Add high-DPI scale factor rounding policy C++ API

This API enables tuning of how Qt rounds fractional scale factors, and
corresponds to the QT_SCALE_FACTOR_ROUNDING_POLICY environment
variable

New API:
	Qt::HighDPiScaleFactorRoundingPolicy
	QGuiApplication::setHighDpiScaleFactorRoundingPolicy()
	QGuiApplication::highDpiScaleFactorRoundingPolicy()

Done-with: Friedemann Kleint <Friedemann.Kleint@qt.io>
Task-number: QTBUG-53022
Change-Id: Ic360f26a173caa757e4ebde35ce08a6b74290b7d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 44 insertions, 0 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 426f2aeece..ddd6726299 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -146,6 +146,8 @@ QString QGuiApplicationPrivate::styleOverride;
 
 Qt::ApplicationState QGuiApplicationPrivate::applicationState = Qt::ApplicationInactive;
 
+Qt::HighDpiScaleFactorRoundingPolicy QGuiApplicationPrivate::highDpiScaleFactorRoundingPolicy =
+    Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor;
 bool QGuiApplicationPrivate::highDpiScalingUpdated = false;
 
 QPointer<QWindow> QGuiApplicationPrivate::currentDragWindow;
@@ -687,6 +689,8 @@ QGuiApplication::~QGuiApplication()
     QGuiApplicationPrivate::lastCursorPosition = {qInf(), qInf()};
     QGuiApplicationPrivate::currentMousePressWindow = QGuiApplicationPrivate::currentMouseWindow = nullptr;
     QGuiApplicationPrivate::applicationState = Qt::ApplicationInactive;
+    QGuiApplicationPrivate::highDpiScaleFactorRoundingPolicy =
+        Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor;
     QGuiApplicationPrivate::highDpiScalingUpdated = false;
     QGuiApplicationPrivate::currentDragWindow = nullptr;
     QGuiApplicationPrivate::tabletDevicePoints.clear();
@@ -3491,6 +3495,46 @@ Qt::ApplicationState QGuiApplication::applicationState()
 }
 
 /*!
+    \since 5.14
+
+    Sets the high-DPI scale factor rounding policy for the application. The
+    policy decides how non-integer scale factors (such as Windows 150%) are
+    handled, for applications that have AA_EnableHighDpiScaling enabled.
+
+    The two principal options are whether fractional scale factors should
+    be rounded to an integer or not. Keeping the scale factor as-is will
+    make the user interface size match the OS setting exactly, but may cause
+    painting errors, for example with the Windows style.
+
+    If rounding is wanted, then which type of rounding should be decided
+    next. Mathematically correct rounding is supported but may not give
+    the best visual results: Consider if you want to render 1.5x as 1x
+    ("small UI") or as 2x ("large UI"). See the Qt::HighDpiScaleFactorRoundingPolicy
+    enum for a complete list of all options.
+
+    This function must be called before creating the application object,
+    and can be overridden by setting the QT_SCALE_FACTOR_ROUNDING_POLICY
+    environment variable. The QGuiApplication::highDpiScaleFactorRoundingPolicy()
+    accessor will reflect the environment, if set.
+
+    The default value is Qt::HighDpiScaleFactorRoundingPolicy::RoundPreferFloor.
+*/
+void QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy policy)
+{
+    QGuiApplicationPrivate::highDpiScaleFactorRoundingPolicy = policy;
+}
+
+/*!
+  \since 5.14
+
+  Returns the high-DPI scale factor rounding policy.
+*/
+Qt::HighDpiScaleFactorRoundingPolicy QGuiApplication::highDpiScaleFactorRoundingPolicy()
+{
+    return QGuiApplicationPrivate::highDpiScaleFactorRoundingPolicy;
+}
+
+/*!
     \since 5.2
     \fn void QGuiApplication::applicationStateChanged(Qt::ApplicationState state)