1 files changed, 66 insertions, 4 deletions

diff --git a/src/widgets/kernel/qopenglwidget.cpp b/src/widgets/kernel/qopenglwidget.cpp
index 7782c4c1d42..dd8dd644704 100644
--- a/src/widgets/kernel/qopenglwidget.cpp
+++ b/src/widgets/kernel/qopenglwidget.cpp
@@ -239,6 +239,28 @@ QT_BEGIN_NAMESPACE
   \note Avoid calling winId() on a QOpenGLWidget. This function triggers the creation of
   a native window, resulting in reduced performance and possibly rendering glitches.
 
+  \section1 Differences to QGLWidget
+
+  Besides the main conceptual difference of being backed by a framebuffer object, there
+  are a number of smaller, internal differences between QOpenGLWidget and the older
+  QGLWidget:
+
+  \list
+
+  \li OpenGL state when invoking paintGL(). QOpenGLWidget sets up the viewport via
+  glViewport(). It does not perform any clearing.
+
+  \li Clearing when starting to paint via QPainter. Unlike regular widgets, QGLWidget
+  defaulted to a value of \c true for
+  \l{QWidget::autoFillBackground()}{autoFillBackground}. It then performed clearing to the
+  palette's background color every time QPainter::begin() was used. QOpenGLWidget does not
+  follow this: \l{QWidget::autoFillBackground()}{autoFillBackground} defaults to false,
+  like for any other widget. The only exception is when being used as a viewport for other
+  widgets like QGraphicsView. In such a case autoFillBackground will be automatically set
+  to true to ensure compatibility with QGLWidget-based viewports.
+
+  \endlist
+
   \section1 Multisampling
 
   To enable multisampling, set the number of requested samples on the
@@ -436,6 +458,7 @@ class QOpenGLWidgetPaintDevice : public QOpenGLPaintDevice
 {
 public:
     QOpenGLWidgetPaintDevice(QOpenGLWidget *widget) : w(widget) { }
+    void beginPaint() Q_DECL_OVERRIDE;
     void ensureActiveTarget() Q_DECL_OVERRIDE;
 
 private:
@@ -454,7 +477,8 @@ public:
           initialized(false),
           fakeHidden(false),
           paintDevice(0),
-          inBackingStorePaint(false)
+          inBackingStorePaint(false),
+          flushPending(false)
     {
         requestedFormat = QSurfaceFormat::defaultFormat();
     }
@@ -478,6 +502,7 @@ public:
     void endBackingStorePainting() Q_DECL_OVERRIDE { inBackingStorePaint = false; }
     void beginCompose() Q_DECL_OVERRIDE;
     void endCompose() Q_DECL_OVERRIDE;
+    void initializeViewportFramebuffer() Q_DECL_OVERRIDE;
     void resizeViewportFramebuffer() Q_DECL_OVERRIDE;
     void resolveSamples() Q_DECL_OVERRIDE;
 
@@ -490,8 +515,28 @@ public:
     QOpenGLPaintDevice *paintDevice;
     bool inBackingStorePaint;
     QSurfaceFormat requestedFormat;
+    bool flushPending;
 };
 
+void QOpenGLWidgetPaintDevice::beginPaint()
+{
+    // NB! autoFillBackground is and must be false by default. Otherwise we would clear on
+    // every QPainter begin() which is not desirable. This is only for legacy use cases,
+    // like using QOpenGLWidget as the viewport of a graphics view, that expect clearing
+    // with the palette's background color.
+    if (w->autoFillBackground()) {
+        QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
+        if (w->testAttribute(Qt::WA_TranslucentBackground)) {
+            f->glClearColor(0, 0, 0, 0);
+        } else {
+            QColor c = w->palette().brush(w->backgroundRole()).color();
+            float alpha = c.alphaF();
+            f->glClearColor(c.redF() * alpha, c.greenF() * alpha, c.blueF() * alpha, alpha);
+        }
+        f->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
+    }
+}
+
 void QOpenGLWidgetPaintDevice::ensureActiveTarget()
 {
     QOpenGLWidgetPrivate *d = static_cast<QOpenGLWidgetPrivate *>(QWidgetPrivate::get(w));
@@ -502,6 +547,11 @@ void QOpenGLWidgetPaintDevice::ensureActiveTarget()
         w->makeCurrent();
     else
         d->fbo->bind();
+
+    // When used as a viewport, drawing is done via opening a QPainter on the widget
+    // without going through paintEvent(). We will have to make sure a glFlush() is done
+    // before the texture is accessed also in this case.
+    d->flushPending = true;
 }
 
 GLuint QOpenGLWidgetPrivate::textureId() const
@@ -572,6 +622,11 @@ void QOpenGLWidgetPrivate::recreateFbo()
 void QOpenGLWidgetPrivate::beginCompose()
 {
     Q_Q(QOpenGLWidget);
+    if (flushPending) {
+        flushPending = false;
+        q->makeCurrent();
+        context->functions()->glFlush();
+    }
     emit q->aboutToCompose();
 }
 
@@ -653,9 +708,10 @@ void QOpenGLWidgetPrivate::invokeUserPaint()
 {
     Q_Q(QOpenGLWidget);
     QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
-    f->glViewport(0, 0, q->width() * q->devicePixelRatio(), q->height() * q->devicePixelRatio());
 
+    f->glViewport(0, 0, q->width() * q->devicePixelRatio(), q->height() * q->devicePixelRatio());
     q->paintGL();
+    f->glFlush();
 }
 
 void QOpenGLWidgetPrivate::render()
@@ -667,7 +723,6 @@ void QOpenGLWidgetPrivate::render()
 
     q->makeCurrent();
     invokeUserPaint();
-    context->functions()->glFlush();
 }
 
 extern Q_GUI_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format, bool include_alpha);
@@ -686,6 +741,14 @@ QImage QOpenGLWidgetPrivate::grabFramebuffer()
     return res;
 }
 
+void QOpenGLWidgetPrivate::initializeViewportFramebuffer()
+{
+    Q_Q(QOpenGLWidget);
+    // Legacy behavior for compatibility with QGLWidget when used as a graphics view
+    // viewport: enable clearing on each painter begin.
+    q->setAutoFillBackground(true);
+}
+
 void QOpenGLWidgetPrivate::resizeViewportFramebuffer()
 {
     Q_Q(QOpenGLWidget);
@@ -929,7 +992,6 @@ void QOpenGLWidget::resizeEvent(QResizeEvent *e)
     d->recreateFbo();
     resizeGL(width(), height());
     d->invokeUserPaint();
-    d->context->functions()->glFlush();
     d->resolveSamples();
 }