High DPI Displays

High DPI Displays

High DPI monitors and displays with superb image quality and crisp font rendering are becoming increasingly popular. For example, 4K monitors may have 3840x2160 pixels, resulting in a logical resolution of 192 DPI, whereas older monitors have around 1920x1080 pixels at 96 DPI.

Issues with High DPI

High DPI Displays cause a number of issues for existing applications:

  • Applications using UI designs with fixed coordinates look small. The combination of font size specification in points and other sizes in pixels is particularly problematic since points are independent of the monitor resolution. For example, a frame of 40x20 pixels around the text "hello" using a 12pt font looks correct on low resolution monitors, but the frame will be too small on high DPI monitors, causing the text to be clipped.
  • Applications must adapt to situations where users have multiple displays with varying resolution. For example, they might use a 4K monitor for the document window of an image editor and a low resolution monitor for the tool box.
  • Backing store and pixmap buffers become quite large so that rendering may slow down.

Operating System Support

The operating systems supported by Qt offer the following support for high DPI displays:

OS X

On OS X, there is a separate, float-based user coordinate system (as opposed to Device Pixels). For high DPI monitors, the operating system will scale the contents using an efficient scaling algorithm that will make everything look very crisp (Retina Display).

Note: The scaling is not applied to Open GL windows.

Microsoft Windows

Scaling

The user can choose a scaling factor from the control panel or via context menu. This works by making the functions for querying the system metrics return different values for standard font sizes, sizes of window borders, and so on. It does not perform any actual scaling.

DPI Awareness

An application on Windows can assume one of the following levels of "DPI Awareness":

DPI Awareness LevelMeaning
DPI UnawareThis level has been introduced in Windows-Vista. Windows will pretend to the application that it is running on a standard display of 96 DPI of 1920x1080 and scale the application accordingly. It is intended to accommodate older applications designed for low DPI displays. Some artifacts may result from this type of scaling.
System-DPI AwareThis level has been introduced in Windows-Vista. It differs from Per-Monitor DPI Aware only when multiple monitors are connected. Windows will calculate a scaling suitable for all connected monitors.
Per-Monitor DPI AwareThis level has been introduced in Windows 8.1. Windows does not perform any scaling at all.

Qt applications by default are Per-Monitor DPI Aware on Windows 8.1 or System-DPI Aware on older versions of Windows. As of Qt 5.4, the level can be specified by passing a parameter to the platform plugin (see Using qt.conf):

<application> -platform windows:dpiawareness=0,1,2

High DPI Support in Qt

  • Ability to provide pixmaps or artwork for high resolution: see Drawing High Resolution Versions of Pixmaps and Images.
  • Qt 5.6 supports cross-platform high-DPI scaling for legacy applications, similar to the scaling done natively by OS X. This allows applications written for low-DPI screens to run unchanged on high-DPI devices. This feature is opt-in, and can be enabled by the following environment variables:
    • QT_AUTO_SCREEN_SCALE_FACTOR [boolean] enables automatic scaling, based on the pixel density of the monitor. This will not change the size of point sized fonts, since point is a physical unit of measure. Multiple screens may get different scale factors.
    • QT_SCALE_FACTOR [numeric] defines a global scale factor for the whole application, including point sized fonts.
    • QT_SCREEN_SCALE_FACTORS [list] specifies scale factors for each screen. This will not change the size of point sized fonts. This environment variable is mainly useful for debugging, or to work around monitors with wrong EDID information(Extended Display Identification Data).

      The format can be either a semicolon-separated list of scale factors in the same order as QGuiApplication::screens(), or a semicolon-separated list of name=value pairs, where name is the same as QScreen::name().

    It is recommended to use the Fusion style.

    Note: Non-integer scale factors may cause significant scaling/painting artifacts.

  • The application attribute Qt::AA_EnableHighDpiScaling, introduced in Qt 5.6, enables automatic scaling based on the pixel density of the monitor.
  • The application attribute Qt::AA_DisableHighDpiScaling, introduced in Qt 5.6, turns off all scaling. This is intended for applications that need to use actual window system coordinates, regardless of environment variables. This attribute takes priority over Qt::AA_EnableHighDpiScaling.
  • An experimental implementation of high-DPI scaling was introduced in Qt 5.4. It was enabled by the environment variable QT_DEVICE_PIXEL_RATIO, which could be set to a numerical scale factor or "auto". This variable is deprecated in Qt 5.6.

Migration of Existing Applications

In order to get an application designed for low DPI values running on a high resolution monitors quickly, consider one of the scaling options (let the application run as DPI Unaware on Windows or set the environment variable QT_AUTO_SCREEN_SCALE_FACTOR to "1". These options may incur some scaling or painting artifacts, though.

In the longer term, the application should be adapted to run unmodified:

  • Always use the qreal versions of the QPainter drawing API.
  • Size windows and dialogs in relation to the screen size.
  • Replace hard-coded sizes in layouts and drawing code by values calculated from font metrics or screen size.

Glossary Of High DPI Terms

TermDefinition
Device Independent PixelsPixels used by application (user space), subject to scaling by the operating system or Qt.
Device PixelsPixels of the display device.
Device Pixel RatioScale factor applied by the operating system or Qt.
Logical DPIResolution used for converting font sizes defined in points to font sizes in pixels. Typically one of the standard values 96, 128, .. 192.
Physical DPIPhysical resolution obtained by dividing the size of the monitor by the number of pixels.
Retina DisplaySee Wikipedia on Retina Displays
User SpaceThe coordinate space the application uses (Device Independent Pixels).

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