how to make updateGL() realtime in QT

Question

I wonder if calling updateGL() in fixed timer intervals may slow down the rendering process. So, I want to try making the render real time. I do not the function to make it execute automatically. Anyone knows that?

secondly, I commented updateGL() in my program, CPU usage reduced to 1% and 0%. Now, when I click on the display widget it jumps to 14%, why? isn't GPU doing all the rendering job? if yes why CPU jumps to 14? how can I resolve this?

Answer 1

Yes, you can make it realtime, with no tearing (e.g., redrawing at exactly 60fps, the refresh rate of your monitor).

For this, you have to enable V_sync, and using a QTimer timer; of interval 0. Enabling V_sync will make the automatically called swapBuffers() command to CPU-wait for the vertical refresh signal from your monitor. Then, the timer will actually be synchronized with the monitor refresh rate.

A related info can be found here: https://www.qt.io/blog/2010/12/02/velvet-and-the-qml-scene-graph. Note the QGLFormat::setSwapInterval(1) to enable V_sync, if not done automatically by your driver settings:

class MyGLWidget: public QGLWidget
{
    // ...
private:
    QTimer timer;
}

QGLFormat desiredFormat()
{
    QGLFormat fmt;
    fmt.setSwapInterval(1);
    return fmt;
}

MyGLWidget::MyGLWidget() :
    QGLWidget(desiredFormat())
{
    // Configure the timer
    connect(&timer, SIGNAL(timeout()), this, SLOT(updateGL()));
    if(format().swapInterval() == -1)
    {
        // V_blank synchronization not available (tearing likely to happen)
        qDebug("Swap Buffers at v_blank not available: refresh at approx 60fps.");
        timer.setInterval(17);
    }
    else
    {
        // V_blank synchronization available
        timer.setInterval(0);
    }
    timer.start();
}

In parallel, you can run a QElapsedTimer to measure how much time has passed between two drawing (normally approx 16.6ms), and use this information to update your scene, for instance.