WPF控件保存成图片不全问题

    问题：需要将缩放和其它处理后的图片保存成图片，采用一般的将控件保存成图片的方法，发现导出的图片内容为空或者显示不全。

• xaml代码

<Window x:Class="Image_Zoom_Save.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:Image_Zoom_Save"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition/>
            <RowDefinition Height="auto"/>
        </Grid.RowDefinitions>
        <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
            <Image Name="imgProcess" Stretch="Uniform" RenderOptions.BitmapScalingMode="HighQuality"/>
        </ScrollViewer>
        <StackPanel Grid.Row="1" Orientation="Horizontal">
            <Button Content="Load" Margin="5" Click="Load_Click"/>
            <Button Content="Save" Margin="5" Click="Save_Click"/>
            <Slider Name="sliderZoom" Minimum="0.2" Maximum="5" Value="1" VerticalAlignment="Center" Width="200"
                    ValueChanged="SliderZoom_ValueChanged"/>
            <Label Name="lblInfo"/>
            <Button Content="Change Position" Margin="5" Click="Position_Click"/>
        </StackPanel>
    </Grid>
</Window>

•  后台代码

using System;
using System.IO;
using System.Windows;
using System.Windows.Media;
using System.Windows.Media.Imaging;

namespace Image_Zoom_Save
{
    /// <summary>
    /// MainWindow.xaml 的交互逻辑
    /// </summary>
    public partial class MainWindow : Window
    {
        private BitmapImage bitmapImage;
        public MainWindow()
        {
            InitializeComponent();
        }

        private void SliderZoom_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
        {
            if (bitmapImage == null)
            {
                return;
            }
            imgProcess.Width = sliderZoom.Value * bitmapImage.Width;
            imgProcess.Height = sliderZoom.Value * bitmapImage.Height;
            lblInfo.Content = "width = " + ((int)imgProcess.Width).ToString() + "; height = " + ((int)imgProcess.Height).ToString();
        }

        private void Load_Click(object sender, RoutedEventArgs e)
        {
            bitmapImage = new BitmapImage();
            bitmapImage.BeginInit();
            bitmapImage.UriSource = new Uri("D:/MyPrograms/图标/qq.png", UriKind.RelativeOrAbsolute);
            bitmapImage.EndInit();
            imgProcess.Source = bitmapImage;
            imgProcess.Width = bitmapImage.Width;
            imgProcess.Height = bitmapImage.Height;
            imgProcess.VerticalAlignment = VerticalAlignment.Center;
            imgProcess.HorizontalAlignment = HorizontalAlignment.Center;
            lblInfo.Content = "width = " + ((int)imgProcess.Width).ToString() + "; height = " + ((int)imgProcess.Height).ToString();
        }

        private void UISaveToImage(FrameworkElement ui, string fileName, int width, int height)
        {
            RenderTargetBitmap bmp = new RenderTargetBitmap(width, height, 96d, 96d, PixelFormats.Pbgra32);
            bmp.Render(ui);
            BitmapEncoder encoder = new PngBitmapEncoder();
            encoder.Frames.Add(BitmapFrame.Create(bmp));
            FileStream fs = new FileStream(fileName, FileMode.Create);
            encoder.Save(fs);
            fs.Close();
        }

        private void Save_Click(object sender, RoutedEventArgs e)
        {
            UISaveToImage(imgProcess, "test.png", (int)imgProcess.Width, (int)imgProcess.Height);
        }

        private void Position_Click(object sender, RoutedEventArgs e)
        {
            imgProcess.VerticalAlignment = VerticalAlignment.Top;
            imgProcess.HorizontalAlignment = HorizontalAlignment.Left;
        }
    }
}

补充说明：放大功能没有利用RenderTransform属性，而是仅仅改变imgProcess的宽度和高度；这种保存控件为图片的方式似乎要求对象位置为父对象的左上角，不过这样显示的效果不尽人意，有什么好的方法吗？发现用Canvas.GetTop(imgProcess)得到的结果是NaN，so weird。