linuxfb: Support transparency

Always clearing to opaque black makes it impossible to create
semi-transparent overlays with linuxfb. Instead, behave like other
platforms' backingstores: if the target image has an alpha channel,
clear to transparent instead and set the correct composition mode.

Task-number: QTBUG-52475
Change-Id: I2db4588e0112e200a3aa5eed49f806c37d7ca8b5
Reviewed-by: Louai Al-Khanji <louai.al-khanji@qt.io>

1 files changed, 10 insertions, 1 deletions

diff --git a/src/platformsupport/fbconvenience/qfbbackingstore.cpp b/src/platformsupport/fbconvenience/qfbbackingstore.cpp
index 2800a81507..5eddf71145 100644
--- a/src/platformsupport/fbconvenience/qfbbackingstore.cpp
+++ b/src/platformsupport/fbconvenience/qfbbackingstore.cpp
@@ -37,6 +37,7 @@
 
 #include <qpa/qplatformwindow.h>
 #include <QtGui/qscreen.h>
+#include <QtGui/qpainter.h>
 
 QT_BEGIN_NAMESPACE
 
@@ -84,9 +85,17 @@ void QFbBackingStore::unlock()
     mImageMutex.unlock();
 }
 
-void QFbBackingStore::beginPaint(const QRegion &)
+void QFbBackingStore::beginPaint(const QRegion &region)
 {
     lock();
+
+    if (mImage.hasAlphaChannel()) {
+        QPainter p(&mImage);
+        p.setCompositionMode(QPainter::CompositionMode_Source);
+        const QVector<QRect> rects = region.rects();
+        for (QVector<QRect>::const_iterator it = rects.begin(); it != rects.end(); ++it)
+            p.fillRect(*it, Qt::transparent);
+    }
 }
 
 void QFbBackingStore::endPaint()