5 files changed, 39 insertions, 2 deletions
diff --git a/examples/quick/window/main.cpp b/examples/quick/window/main.cpp
index 2223c607b4..7872c7183c 100644
--- a/examples/quick/window/main.cpp
+++ b/examples/quick/window/main.cpp
@@ -41,6 +41,7 @@
 #include <QtGui/QGuiApplication>
 #include <QtQml/QQmlEngine>
 #include <QtQml/QQmlComponent>
+#include <QtQuick/QQuickWindow>
 #include <QtCore/QUrl>
 #include <QDebug>
 
@@ -49,6 +50,7 @@ int main(int argc, char* argv[])
     QGuiApplication app(argc, argv);
     QQmlEngine engine;
     QQmlComponent component(&engine);
+    QQuickWindow::setDefaultAlphaBuffer(true);
     component.loadUrl(QUrl("qrc:///window/window.qml"));
     if ( component.isReady() )
         component.create();
diff --git a/src/quick/items/qquickwindow.cpp b/src/quick/items/qquickwindow.cpp
index 4a34adabe8..7137bb165e 100644
--- a/src/quick/items/qquickwindow.cpp
+++ b/src/quick/items/qquickwindow.cpp
@@ -74,6 +74,8 @@ QT_BEGIN_NAMESPACE
 
 extern Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format, bool include_alpha);
 
+bool QQuickWindowPrivate::defaultAlphaBuffer(0);
+
 void QQuickWindowPrivate::updateFocusItemTransform()
 {
     Q_Q(QQuickWindow);
@@ -2969,7 +2971,7 @@ QSGTexture *QQuickWindow::createTextureFromId(uint id, const QSize &size, Create
     Setting the clear color has no effect when clearing is disabled.
     By default, the clear color is white.
 
-    \sa setClearBeforeRendering()
+    \sa setClearBeforeRendering(), setDefaultAlphaBuffer()
  */
 
 void QQuickWindow::setColor(const QColor &color)
@@ -2979,7 +2981,7 @@ void QQuickWindow::setColor(const QColor &color)
         return;
 
     if (color.alpha() != d->clearColor.alpha()) {
-        QSurfaceFormat fmt = format();
+        QSurfaceFormat fmt = requestedFormat();
         if (color.alpha() < 255)
             fmt.setAlphaBufferSize(8);
         else
@@ -2997,6 +2999,31 @@ QColor QQuickWindow::color() const
 }
 
 /*!
+    \brief Returns whether to use alpha transparency on newly created windows.
+
+    \since Qt 5.1
+    \sa setDefaultAlphaBuffer()
+ */
+bool QQuickWindow::hasDefaultAlphaBuffer()
+{
+    return QQuickWindowPrivate::defaultAlphaBuffer;
+}
+
+/*!
+    \brief \a useAlpha specifies whether to use alpha transparency on newly created windows.
+    \since Qt 5.1
+
+    In any application which expects to create translucent windows, it's
+    necessary to set this to true before creating the first QQuickWindow,
+    because all windows will share the same \l QOpenGLContext. The default
+    value is false.
+ */
+void QQuickWindow::setDefaultAlphaBuffer(bool useAlpha)
+{
+    QQuickWindowPrivate::defaultAlphaBuffer = useAlpha;
+}
+
+/*!
     \qmlproperty string QtQuick.Window2::Window::title
 
     The window's title in the windowing system.
diff --git a/src/quick/items/qquickwindow.h b/src/quick/items/qquickwindow.h
index fc148aabe6..a0bc832334 100644
--- a/src/quick/items/qquickwindow.h
+++ b/src/quick/items/qquickwindow.h
@@ -115,6 +115,9 @@ public:
     void setColor(const QColor &color);
     QColor color() const;
 
+    static bool hasDefaultAlphaBuffer();
+    static void setDefaultAlphaBuffer(bool useAlpha);
+
     void setPersistentOpenGLContext(bool persistent);
     bool isPersistentOpenGLContext() const;
 
diff --git a/src/quick/items/qquickwindow_p.h b/src/quick/items/qquickwindow_p.h
index 9e3251b240..2465629778 100644
--- a/src/quick/items/qquickwindow_p.h
+++ b/src/quick/items/qquickwindow_p.h
@@ -219,6 +219,8 @@ public:
 
     mutable QQuickWindowIncubationController *incubationController;
 
+    static bool defaultAlphaBuffer;
+
     static bool dragOverThreshold(qreal d, Qt::Axis axis, QMouseEvent *event);
 
     // data property
diff --git a/src/quick/scenegraph/qsgcontext.cpp b/src/quick/scenegraph/qsgcontext.cpp
index d3710c7bd5..4ebae38699 100644
--- a/src/quick/scenegraph/qsgcontext.cpp
+++ b/src/quick/scenegraph/qsgcontext.cpp
@@ -54,6 +54,7 @@
 
 #include <QGuiApplication>
 #include <QOpenGLContext>
+#include <QQuickWindow>
 #include <QtGui/qopenglframebufferobject.h>
 
 #include <private/qqmlglobal_p.h>
@@ -398,6 +399,8 @@ QSurfaceFormat QSGContext::defaultSurfaceFormat() const
     QSurfaceFormat format;
     format.setDepthBufferSize(24);
     format.setStencilBufferSize(8);
+    if (QQuickWindow::hasDefaultAlphaBuffer())
+        format.setAlphaBufferSize(8);
     format.setSwapBehavior(QSurfaceFormat::DoubleBuffer);
     return format;
 }