Fuzzy select fill simple (Avalonia UI)

MainWindow.axaml.cs

using Avalonia.Controls;
using Avalonia.Media.Imaging;
using System.Collections.Generic;
using System;
using System.IO;
using System.ComponentModel;
using System.Runtime.CompilerServices;
using Avalonia;
using Avalonia.Media;
using Avalonia.Platform;
using System.Runtime.InteropServices;
using System.Linq;

namespace floodfillava
{

    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        const string TEST_IMAGE_PATH = @"t1.jpg";

        private WriteableBitmap _ImSrc;
        public WriteableBitmap ImSrc { get { return _ImSrc; } set { _ImSrc = value; NotifyPropertyChanged(); } }

        private double _Threshold = 50;
        public double Threshold { get { return _Threshold; } set { _Threshold = value; NotifyPropertyChanged(); } }

        public MainWindow()
        {
            InitializeComponent();
            DataContext = this;
            ctlImSrc.PointerPressed += ImSrc_PointerPressed;
        }

        private void ImSrc_PointerPressed(object? sender, Avalonia.Input.PointerPressedEventArgs e)
        {
            Point pt_click = e.GetPosition(ctlImSrc);
            Point pt_im = new Point((ImSrc.Size.Width / ctlImSrc.Bounds.Width) * pt_click.X, (ImSrc.Size.Height / ctlImSrc.Bounds.Height) * pt_click.Y);
            FloodFill(pt_im, (int)Threshold);
        }

        void FloodFill(Point start, int thresh)
        {
            
            int im_w = (int)ImSrc.Size.Width;
            int im_h = (int)ImSrc.Size.Height;
            UInt32[,] imdat = new UInt32[im_h, im_w];

            var imp = ImSrc.Lock();
            unsafe
            {
                fixed (UInt32* ptr = imdat)
                {
                    int sz = imp.RowBytes * im_h * 4;
                    CopyMemory((void*)imp.Address, ptr, imp.RowBytes * im_h);
                }
            }

            Color targetColor = imdat.GetColor((int)start.X, (int)start.Y);
            HashSet<Point> selectedPoints = new HashSet<Point>();
            Queue<Point> pointsToCheck = new Queue<Point>();

            pointsToCheck.Enqueue(start);

            while (pointsToCheck.Count > 0)
            {
                Point currentPoint = pointsToCheck.Dequeue();

                if (selectedPoints.Contains(currentPoint)) continue;

                Color currentColor = imdat.GetColor((int)currentPoint.X, (int)currentPoint.Y);
                int colorDifference = ColorDifference(targetColor, currentColor);

                if (colorDifference <= thresh)
                {
                    selectedPoints.Add(currentPoint);

                    IEnumerable<Point> neighbors = GetNeighbors(currentPoint, im_w, im_h);
                    foreach (Point neighbor in neighbors)
                    {
                        if (!selectedPoints.Contains(neighbor))
                        {
                            pointsToCheck.Enqueue(neighbor);
                        }
                    }
                }

            }

            Color color = new Color(255, 255, 0, 0);
            
            foreach (var pt in selectedPoints)
                imp.SetPixel((int)pt.X, (int)pt.Y, color);
            
            imp.Dispose();
            ctlImSrc.Source = null;
            ctlImSrc.Source = ImSrc;
        }

        private static IEnumerable<Point> GetNeighbors(Point point, int width, int height)
        {
            List<Point> neighbors = new List<Point>();

            var x = point.X;
            var y = point.Y;

            if (x > 0) neighbors.Add(new Point(x - 1, y));
            if (x < width - 1) neighbors.Add(new Point(x + 1, y));
            if (y > 0) neighbors.Add(new Point(x, y - 1));
            if (y < height - 1) neighbors.Add(new Point(x, y + 1));

            return neighbors;
        }

        private static int ColorDifference(Color color1, Color color2)
        {
            int redDifference = Math.Abs(color1.R - color2.R);
            int greenDifference = Math.Abs(color1.G - color2.G);
            int blueDifference = Math.Abs(color1.B - color2.B);

            return redDifference + greenDifference + blueDifference;
        }

        private async void Load_Click(object? sender, Avalonia.Interactivity.RoutedEventArgs e)
        {

            var ofd = new OpenFileDialog() { Title = "Load Image", AllowMultiple = false, Filters = new List<FileDialogFilter>() { new FileDialogFilter() { Extensions = new List<string>() { "jpg", "jpeg", "JPG" }, Name = "JPG" } } };
            var file = await ofd.ShowAsync(this);
            if (file != null && File.Exists(file[0]))
            {
                loadtestim(file[0]);
            }
        }

        void loadtestim(string fpath)
        {
            WriteableBitmap wb = null;
            using (FileStream fs = new FileStream(TEST_IMAGE_PATH, FileMode.Open))
            {
                ImSrc = WriteableBitmap.Decode(fs);
            }
            ctlImSrc.Source = ImSrc;

        }

        public event PropertyChangedEventHandler PropertyChanged;
        private void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
        {
            if (PropertyChanged != null)
            {
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public unsafe static void CopyMemory(void* srcPtr, void* dstPtr, int count) => new Span<byte>(srcPtr, count).CopyTo(new Span<byte>(dstPtr, count));
    }

    public static class Extensions
    { 
        public static Color GetColor(this UInt32[,] dat, int x, int y)
        {
            return Color.FromArgb((byte)(dat[y,x] >> 24), (byte)(dat[y, x] >> 16), (byte)(dat[y, x] >> 8), (byte)dat[y, x]);
        }
    }
}

MainWindow.xaml

<Window xmlns="https://github.com/avaloniaui"
        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"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="floodfillava.MainWindow"
        Title="floodfillava">
  <Grid RowDefinitions="50,*" ColumnDefinitions="*,*">
    <StackPanel Orientation="Horizontal" Grid.Row="0" Grid.Column="0" HorizontalAlignment="Stretch" VerticalAlignment="Center" Margin="20,0,0,0">
      <Button Content="Load" Click="Load_Click" Margin="0,0,30,0"/>
      <Slider Minimum="0" Maximum="255" Value="{Binding Threshold, Mode=TwoWay}" Width="300" Margin="0,0,10,0"/>
      <TextBlock Text="{Binding Threshold, StringFormat={}{0:N0}}" VerticalAlignment="Center" Width="60"/>
      <TextBlock Text="Threshold" VerticalAlignment="Center"/>
    </StackPanel>
      <Image x:Name="ctlImSrc" Stretch="Uniform" StretchDirection="DownOnly" Grid.Row="1" Grid.Column="0"/>
    <Image x:Name="ctlImDst" Stretch="Uniform" StretchDirection="DownOnly" Grid.Row="1" Grid.Column="1"/>
  </Grid>
    
</Window>