wpf、c#、renderTargetBitmap的viewport3D，而不将其指定给窗口

我在代码中创建了一个Viewport3D：

Viewport3D CurViewport3D = new Viewport3D();
CurViewport3D = CreateViewportWithContent(); //fill the viewport with some content
BitmapSource bmp;
var renderTargetBitmap = new RenderTargetBitmap(600, 300, 96, 96, PixelFormats.Default);

renderTargetBitmap.Render(CurViewport3D);
bmp = (BitmapSource)renderTargetBitmap;
PngBitmapEncoder png = new PngBitmapEncoder();
png.Frames.Add(BitmapFrame.Create(bmp));
using (Stream stm = File.Create("outp.png")){ png.Save(stm);}

但不幸的是，outp.png是空的，没有任何内容。 如果我将视口应用于窗口：

MainGrid.Children.Add(CurViewport3D); //MainGrid is part of the window

一切都很完美。outp.png不是空的。 有人知道如何在不将视口应用于窗口的情况下获得正确的图像吗

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问题解决了！我不能回答我的问题-不知道为什么。。。 我添加了以下序列：

int width = 500;
int height = 300;
CurViewport3D.Width = width;
CurViewport3D.Height = height;
CurViewport3D.Measure(new Size(width, height));
CurViewport3D.Arrange(new Rect(0, 0, width, height));

int width = 500;
int height = 300;
CurViewport3D.Width = width;
CurViewport3D.Height = height;
CurViewport3D.Measure(new Size(width, height));
CurViewport3D.Arrange(new Rect(0, 0, width, height));

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问题解决了！我不能回答我的问题-不知道为什么。。。我添加了以下序列：

int width = 500;
int height = 300;
CurViewport3D.Width = width;
CurViewport3D.Height = height;
CurViewport3D.Measure(new Size(width, height));
CurViewport3D.Arrange(new Rect(0, 0, width, height));

int width = 500;
int height = 300;
CurViewport3D.Width = width;
CurViewport3D.Height = height;
CurViewport3D.Measure(new Size(width, height));
CurViewport3D.Arrange(new Rect(0, 0, width, height));