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How could I know the effect of ViewportScaleFactor of HolographicCamera?

In https://developer.microsoft.com/en-us/windows/mixed-reality/performance_recommendations_for_hololens_apps , the author mentioned like below:

"To easily distinguish a vertex bottleneck from a pixel bottleneck, use the ViewportScaleFactor to reduce your output resolution. If you see a performance gain, you have a pixel bottleneck, if you do not see a gain, you likely have a vertex shader bottleneck."

So I tried to change ViewportScaleFactor, but I couldn't know whether it works or not.

I added scaling at the beginning of AppNameMain::OnCameraAdded():

    Deferral^ deferral = args->GetDeferral();
    HolographicCamera^ holographicCamera = args->Camera;
    holographicCamera->ViewportScaleFactor = 0.5f;

And I checked below snippet by debugger,

void DX::CameraResources::UpdateViewProjectionBuffer(
    std::shared_ptr<DX::DeviceResources> deviceResources,
    HolographicCameraPose^ cameraPose,
    SpatialCoordinateSystem^ coordinateSystem
    )
{
    // The system changes the viewport on a per-frame basis for system optimizations.
    m_d3dViewport = CD3D11_VIEWPORT(
        cameraPose->Viewport.Left,
        cameraPose->Viewport.Top,
        cameraPose->Viewport.Width,
        cameraPose->Viewport.Height
        );

but cameraPose->Viewport.Width and Height didn't changed. It was same with ViewportScaleFactor = 1.0f; setting. And the rendered scene felt same (but it's just my feeling)

My question is,

1) How can I know ViewportScaleFactor is working?
2) Where should I put the code changing ViewportScaleFactor?
3) Or is there another method to change the resolution of HoloLens app?

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