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

1 files changed, 7 insertions, 3 deletions

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"))) {