StepTimer.GetTotalSeconds() produces values that do not always increase

Question

I'm using DirectX 11. For simplicity, and to reproduce the problem, I narrowed it down to the following steps:

1. Create a new "DirectX and XAML App (UWP)" in Visual Studio (I'm using VS 2017).

2. Replace the Sample3DSceneRenderer::Update method with the following code:

   void Sample3DSceneRenderer::Update(DX::StepTimer const& timer)
   {
       if (!m_tracking)
       {
           double total = timer.GetTotalSeconds();
   
           // Convert degrees to radians, then convert seconds to rotation angle
           float radiansPerSecond = XMConvertToRadians(m_degreesPerSecond);
           double totalRotation = total * radiansPerSecond;
           float radians = static_cast<float>(fmod(totalRotation, XM_2PI));
   
           DX::DebugTrace(L"num = %4.2f\t%4.2f\n", radians, total);
   
           Rotate(radians);
       }
   }
   

3. Add the following function to trace the values in the Output window:

   inline void DebugTrace(const wchar_t *format, ...)
   {
       // Generate the message string.
       va_list args;
       va_start(args, format); // initialize the argument list
       wchar_t buffer[1024];
       va_end(args);
   
       OutputDebugStringW(buffer); // this is a Windows function
   }
   

When I run the app, the Output window shows the following values:

    num = 0.01  0.01
    num = 0.02  0.02
    num = 0.00  0.00 // decreased
    num = 0.00  0.01 // decreased
    num = 0.03  0.04
    num = 0.05  0.06
    num = 0.02  0.02 // decreased
    num = 0.06  0.07
    num = 0.03  0.04 // decreased
    num = 0.07  0.09
    num = 0.04  0.06 // decreased
    num = 0.08  0.11
    num = 0.06  0.07 // decreased
    num = 0.10  0.12 
    num = 0.07  0.09 // decreased
    num = 0.11  0.14
    num = 0.08  0.11 // decreased
    num = 0.12  0.16
    num = 0.10  0.12 // decreased
    num = 0.11  0.14 
    num = 0.14  0.17
    num = 0.12  0.16 // decreased
    num = 0.15  0.19
    num = 0.16  0.21
    num = 0.14  0.17 // decreased
    num = 0.18  0.22
    num = 0.15  0.19 // decreased
    num = 0.16  0.21 
    num = 0.19  0.24
    num = 0.20  0.26
    num = 0.18  0.22 // decreased
    etc.

Question: Why does the TotalSeconds values increase and then decrease and then increase again etc.? For example: 0.01, 0.02, 0.00, 0.01. Shouldn't they always increase?

Answer 1

The bug is in App::OnLaunched. The version in the template is creating two DirectXPages:

• Direct assignment: m_directXPage = ref new DirectXPage();
• Navigation: rootFrame->Navigate(TypeName(DirectXPage::typeid), e->Arguments);

I have a modified version that only creates a single version by taking a reference to the one created during navigation, but I haven't spent much time checking on error cases.

void App::OnLaunched(Windows::ApplicationModel::Activation::LaunchActivatedEventArgs^ e)
{
#if _DEBUG
       if (IsDebuggerPresent())
       {
              DebugSettings->EnableFrameRateCounter = true;
       }
#endif

       auto rootFrame = dynamic_cast<Frame^>(Window::Current->Content);

       // Do not repeat app initialization when the Window already has content,
       // just ensure that the window is active
       if (rootFrame == nullptr)
       {
              // Create a Frame to act as the navigation context and associate it with
              // a SuspensionManager key
              rootFrame = ref new Frame();

              rootFrame->NavigationFailed += ref new Windows::UI::Xaml::Navigation::NavigationFailedEventHandler(this, &App::OnNavigationFailed);

              // Place the frame in the current Window
              Window::Current->Content = rootFrame;
       }

       if (rootFrame->Content == nullptr)
       {
              // When the navigation stack isn't restored navigate to the first page,
              // configuring the new page by passing required information as a navigation
              // parameter
              rootFrame->Navigate(TypeName(DirectXPage::typeid), e->Arguments);
       }

       if (m_directXPage == nullptr)
       {
              m_directXPage = dynamic_cast<DirectXPage^>(rootFrame->Content);
       }

       if (e->PreviousExecutionState == ApplicationExecutionState::Terminated)
       {
              m_directXPage->LoadInternalState(ApplicationData::Current->LocalSettings->Values);
       }

       // Ensure the current window is active
       Window::Current->Activate();
}