Refactor QHighDpiScaling internals and usage

Overall goal is to simplify, separate concerns, and
improve cross-platform-ness and testability.

QT_SCALE_FACTOR is now a pure cross-platform global
scale factor setter has no "auto". "auto" requires
input from the platform plugin via QPlatformScreen::
pixelDensity() and gets a separate environment variable:
QT_AUTO_SCREEN_SCALE_FACTOR

The effective scale factor (aka devicePixelRatio) is now
computed as the product of the global, screen, and
window scale factors. This matches how devicePixelRatio
is computed in general (the window system devicePixelRatio
is also a factor), and makes QT_SCALE_FACTOR
work consistently regardless if there is a window
scale factor set or not. This also means we can remove
the if/else casing from the nativePixels conversion
functions.

Add QHighDpiScaling initializer which reads the environment
variables and sets the "active" variables. Call it
during QGuiApplication construction, before the platform
plugin is created

Add per-screen scale factor setting capability to
the manual test. This makes it possible to test
this logic on all platforms. The command line argument
is --screen-scale-factor.

Change-Id: I054337cbae37a01cdd731d26d9cb628fcecdb652
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

1 files changed, 15 insertions, 3 deletions

diff --git a/src/gui/kernel/qplatformscreen.cpp b/src/gui/kernel/qplatformscreen.cpp
index bda7280e81..ccb0452629 100644
--- a/src/gui/kernel/qplatformscreen.cpp
+++ b/src/gui/kernel/qplatformscreen.cpp
@@ -159,9 +159,11 @@ 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 the
-    device-independent pixels of the windowing system.
+    device-independent pixels of the windowing system. The default
+    implementation returns 1.0.
 
-    \sa QPlatformWindow::devicePixelRatio();
+    \sa QPlatformWindow::devicePixelRatio()
+    \sa QPlatformScreen::pixelDensity()
 */
 qreal QPlatformScreen::devicePixelRatio() const
 {
@@ -171,7 +173,17 @@ 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.
+    usable on this screen. The default implementation returns 1.0. In
+    addition the platform integration must return true for the PlatformScreenPixelDensity
+    Capability for this value to have an effect.
+
+    Returning something else than 1 from this function causes Qt to
+    apply the scale factor to the application's coordinate system.
+    This is different from devicePixelRatio, which reports a scale
+    factor already applied by the windowing system. A platform plugin
+    typically implements one (or none) of these two functions.
+
+    \sa QPlatformWindow::devicePixelRatio()
 */
 qreal QPlatformScreen::pixelDensity()  const
 {