WPF中RenderTargetBitmap的时间不是固定的

如何解决WPF中RenderTargetBitmap的时间不是固定的

我在 WPF 中创建了这个测试程序

主窗口.xaml

<Window x:Class="TestrendertargetBitmap.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"
        mc:Ignorable="d"
        Title="MainWindow" Height="1080" Width="1920">
    <Canvas x:Name="canvas">
        <Rectangle x:Name="rectangle" Height="1080" Width="1920" >
            <Rectangle.Fill>
                <ImageBrush ImageSource="IMG_00000009.jpg"/>
            </Rectangle.Fill>
        </Rectangle>
    </Canvas>
</Window>

MainWindow.xaml.cs

using System.Diagnostics;
using System.Threading;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Imaging;

namespace TestrendertargetBitmap
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        Stopwatch stopwatch = new Stopwatch();
        rendertargetBitmap rendertargetBitmap;

        public MainWindow()
        {
            InitializeComponent();

            Timer timer = new Timer(tick,null,1000);

            rendertargetBitmap = new rendertargetBitmap(1920,1080,96,PixelFormats.Pbgra32);
        }

        void tick(object o)
        {
            dispatcher.Invoke(() =>
            {
                stopwatch.Restart();

                rendertargetBitmap.Render(rectangle);

                Trace.WriteLine(stopwatch.Elapsed);
            },System.Windows.Threading.dispatcherPriority.Send);
        }
    }
}

rendertargetBitmap 的时间不是恒定的（从 7ms 到 27ms）

00:00:00.0079500 00:00:00.0075308 00:00:00.0074662 00:00:00.0074894 00:00:00.0201755 00:00:00.0076335 00:00:00.0074838 00:00:00.0083659 00:00:00.0254968 00:00:00.0075025 00:00:00.0080511 00:00:00.0076793 00:00:00.0075073 00:00:00.0274513 00:00:00.0075937 00:00:00.0084706

我怎样才能有固定的时间？