关于WPF WriteableBitmap类直接操作像素点的问题

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*"></RowDefinition>
            <RowDefinition Height="10*" ></RowDefinition>
        </Grid.RowDefinitions>
 
        <Button Name="button" Grid.Row="0" HorizontalAlignment="Center" Content="generate_bitmap" MinWidth="120"  MinHeight="30" Click="Button_Click"></Button>
        <Grid x:Name="imgGrid" Grid.Row="1">
            <Viewbox>
                <Image x:Name="img"  Width="{Binding RelativeSource={RelativeSource Self}, Path=Source.PixelWidth}"
                       Height="{Binding RelativeSource={RelativeSource Self}, Path=Source.PixelHeight}"
                        Source="{Binding CtrlImage, IsAsync=True}"
                      Stretch="None" />
            </Viewbox>
        </Grid>
    </Grid>
</Window>

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
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.Drawing;
using System.Drawing.Drawing2D;
 
namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        public void Button_Click(object sender, RoutedEventArgs e)
            WriteableBitmap wb = new WriteableBitmap((int)imgGrid.ActualWidth, (int)imgGrid.ActualHeight, 96, 96, PixelFormats.Pbgra32, null);
            wb.Lock();
            Bitmap backBitmap = new Bitmap((int)imgGrid.ActualWidth, (int)imgGrid.ActualHeight, wb.BackBufferStride, System.Drawing.Imaging.PixelFormat.Format32bppArgb, wb.BackBuffer);
            Int32Rect rect = new Int32Rect(0, 0, (int)imgGrid.ActualWidth, (int)imgGrid.ActualHeight);
            byte[] pixels = new byte[(int)imgGrid.ActualWidth * (int)imgGrid.ActualHeight * wb.Format.BitsPerPixel / 8];
            Random rand = new Random();
            for (int y = 0; y < wb.PixelHeight; y++)
            {
                for (int x = 0; x < wb.PixelWidth; x++)
                {
                    int alpha = 0;
                    int red = 0;
                    int green = 0;
                    int blue = 0;
                    if ((x % 5 == 0) || (y % 7 == 0))
                    {
                        red = (int)((double)y / wb.PixelHeight * 255);
                        green = rand.Next(100, 255);
                        blue = (int)((double)x / wb.PixelWidth * 255);
                        alpha = 255;
                    }
                    else
                        red = (int)((double)x / wb.PixelWidth * 255);
                        blue = (int)((double)y / wb.PixelHeight * 255);
                        alpha = 50;
                    int pixeloffset = (x + y * wb.PixelWidth) * wb.Format.BitsPerPixel / 8;
                    pixels[pixeloffset] = (byte)blue;
                    pixels[pixeloffset + 1] = (byte)green;
                    pixels[pixeloffset + 2] = (byte)red;
                    pixels[pixeloffset + 3] = (byte)alpha;
                }
                int stride = (wb.PixelWidth * wb.Format.BitsPerPixel) / 8;
                wb.WritePixels(rect, pixels, stride, 0);
            }
            wb.Unlock();
            backBitmap.Dispose();
            backBitmap = null;
            img.Source = wb;
    }
}