Add flag for forcibly propagating backing store alpha to framebuffer

Previously, the following rules applies to the creation and blitting of
the Symbian raster backing store:

    1. Creation of backing store with an alpha channel:
	   Backing store has an alpha channel only if !QWidget::isOpaque.

	2. Pre-filling of backing store prior to paint loop:
	   Backing store is filled with transparent pixels if
	   !QWidget::isOpaque.

	3. Blitting of backing store:
	   CGraphicsContext::EDrawModeWriteAlpha is used (meaning that backing
	   store alpha values are propagated into the frame buffer), if
	   QWidget::isOpaque.

In order for the direct camera viewfinder to be visible on DSA devices,
alpha=0 must be written into the framebuffer in the region where the
viewfinder will be displayed.  This requires a backing store with an alpha
channel (1), use of CGraphicsContext::EDrawModeWriteAlpha (3), but not
pre-filling of the entire backing store (2).

This patch adds a new enum value, QWExtra::BlitWriteAlpha,  which can be
used by camera backends to achieve the desired behaviour.

Task-number: QTMOBILITY-1278
Reviewed-by: Jani Hautakangas

4 files changed, 33 insertions, 13 deletions

diff --git a/src/gui/kernel/qapplication_s60.cpp b/src/gui/kernel/qapplication_s60.cpp
index 43b9b01..2b10d63 100644
--- a/src/gui/kernel/qapplication_s60.cpp
+++ b/src/gui/kernel/qapplication_s60.cpp
@@ -1132,7 +1132,8 @@ void QSymbianControl::Draw(const TRect& controlRect) const
             // Do nothing
             break;
         case QWExtra::Blit:
-            if (qwidget->d_func()->isOpaque)
+        case QWExtra::BlitWriteAlpha:
+            if (qwidget->d_func()->isOpaque || nativePaintMode == QWExtra::BlitWriteAlpha)
                 gc.SetDrawMode(CGraphicsContext::EDrawModeWriteAlpha);
             gc.BitBlt(controlRect.iTl, bitmap, backingStoreRect);
             break;
diff --git a/src/gui/kernel/qwidget_p.h b/src/gui/kernel/qwidget_p.h
index 5235dc4..13e2349 100644
--- a/src/gui/kernel/qwidget_p.h
+++ b/src/gui/kernel/qwidget_p.h
@@ -315,6 +315,11 @@ struct QWExtra {
          */
         ZeroFill,
 
+        /**
+         * Blit backing store, propagating alpha channel into the framebuffer.
+         */
+        BlitWriteAlpha,
+
         Default = Blit
     };
 
diff --git a/src/gui/kernel/qwidget_s60.cpp b/src/gui/kernel/qwidget_s60.cpp
index 62d09fe..b65ae4d 100644
--- a/src/gui/kernel/qwidget_s60.cpp
+++ b/src/gui/kernel/qwidget_s60.cpp
@@ -805,9 +805,14 @@ void QWidgetPrivate::s60UpdateIsOpaque()
 {
     Q_Q(QWidget);
 
-    if (!q->testAttribute(Qt::WA_WState_Created) || !q->testAttribute(Qt::WA_TranslucentBackground))
+    if (!q->testAttribute(Qt::WA_WState_Created))
         return;
 
+    const bool writeAlpha = extraData()->nativePaintMode == QWExtra::BlitWriteAlpha;
+    if (!q->testAttribute(Qt::WA_TranslucentBackground) && !writeAlpha)
+        return;
+    const bool requireAlphaChannel = !isOpaque || writeAlpha;
+
     createTLExtra();
 
