如何将Viewport3D保存为图像文件？

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

public void SaveViewport3DAsImage(Viewport3D viewport)
{
    RenderTargetBitmap renderTargetBitmap = new RenderTargetBitmap(
        (int)viewport.ActualWidth, (int)viewport.ActualHeight, 96, 96, PixelFormats.Pbgra32);

    renderTargetBitmap.Render(viewport);

    PngBitmapEncoder encoder = new PngBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(renderTargetBitmap));

    using (FileStream fileStream = new FileStream("output.png", FileMode.Create))
    {
        encoder.Save(fileStream);
    }
}