Add support for multisampling in QQuickWidget

Use a multisampled fbo when the requested format has samples > 0.
Resolving happens after each rendering of the scene.

The blit to the temporary non-multisampled fbo could be avoided,
in theory, by sending the fbo instead of the texture id down the
stack and performing a blit directly to fbo #0. This however
involves a number of potential issues, for example due to the
non-sharability of FBOs between contexts. Hence it is left as a
future exercise.

Task-number: QTBUG-39187
Change-Id: Iae98b969bcbc3bb57e6d73288496f5428913c826
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>

2 files changed, 28 insertions, 5 deletions

diff --git a/src/quickwidgets/qquickwidget.cpp b/src/quickwidgets/qquickwidget.cpp
index 30c35c55dd..439b039266 100644
--- a/src/quickwidgets/qquickwidget.cpp
+++ b/src/quickwidgets/qquickwidget.cpp
@@ -143,6 +143,7 @@ QQuickWidgetPrivate::QQuickWidgetPrivate()
     , offscreenSurface(0)
     , renderControl(0)
     , fbo(0)
+    , resolvedFbo(0)
     , context(0)
     , resizeMode(QQuickWidget::SizeViewToRootObject)
     , initialSize(0,0)
@@ -163,6 +164,7 @@ QQuickWidgetPrivate::~QQuickWidgetPrivate()
     Q_ASSERT(!context || (QOpenGLContext::currentContext() == context && context->surface() == offscreenSurface));
     delete offscreenWindow;
     delete renderControl;
+    delete resolvedFbo;
     delete fbo;
 
     destroyContext();
@@ -226,6 +228,12 @@ void QQuickWidgetPrivate::renderSceneGraph()
     renderControl->sync();
     renderControl->render();
     glFlush();
+
+    if (resolvedFbo) {
+        QRect rect(QPoint(0, 0), fbo->size());
+        QOpenGLFramebufferObject::blitFramebuffer(resolvedFbo, rect, fbo, rect);
+    }
+
     context->doneCurrent();
     q->update();
 }
@@ -669,11 +677,23 @@ void QQuickWidget::createFramebufferObject()
 
     context->makeCurrent(d->offscreenSurface);
 
+    int samples = d->offscreenWindow->requestedFormat().samples();
+    if (!QOpenGLExtensions(context).hasOpenGLExtension(QOpenGLExtensions::FramebufferMultisample))
+        samples = 0;
+
+    QOpenGLFramebufferObjectFormat format;
+    format.setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
+    format.setSamples(samples);
+
+    QSize fboSize = size() * window()->devicePixelRatio();
+
     delete d->fbo;
-    d->fbo = new QOpenGLFramebufferObject(size() * window()->devicePixelRatio());
-    d->fbo->setAttachment(QOpenGLFramebufferObject::CombinedDepthStencil);
+    d->fbo = new QOpenGLFramebufferObject(fboSize, format);
     d->offscreenWindow->setRenderTarget(d->fbo);
 
+    if (samples > 0)
+        d->resolvedFbo = new QOpenGLFramebufferObject(fboSize);
+
     // Sanity check: The window must not have an underlying platform window.
     // Having one would mean create() was called and platforms that only support
     // a single native window were in trouble.
@@ -683,9 +703,10 @@ void QQuickWidget::createFramebufferObject()
 void QQuickWidget::destroyFramebufferObject()
 {
     Q_D(QQuickWidget);
-    if (d->fbo)
-        delete d->fbo;
+    delete d->fbo;
     d->fbo = 0;
+    delete d->resolvedFbo;
+    d->resolvedFbo = 0;
 }
 
 QQuickWidget::ResizeMode QQuickWidget::resizeMode() const
@@ -770,7 +791,8 @@ GLuint QQuickWidgetPrivate::textureId() const
                    << "Consider setting Qt::AA_DontCreateNativeWidgetSiblings";
         return 0;
     }
-    return fbo ? fbo->texture() : 0;
+    return resolvedFbo ? resolvedFbo->texture()
+        : (fbo ? fbo->texture() : 0);
 }
 
 /*!
diff --git a/src/quickwidgets/qquickwidget_p.h b/src/quickwidgets/qquickwidget_p.h
index d7652432b7..356c34e7f5 100644
--- a/src/quickwidgets/qquickwidget_p.h
+++ b/src/quickwidgets/qquickwidget_p.h
@@ -107,6 +107,7 @@ public:
     QOffscreenSurface *offscreenSurface;
     QQuickRenderControl *renderControl;
     QOpenGLFramebufferObject *fbo;
+    QOpenGLFramebufferObject *resolvedFbo;
     QOpenGLContext *context;
 
     QQuickWidget::ResizeMode resizeMode;