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Why are the maximum X and Y touch coordinates on the Surface Pro is different from native display resolution?

I have noticed that the Surface Pro and I believe the Sony Vaio Duo 11 are reporting maximum touch coordinates of 1366x768, which is surprising to me since their native display resolution is 1920x1080. Does anyone know of a way to find out at runtime what the maximum touch coordinates are? I'm running a DirectX app underneath the XAML, so I have to scale the touch coordinates into my own world coordinates and I cannot do this without knowing what the scale factor is.

Here is the code that I'm running that looks at the touch coordinates:

From DirectXPage.xaml

<Grid PointerPressed="OnPointerPressed"></Grid>

From DirectXPage.xaml.cpp

void DirectXPage::OnPointerPressed(Platform::Object^ sender, Windows::UI::Xaml::Input::PointerRoutedEventArgs^ args)
{
   auto pointerPoint = args->GetCurrentPoint(nullptr);
   // the x value ranges between 0 and 1366
   auto x = pointerPoint->Position.X;
   // the y value ranges between 0 and 768
   auto y = pointerPoint->Position.Y;
}

Also, here is a sample project setup that can demonstrate this issue if run on a Surface Pro: http://andrewgarrison.com/files/TouchTester.zip

Solution

  • Everything on XAML side is measured in device independent pixels. Ideally you should never have to worry about actual physical pixels and let winrt do its magic in the background.

    If for some season you do need to find you current scale factor you can use DisplayProperties.ResolutionScale and use it to convert DIPs into screen pixels.