FreeType: Support RGB rendering when not using FontConfig

Windows+FreeType, Linux with -no-fontconfig and the forthcoming
OSX FreeType engine can now use sub pixel rendering.

The function to get the subpixel type is in QPlatformScreen because
we're moving to per screen font settings in the future.

This patch is safe, as no functionality is changed for existing users,
if one wants sub pixel rendering they'll still have to pass -DFT_CONFIG_OPTION_SUBPIXEL_RENDERING
to configure.

Task-number: QTBUG-44269
Change-Id: Ib6c22d48a1b7c7b85ee316d5d9e3b6eae0c1ecc0
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/src/gui/kernel/qplatformscreen.cpp b/src/gui/kernel/qplatformscreen.cpp
index 745b89018e..edf546799f 100644
--- a/src/gui/kernel/qplatformscreen.cpp
+++ b/src/gui/kernel/qplatformscreen.cpp
@@ -393,4 +393,31 @@ QRect QPlatformScreen::mapBetween(Qt::ScreenOrientation a, Qt::ScreenOrientation
     return rect;
 }
 
+/*!
+  Returns a hint about this screen's subpixel layout structure.
+
+  The default implementation queries the \b{QT_SUBPIXEL_AA_TYPE} env variable.
+  This is just a hint because most platforms don't have a way to retrieve the correct value from hardware
+  and instead rely on font configurations.
+*/
+QPlatformScreen::SubpixelAntialiasingType QPlatformScreen::subpixelAntialiasingTypeHint() const
+{
+    static int type = -1;
+    if (type == -1) {
+        QByteArray env = qgetenv("QT_SUBPIXEL_AA_TYPE");
+        if (env == "RGB")
+            type = QPlatformScreen::Subpixel_RGB;
+        else if (env == "BGR")
+            type = QPlatformScreen::Subpixel_BGR;
+        else if (env == "VRGB")
+            type = QPlatformScreen::Subpixel_VRGB;
+        else if (env == "VBGR")
+            type = QPlatformScreen::Subpixel_VBGR;
+        else
+            type = QPlatformScreen::Subpixel_None;
+    }
+
+    return static_cast<QPlatformScreen::SubpixelAntialiasingType>(type);
+}
+
 QT_END_NAMESPACE