Fix msaa in QOpenGLWidget

Take the correct number of samples from the widget's context, not the
tlw's context.

The original implementation did the blit only after paintGL(). This is
not sufficient for applications that override paintEvent() and do QPainter
calls in there (e.g. the 2dpainting example). Therefore the approach is
changed to perform the resolving of the samples in a function that is
invoked by QWidgetPrivate every time a paint event is sent.

Change-Id: Iae0b2c30f6070ec75201339a848854e4582a9c0c
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -228,6 +228,18 @@ QT_BEGIN_NAMESPACE
   regarding stacking orders for example. QOpenGLWidget avoids this by not
   creating a separate native window.
 
+  \section1 Multisampling
+
+  To enable multisampling, set the number of requested samples on the
+  QSurfaceFormat that is passed to setFormat(). On systems that do not support
+  it the request may get ignored.
+
+  Multisampling support requires support for multisampled renderbuffers and
+  framebuffer blits. On OpenGL ES 2.0 implementations it is likely that these
+  will not be present. This means that multisampling will not be available. With
+  modern OpenGL versions and OpenGL ES 3.0 and up this is usually not a problem
+  anymore.
+
   \section1 Threading
 
   Performing offscreen rendering on worker threads, for example to generate
@@ -435,6 +447,7 @@ public:
     void beginCompose() Q_DECL_OVERRIDE;
     void endCompose() Q_DECL_OVERRIDE;
     void resizeViewportFramebuffer() Q_DECL_OVERRIDE;
+    void resolveSamples() Q_DECL_OVERRIDE;
 
     QOpenGLContext *context;
     QOpenGLFramebufferObject *fbo;
@@ -491,9 +504,11 @@ void QOpenGLWidgetPrivate::recreateFbo()
     context->makeCurrent(surface);
 
     delete fbo;
+    fbo = 0;
     delete resolvedFbo;
+    resolvedFbo = 0;
 
-    int samples = get(q->window())->shareContext()->format().samples();
+    int samples = context->format().samples();
     QOpenGLExtensions *extfuncs = static_cast<QOpenGLExtensions *>(context->functions());
     if (!extfuncs->hasOpenGLExtension(QOpenGLExtensions::FramebufferMultisample))
         samples = 0;
@@ -570,6 +585,16 @@ void QOpenGLWidgetPrivate::initialize()
     q->initializeGL();
 }
 
+void QOpenGLWidgetPrivate::resolveSamples()
+{
+    Q_Q(QOpenGLWidget);
+    if (resolvedFbo) {
+        q->makeCurrent();
+        QRect rect(QPoint(0, 0), fbo->size());
+        QOpenGLFramebufferObject::blitFramebuffer(resolvedFbo, rect, fbo, rect);
+    }
+}
+
 void QOpenGLWidgetPrivate::invokeUserPaint()
 {
     Q_Q(QOpenGLWidget);
@@ -577,11 +602,6 @@ void QOpenGLWidgetPrivate::invokeUserPaint()
     f->glViewport(0, 0, q->width() * q->devicePixelRatio(), q->height() * q->devicePixelRatio());
 
     q->paintGL();
-
-    if (resolvedFbo) {
-        QRect rect(QPoint(0, 0), fbo->size());
-        QOpenGLFramebufferObject::blitFramebuffer(resolvedFbo, rect, fbo, rect);
-    }
 }
 
 void QOpenGLWidgetPrivate::render()
@@ -605,6 +625,7 @@ QImage QOpenGLWidgetPrivate::grabFramebuffer()
         return QImage();
 
     render();
+    resolveSamples();
     q->makeCurrent();
     QImage res = qt_gl_read_framebuffer(q->size() * q->devicePixelRatio(), false, false);
 

src/widgets/kernel/qwidget.cpp

@@ -5595,6 +5595,11 @@ void QWidgetPrivate::sendPaintEvent(const QRegion &toBePainted)
     Q_Q(QWidget);
     QPaintEvent e(toBePainted);
     QCoreApplication::sendSpontaneousEvent(q, &e);
+
+#ifndef QT_NO_OPENGL
+    if (renderToTexture)
+        resolveSamples();
+#endif // QT_NO_OPENGL
 }
 
 void QWidgetPrivate::render(QPaintDevice *target, const QPoint &targetOffset,

src/widgets/kernel/qwidget_p.h

@@ -653,6 +653,8 @@ public:
     // filtered away from the widget. This is fine for QGLWidget but bad for QOpenGLWidget
     // since the fbo must be resized. We need an alternative way to notify.
     virtual void resizeViewportFramebuffer() { }
+    // Called after each paint event.
+    virtual void resolveSamples() { }
 #endif
 
     // Variables.