QGLWidget on ubuntu shows nothing - qt

Question

I'm using QGLWidget in a project with Qt5 on ubuntu. The class that extends QGLWidget is very simple, just overiding its two virtual protected functions:
Header:
class MyGLWidget : public QGLWidget {
Q_OBJECT
public:
explicit MyGLWidget(QWidget *parent = 0);
protected:
void initializeGL();
void paintGL();
};
Implementation:
MyGLWidget::MyGLWidget(QWidget *parent) :
QGLWidget(parent) {
}
void MyGLWidget::initializeGL() {
glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
}
void MyGLWidget::paintGL() {
glClear(GL_COLOR_BUFFER_BIT);
}
I created an instance of it and called its show method, expecting a red window to show up, but what I got is:
The window is transparent, it displays whatever underneath it.
What indeed the problem is, and how can I fix it? (I'm using ubuntu 14.04 64bit, the Qt5.3 is installed from qt-project.org directly)

Answer 1

I found the same problem on the same version of Ubuntu and Qt 5.4. I do not know the exact reason but I think it's because the glClear command is not being received or executed by the graphics card unless it is followed by other draw commands. I expanded the paintGL function to include some classic test code like this:
void MyGLWidget::paintGL()
{
glViewport(0, 0, width(), height());
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-20, 20, 20, -20, -1, 1);
glColor3f(1.0f, 0.0f, 0.0f);
glBegin(GL_TRIANGLES);
glVertex3f(10, 10, 0);
glVertex3f(-10, 10, 0);
glVertex3f(0, -10, 0);
glEnd();
}
With the additional code to render a red triangle, I found that the clear began to work for me when it had not before.