High DPI displays have increased pixel density, compared to standard DPI displays.

Pixel density is measured in Dots per Inch (DPI) or Pixels per Inch (PPI), and is determined by the number of display pixels and their size. Consequently, the number of pixels alone isn't enough to determine if a display falls into the high-DPI category.

A 4K monitor has a fixed number of pixels (~8M), however its DPI varies between 185 (23 inches) and 110 (40 inches). The former is around twice the standard 96 DPI desktop resolution; the latter barely exceeds this resolution.

Challenges with High DPI

High DPI displays bring about some challenges for existing applications:

• Applications using UI designs with fixed coordinates look small
  The combination of specifying font size in points and other sizes in pixels is particularly problematic because points are independent from the monitor's resolution. For example, suppose we have a frame of 40x20 pixels around the text "hello". If we use a 12pt font, it would look correct on low resolution monitors. But, on high DPI monitors, the frame would be too small, resulting in the text being clipped.
• Applications must adapt to situations where users have multiple displays with varying resolutions
  For example, a user might use a 4K monitor for the document window of an image editor but a low resolution monitor for the tool box.

Traditionally, to support high DPI, Qt scales fonts automatically and provides a DPI value that application code can use to scale the rest of the UI.

High DPI Support on a System Level

Qt supports a high DPI mode where the main coordinate system is virtualized and made independent from the display pixel density. Some operating systems, like macOS and iOS implement this mode. Additionally, if an operating system doesn't support this mode, Qt has an implementation to fallback on.

Now, geometry is specified in device independent pixels. This includes widget and item geometry, event geometry, desktop, window and screen geometry, as well as animation velocities. The output is rendered in device pixels, which corresponds to the display resolution. The devicePixelRatio is the ratio between the device independent pixels and the device pixel coordinate system.

Typically, most applications work with device independent pixels; except for OpenGL and code for raster graphics.

Operating System Support

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

macOS and iOS

The Apple platforms implement scaling and coordinate system virtualization in the operating system. Normally, no special configuration is required.

On macOS, high-DPI support is enabled by settings in the Info.plist file; so make sure these settings are present.

<key>NSPrincipalClass</key>
<string>NSApplication</string>
<key>NSHighResolutionCapable</key>
<string>True</string>

Newer versions of qmake will generate an Info.plist file with the NSPrincipalClass key; this is sufficient since NSHighResolutionCapable is true by default.

Note: Both macOS and iOS may apply further virtualization, such that device pixels no longer correspond to display pixels 1:1. This happens on the iPhone 6+ and on macOS configured with "display scaling" enabled.

Microsoft Windows

Scaling

Users choose a scaling factor from the Control Panel or via the 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 doesn't perform any actual scaling.

DPI Awareness

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

By default, Qt applications are set to Per-Monitor DPI Aware on Windows 8.1 or System-DPI Aware on older Windows versions. As of Qt 5.4, this level can be specified via a parameter to the platform plugin:

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

For more information, see Using qt.conf.

High DPI Support in Qt

Qt provides the following ways for you to handle high DPI support in your application.

• The ability to provide pixmaps or artwork for high resolution. For more details, 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 in macOS. This allows applications written for low DPI screens to run unchanged on high DPI devices. This feature is opt-in, and can be enabled using the following environment variables:
  • QT_AUTO_SCREEN_SCALE_FACTOR [boolean] enables automatic scaling, based on the monitor's pixel density. This won't change the size of point-sized fonts, since point is a physical measurement unit. 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 won't change the size of point-sized fonts. The environment variable is mainly useful for debugging, or to work around monitors with wrong EDID information(Extended Display Identification Data).

    The format can either be 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().

  While the macOS style fully supports high-DPI, the Windows desktop style currently has some limitations with certain scale factors. In these cases, consider using the Fusion style instead, which supports high-DPI in all cases.

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

• The Qt::AA_EnableHighDpiScaling application attribute, introduced in Qt 5.6, enables automatic scaling based on the monitor's pixel density.
• The Qt::AA_DisableHighDpiScaling application attribute, introduced in Qt 5.6, turns off all scaling. This is intended for applications that require actual window system coordinates, regardless of environment variables. This attribute takes priority over Qt::AA_EnableHighDpiScaling.
• The QT_ENABLE_HIGHDPI_SCALING environment variable, introduced in Qt 5.14, enables automatic scaling based on the pixel density of the monitor. Replaces QT_AUTO_SCREEN_SCALE_FACTOR.
• The QT_SCALE_FACTOR_ROUNDING_POLICY environment variable and QGuiApplication::highDpiScaleFactorRoundingPolicy API, introduced in Qt 5.14, makes it possible to control if and how the device pixel ratio should be rounded to the nearest integer. This is relevant for configurations like Windows at 150% scale. Possible values are Round, Ceil, Floor, RoundPreferFloor, PassThrough. See the Qt::HighDpiScaleFactorRoundingPolicy enum documentation for a full description of the options.
• In Qt 5.4, there was an experimental implementation of high DPI scaling introduced via the QT_DEVICE_PIXEL_RATIO environment variable, that you could set to a numerical scale factor or auto. This variable was deprecated in Qt 5.6.

Migrate Existing Applications

To get an application designed for low DPI values running on high resolution monitors quickly, consider one of the following:

• let the application run as DPI Unaware on Windows
• set the QT_AUTO_SCREEN_SCALE_FACTOR environment variable to 1.

However, these options may result in some scaling or painting artifacts.

In the long 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 corresponding screen size.
• Replace hard-coded sizes in layouts and drawing code with values calculated from font metrics or screen size.

Glossary Of High DPI Terms