     RWindow *const window = static_cast<RWindow *>(q->effectiveWinId()->DrawableWindow());
@@ -819,12 +824,11 @@ void QWidgetPrivate::s60UpdateIsOpaque()
         return;
     }
 #endif
-    if (!isOpaque) {
+    if (requireAlphaChannel) {
         const TDisplayMode displayMode = static_cast<TDisplayMode>(window->SetRequiredDisplayMode(EColor16MA));
         if (window->SetTransparencyAlphaChannel() == KErrNone) {
             window->SetBackgroundColor(TRgb(255, 255, 255, 0));
             extra->topextra->nativeWindowTransparencyEnabled = 1;
-
             if (extra->topextra->backingStore.data() && (
                     QApplicationPrivate::graphics_system_name == QLatin1String("openvg")
                     || QApplicationPrivate::graphics_system_name == QLatin1String("opengl"))) {
diff --git a/src/gui/painting/qwindowsurface_s60.cpp b/src/gui/painting/qwindowsurface_s60.cpp
index 9f371a8..cb53ea0 100644
--- a/src/gui/painting/qwindowsurface_s60.cpp
+++ b/src/gui/painting/qwindowsurface_s60.cpp
@@ -63,7 +63,6 @@ struct QS60WindowSurfacePrivate
 
 TDisplayMode displayMode(bool opaque)
 {
-
     TDisplayMode mode = S60->screenDevice()->DisplayMode();
     if (opaque) {
         mode = EColor16MU;
@@ -76,10 +75,18 @@ TDisplayMode displayMode(bool opaque)
     return mode;
 }
 
+bool blitWriteAlpha(QWidgetPrivate *widgetPrivate)
+{
+    QWExtra *extra = widgetPrivate->extraData();
+    return extra ? extra->nativePaintMode == QWExtra::BlitWriteAlpha : false;
+}
+
 QS60WindowSurface::QS60WindowSurface(QWidget* widget)
     : QWindowSurface(widget), d_ptr(new QS60WindowSurfacePrivate)
 {
-    TDisplayMode mode = displayMode(qt_widget_private(widget)->isOpaque);
+    QWidgetPrivate *widgetPrivate = qt_widget_private(widget);
+    const bool opaque = widgetPrivate->isOpaque && !blitWriteAlpha(widgetPrivate);
+    TDisplayMode mode = displayMode(opaque);
     // We create empty CFbsBitmap here -> it will be resized in setGeometry
     CFbsBitmap *bitmap = q_check_ptr(new CFbsBitmap);	// CBase derived object needs check on new
     qt_symbian_throwIfError( bitmap->Create( TSize(0, 0), mode ) );
@@ -123,7 +130,8 @@ void QS60WindowSurface::beginPaint(const QRegion &rgn)
     S60->wsSession().Finish();
 #endif
 
-    if (!qt_widget_private(window())->isOpaque) {
+    QWidgetPrivate *windowPrivate = qt_widget_private(window());
+    if (!windowPrivate->isOpaque || blitWriteAlpha(windowPrivate)) {
         QS60PixmapData *pixmapData = static_cast<QS60PixmapData *>(d_ptr->device.data_ptr().data());
 
         TDisplayMode mode = displayMode(false);
@@ -132,12 +140,14 @@ void QS60WindowSurface::beginPaint(const QRegion &rgn)
 
         pixmapData->beginDataAccess();
 
-        QPainter p(&pixmapData->image);
-        p.setCompositionMode(QPainter::CompositionMode_Source);
-        const QVector<QRect> rects = rgn.rects();
-        const QColor blank = Qt::transparent;
-        for (QVector<QRect>::const_iterator it = rects.begin(); it != rects.end(); ++it) {
-            p.fillRect(*it, blank);
+        if (!windowPrivate->isOpaque) {
+            QPainter p(&pixmapData->image);
+            p.setCompositionMode(QPainter::CompositionMode_Source);
+            const QVector<QRect> rects = rgn.rects();
+            const QColor blank = Qt::transparent;
+            for (QVector<QRect>::const_iterator it = rects.begin(); it != rects.end(); ++it) {
+                p.fillRect(*it, blank);
+            }
         }
 
         pixmapData->endDataAccess();