vtkPNGwriter problem of SetScale() larger than 1

Hi there, I tried to do screenshot with 'vtkWindowToImage` class, with the code below:

	vtkSmartPointer<vtkWindowToImageFilter> shot = vtkSmartPointer<vtkWindowToImageFilter>::New();
	shot->SetInputBufferTypeToRGB(); //RGBA format
	shot->SetScale(1);  // here is the problem
	shot->SetViewport(0, 0, 1, 1);

	const size_t buf = 256;
	char buffer[buf];
	size_t convertedChars = 0;
	wcstombs_s(&convertedChars, buffer, buf, m_strpwzPath, _TRUNCATE);
	const char* narrowString = buffer;

	vtkNew<vtkPNGWriter> writer;

When I tried to set setScale() with 2, the saved image shows black crosshairs(could be transparent when I save as RGBA)

When I set back to 1 like setScale(1), the image is correct, but the resolution is bad.

Could you please give me some advice if I want to save the PNG picture with higher resolution but without that crosshairs? Great Thanks!