QOpenGLWidget: Call glClear in the default paintGL implementation

Amends 8cc8bbb466fb871585bfa1d5c56fc8bc6d6c8a96.

With the referenced patch framebuffer invalidation is preferred
over clearing with a wider range of OpenGL implementations.
This is good and is in line with the documentation. If an existing
application does not do glClear() in their reimplemented paintGL(),
that we cannot fix, and the docs never actually promised that a
glClear() is done automatically.

However, there is a special case: when one does not reimplement
paintGL(). A QOpenGLWidget is expected to be usable without
reimplementing any of its virtuals (i.e., QOpenGLWidget is
constructible as-is without subclassing). It is then ideal
if this does not lead to showing rendering artifacts due to
having potentially random content in the (invalidated,
not cleared) color buffer. This problem was never encountered
before due to hitting the invalidate-instead-of-clear code path
on a limited set of platforms (like with OpenGL ES) only.

Do a glClear() in paintGL(). This won't effect the vast majority
of applications, that reimplement this function, but plays nice
for when it is not.

It is in fact also in line with Qt 4's QGLWidget: that also used to
do a glClear() in the default paintGL() implementation.

Change-Id: I117ce1d3340f6d4d82cde1f52769a3b13ce367fb
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/openglwidgets/qopenglwidget.cpp b/src/openglwidgets/qopenglwidget.cpp
index b85e699fa4..e4438d226d 100644
--- a/src/openglwidgets/qopenglwidget.cpp
+++ b/src/openglwidgets/qopenglwidget.cpp
@@ -1457,6 +1457,10 @@ void QOpenGLWidget::resizeGL(int w, int h)
   other state is set and no clearing or drawing is performed by the
   framework.
 
+  The default implementation performs a glClear(). Subclasses are not expected
+  to invoke the base class implementation and should perform clearing on their
+  own.
+
   \note To ensure portability, do not expect that state set in initializeGL()
   persists. Rather, set all necessary state, for example, by calling
   glEnable(), in paintGL(). This is because some platforms, such as WebAssembly
@@ -1476,6 +1480,9 @@ void QOpenGLWidget::resizeGL(int w, int h)
 */
 void QOpenGLWidget::paintGL()
 {
+    Q_D(QOpenGLWidget);
+    if (d->initialized)
+        d->context->functions()->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
 }
 
 /*!