Per-screen scaleFactor

To enable, set QT_SCALE_FACTOR=auto

Tons of bugs, since this does not port all the QT_DEVICE_PIXEL_RATIO
fixes from xcb.

5 files changed, 32 insertions, 7 deletions

diff --git a/src/gui/kernel/qhighdpiscaling.cpp b/src/gui/kernel/qhighdpiscaling.cpp
index eeeb995d43..0d847a243f 100644
--- a/src/gui/kernel/qhighdpiscaling.cpp
+++ b/src/gui/kernel/qhighdpiscaling.cpp
@@ -77,6 +77,7 @@ static inline qreal initialScaleFactor()
 */
 
 qreal QHighDpiScaling::m_factor = initialScaleFactor();
+bool QHighDpiScaling::m_autoFactor = qgetenv("QT_SCALE_FACTOR").toLower() == "auto";
 bool QHighDpiScaling::m_active = !qFuzzyCompare(QHighDpiScaling::m_factor, qreal(1));
 bool QHighDpiScaling::m_perWindowActive = false;
 
@@ -103,13 +104,24 @@ void QHighDpiScaling::setWindowFactor(QWindow *window, qreal factor)
     window->setProperty(scaleFactorProperty, QVariant(factor));
 }
 
+qreal QHighDpiScaling::factor(const QScreen *screen)
+{
+    if (m_autoFactor && screen && screen->handle())
+        return screen->handle()->pixelDensity();
+    return m_factor;
+}
+
+
 qreal QHighDpiScaling::factor(const QWindow *window)
 {
+    qreal f = m_factor;
+    if (m_autoFactor && window && window->screen() && window->screen()->handle())
+        f = window->screen()->handle()->pixelDensity();
     if (!m_perWindowActive || window == 0)
-        return m_factor;
+        return f;
 
     QVariant windowFactor = window->property(scaleFactorProperty);
-    return m_factor * (windowFactor.isValid() ? windowFactor.toReal() : 1);
+    return f * (windowFactor.isValid() ? windowFactor.toReal() : 1);
 }
 
 Q_GUI_EXPORT QSize QHighDpi::toDevicePixelsConstrained(const QSize &size, const QWindow *window)
diff --git a/src/gui/kernel/qhighdpiscaling_p.h b/src/gui/kernel/qhighdpiscaling_p.h
index 6929b3ac83..5827bc1565 100644
--- a/src/gui/kernel/qhighdpiscaling_p.h
+++ b/src/gui/kernel/qhighdpiscaling_p.h
@@ -79,11 +79,13 @@ Q_DECLARE_LOGGING_CATEGORY(lcScaling);
 class Q_GUI_EXPORT QHighDpiScaling {
 public:
     static bool isActive() { return m_active; }
-    static qreal factor(const QWindow *window = 0);
+    static qreal factor(const QWindow *window);
+    static qreal factor(const QScreen *screen);
     static void setFactor(qreal factor);
     static void setWindowFactor(QWindow *window, qreal factor);
 private:
     static qreal m_factor;
+    static bool m_autoFactor;
     static bool m_active;
     static bool m_perWindowActive;
 };
diff --git a/src/gui/kernel/qplatformscreen.cpp b/src/gui/kernel/qplatformscreen.cpp
index e2c84fbc2c..6088e6495e 100644
--- a/src/gui/kernel/qplatformscreen.cpp
+++ b/src/gui/kernel/qplatformscreen.cpp
@@ -157,9 +157,9 @@ QDpi QPlatformScreen::logicalDpi() const
 }
 
 /*!
-    Reimplement this function in subclass to return the device pixel
-    ratio for the screen. This is the ratio between physical pixels
-    and device-independent pixels.
+    Reimplement this function in subclass to return the device pixel ratio
+    for the screen. This is the ratio between physical pixels and the
+    device-independent pixels of the windowing system.
 
     \sa QPlatformWindow::devicePixelRatio();
 */
@@ -169,6 +169,16 @@ qreal QPlatformScreen::devicePixelRatio() const
 }
 
 /*!
+    Reimplement this function in subclass to return the pixel density of the
+    screen. This is the scale factor needed to make a low-dpi application
+    usable on this screen. The default implementation returns 1.0.
+*/
+qreal QPlatformScreen::pixelDensity()  const
+{
+    return 1.0;
+}
+
+/*!
     Reimplement this function in subclass to return the vertical refresh rate
     of the screen, in Hz.
 
diff --git a/src/gui/kernel/qplatformscreen.h b/src/gui/kernel/qplatformscreen.h
index ad1cb6f9b3..8dceef26eb 100644
--- a/src/gui/kernel/qplatformscreen.h
+++ b/src/gui/kernel/qplatformscreen.h
@@ -96,6 +96,7 @@ public:
     virtual QSizeF physicalSize() const;
     virtual QDpi logicalDpi() const;
     virtual qreal devicePixelRatio() const;
+    virtual qreal pixelDensity()  const;
 
     virtual qreal refreshRate() const;
 
diff --git a/src/gui/kernel/qscreen.cpp b/src/gui/kernel/qscreen.cpp
index 684b298631..91cef9640b 100644
--- a/src/gui/kernel/qscreen.cpp
+++ b/src/gui/kernel/qscreen.cpp
@@ -259,7 +259,7 @@ qreal QScreen::logicalDotsPerInch() const
 qreal QScreen::devicePixelRatio() const
 {
     Q_D(const QScreen);
-    return d->platformScreen->devicePixelRatio() * QHighDpiScaling::factor();
+    return d->platformScreen->devicePixelRatio() * QHighDpiScaling::factor(this);
 }
 
 /*!