Screenshot refer to the spacing of the volume

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Hi, there!
I am doing screenshot in the MIP module, and here is the code:

	vtkSmartPointer<vtkWindowToImageFilter> shot = vtkSmartPointer<vtkWindowToImageFilter>::New();
	shot->SetInput(m_pRenderWnd);
	shot->SetInputBufferTypeToRGB(); //RGBA format
	shot->ReadFrontBufferOff();
	shot->SetViewport(0, 0, 1, 1);
	shot->Update();

	vtkSmartPointer<vtkImageData> shotdata = shot->GetOutput();
	double* spacing = shotdata->GetSpacing();

No matter how I rotate and zoom in/out the MIP volume, the shotdata->GetSpacing() keeps (1, 1, 1), seems that vtkWindowToImageFilter just read something directly.
Are there any compressed code to get the specify spacing screenshot output? Great thanks!