = e.GetPosition(this);
                Vector newVec = Point.Subtract(newPt, originPt);
                double rotatedAngle = Vector.AngleBetween(prevVec, newVec);
                elpRotater.Angle += rotatedAngle;
                this.Title = elpRotater.Angle.ToString();
                prevPt = newPt;
            }
        }

        private void elp_MouseUp(object sender, MouseButtonEventArgs e)
        {
            if (e.LeftButton == MouseButtonState.Released)
            {
                elp.ReleaseMouseCapture();
            }
        }
    }
}