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>

3 files changed, 19 insertions, 4 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()
diff --git a/src/platformsupport/fbconvenience/qfbscreen.cpp b/src/platformsupport/fbconvenience/qfbscreen.cpp
index d00954375d..ad4c62f4a6 100644
--- a/src/platformsupport/fbconvenience/qfbscreen.cpp
+++ b/src/platformsupport/fbconvenience/qfbscreen.cpp
@@ -233,6 +233,7 @@ QRegion QFbScreen::doRedraw()
 
     if (!mCompositePainter)
         mCompositePainter = new QPainter(mScreenImage);
+
     for (int rectIndex = 0; rectIndex < mRepaintRegion.rectCount(); rectIndex++) {
         QRegion rectRegion = rects[rectIndex];
 
@@ -250,7 +251,8 @@ QRegion QFbScreen::doRedraw()
             foreach (const QRect &rect, intersect.rects()) {
                 bool firstLayer = true;
                 if (layer == -1) {
-                    mCompositePainter->fillRect(rect, Qt::black);
+                    mCompositePainter->setCompositionMode(QPainter::CompositionMode_Source);
+                    mCompositePainter->fillRect(rect, mScreenImage->hasAlphaChannel() ? Qt::transparent : Qt::black);
                     firstLayer = false;
                     layer = mWindowStack.size() - 1;
                 }
@@ -283,6 +285,7 @@ QRegion QFbScreen::doRedraw()
 
     QRect cursorRect;
     if (mCursor && (mCursor->isDirty() || mRepaintRegion.intersects(mCursor->lastPainted()))) {
+        mCompositePainter->setCompositionMode(QPainter::CompositionMode_SourceOver);
         cursorRect = mCursor->drawCursor(*mCompositePainter);
         touchedRegion += cursorRect;
     }
diff --git a/src/plugins/platforms/linuxfb/qlinuxfbscreen.cpp b/src/plugins/platforms/linuxfb/qlinuxfbscreen.cpp
index 8c3e73fd80..13e06ef104 100644
--- a/src/plugins/platforms/linuxfb/qlinuxfbscreen.cpp
+++ b/src/plugins/platforms/linuxfb/qlinuxfbscreen.cpp
@@ -405,9 +405,12 @@ QRegion QLinuxFbScreen::doRedraw()
     if (!mBlitter)
         mBlitter = new QPainter(&mFbScreenImage);
 
-    QVector<QRect> rects = touched.rects();
-    for (int i = 0; i < rects.size(); i++)
+    const QVector<QRect> rects = touched.rects();
+    mBlitter->setCompositionMode(QPainter::CompositionMode_Source);
+
+    for (int i = 0; i < rects.size(); ++i)
         mBlitter->drawImage(rects[i], *mScreenImage, rects[i]);
+
     return touched;
 }