/****************************************************************************
**
** Copyright (C) 2016 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and The Qt Company. For licensing terms
** and conditions see https://www.qt.io/terms-conditions. For further
** information use the contact form at https://www.qt.io/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: https://www.gnu.org/licenses/fdl-1.3.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
    \page highdpi.html
    \title High DPI Displays
    \brief Describes the concepts in high DPI displays.

    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.

    \section2 Challenges with High DPI

    High DPI displays bring about some challenges for existing applications:

    \list
        \li \b{Applications using UI designs with fixed coordinates look small}
            \br 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.

        \li \b{Applications must adapt to situations where users have multiple displays with
            varying resolutions}
            \br 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.
    \endlist

    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.

    \section2 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 \e 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.

    \section2 Operating System Support

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

    \section3 \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 \c{Info.plist} file; so make sure
    these settings are present.

    \code
        <key>NSPrincipalClass</key>
        <string>NSApplication</string>
        <key>NSHighResolutionCapable</key>
        <string>True</string>
    \endcode

    Newer versions of qmake will generate an \c{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.

    \section3 Microsoft Windows

    \b Scaling

    Users choose a scaling factor from the \uicontrol{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.

    \b DPI Awareness

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

    \table
        \header
            \li DPI Awareness Level
            \li Meaning

        \row
            \li DPI Unaware
            \li This level was introduced in Windows Vista. To the application, Windows pretends
                as if it's running on a standard display of 96 DPI of 1920x1080 and scales the
                application accordingly. It's intended to accommodate older applications designed
                for low DPI displays. This type of scaling may result in some artifacts.

        \row
            \li System-DPI Aware
            \li This level was introduced in Windows Vista. It differs from
                \e{Per-Monitor DPI Aware} only when multiple monitors are connected. Windows
                calculates a scaling that's suitable for all monitors connected.

        \row
            \li Per-Monitor DPI Aware
            \li This level was introduced in Windows 8.1. Windows does not perform any scaling at
                all.
    \endtable

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

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

    For more information, see \l{Using qt.conf}.

    \section2 High DPI Support in Qt

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

    \list
        \li The ability to provide pixmaps or artwork for high resolution. For more details,
            see \l{Drawing High Resolution Versions of Pixmaps and Images}.

        \li 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:
            \list
                \li \c 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.

                \li \c QT_SCALE_FACTOR [numeric] defines a global scale factor for the whole
                application, including point-sized fonts.

                \li \c 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
                \l {https://en.wikipedia.org/wiki/Extended_Display_Identification_Data}
                {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 \c name=value
                pairs, where \c name is the same as QScreen::name().
            \endlist

            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.

        \li The \c Qt::AA_EnableHighDpiScaling application attribute, introduced in Qt 5.6,
            enables automatic scaling based on the monitor's pixel density.

        \li The \c 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.

        \li In Qt 5.4, there was an experimental implementation of high DPI scaling introduced via
            the \c QT_DEVICE_PIXEL_RATIO environment variable, that you could set to a numerical
            scale factor or \c auto. This variable was deprecated in Qt 5.6.
        \endlist

    \section2 Migrate Existing Applications

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

    \list
        \li let the application run as \e{DPI Unaware} on Windows
        \li set the \c QT_AUTO_SCREEN_SCALE_FACTOR environment variable to \c 1.

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

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

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

    \section2 Glossary

    \table
        \header
            \li Term
            \li Definition

        \row
            \li Device Independent Pixels
            \li The pixels that an application uses (user space), subject to scaling by the operating
                system or Qt.

        \row
            \li Device Pixels
            \li The display device's pixels.

        \row
            \li Device Pixel Ratio
            \li The scale factor that either the operating system or Qt applies.

        \row
            \li Logical DPI
            \li The resolution used to convert font sizes defined in points to font sizes in pixels.
                The standard values are 96, 128, ... 192.

        \row
            \li Physical DPI
            \li The physical resolution obtained by dividing the size of the monitor by the number
                of pixels.
        \row
            \li User Space
            \li The coordinate space that an application uses in Device Independent Pixels.
    \endtable
*/
