How to force QGLWidget to update screen?

Question

I’m drawing a simple scene with Open GL. I’ve subclassed QGLWidget and overriden paintGL(). Nothing fancy there:

void CGLWidget::paintGL()
 {
    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
    glClear(GL_COLOR_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();

    gluLookAt (120.0, 160.0, -300.0, 0.0 + 120.0, 0.0 + 160.0, 2.0 - 300.0, 0.0, 1.0, 0.0);
    glScalef(1.0f/300.0f, 1.0f/300.0f, 1.0f/300.0f);
    glClear(GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(80.0, width()/(double)height(), 5.0, 100000.0);

    glMatrixMode(GL_MODELVIEW);
    glBegin(GL_POINTS);
    glColor3f(1.0f,0.6f,0.0f);
    glVertex3d(x, y, z);

    // ...drawing some more points...

    glEnd();

 }

I have a timer in main window which triggers updateGL() of GL widget. I’ve verified that it results in paintGL() being called. However, the actual picture on the screen is only updated very rarely. Even if I resize the window, scene is not updated. Why is that and how can I force it to update?

Solution 1

Don't call updateGL() from your timer, call update() instead to ensure that the view gets a paint event.

Solution 2

inside your CGLWidget constructor

QTimer *timer = new QTimer(this);
connect(timer, SIGNAL(timeout()), this, SLOT(update()));
timer->start(10);

timer -> start(VALUE);

play with `VALUE 10 is just an example.

Adorn

Comments

[Violet Giraffe]

It doesn't work, paintGL() is not called at all then. Tried calling both, too - also doesn't work. But thanks for the hint.

[Pete]

Try sticking a breakpoint in QGLWidget::paintEvent and see if it gets there after calling update() and what happens after that.

[Violet Giraffe]

It's only called when I resize the window.

[Pete]

Have you set any window flags in the constructor of your view? update SHOULD cause you to get a paint event at some point in the near future after calling it, providing you are letting the event loop run correctly and you have not disabled updates on the widget. Not getting a paint event is your problem.

[Violet Giraffe]

Thanks a lot, found the problem! Was actually just a logical problem on my part, but your answers helped :)

[activatedgeek]

Hey, I have used this technique it works. Is there somehow I can immediately update the scene as soon as the previous scene is rendered because right now I have a bottleneck of 10ms from the timer. I can't go faster than that.

[Adorn]

play with `VALUE 10 is just an example. if u put 1 there, it would be 1ms, point is not however, when you want to refresh the screen, point is when your scene is ready..

[activatedgeek]

Exactly what I said. As soon as my scene is ready, I want update to be triggered.

[Adorn]

then make sure if you really need a timer. also timer(0) will shoot out the timer immediately

[activatedgeek]

Well, this is what I need to know, how do I remove the timer from the application itself. I would prefer not using it.

[BjkOcean]

What was the problem actually? I am having a similar problem and I couldn't figure it out. Thanks in advance.