Fast Timer Updates vs. Monitor Refresh Rates

[wysota]

It may sound as a dumb question but why do you want to update the widget every 6ms? This gives about 167 frames per second, I can hardly think of a reason why anyone would want so many ui updates per second. Especially with a 60Hz display. I can't even think of a hardware that would be capable of doing as many updates and still be able to provide some data to update the ui with.

[artoonie]

It may sound as a dumb question but why do you want to update the widget every 6ms? This gives about 167 frames per second, I can hardly think of a reason why anyone would want so many ui updates per second. Especially with a 60Hz display. I can't even think of a hardware that would be capable of doing as many updates and still be able to provide some data to update the ui with.

Haha, silly typo, my bad-
I meant 16ms, or more precisely, 16.666...ms.
I need 60Hz, meaning refresh every time the screen refreshes.

[wysota]

Ok, so how exactly do you force your widget to redraw?

[artoonie]

I'm using a QTimer...is there another way, maybe blocking until a vertical sync event? I've tried using swapBuffers but I can't get it to work without a timer.

[wysota]

I'm using a QTimer...

Show us the code.

[artoonie]

I'm using a QTimer...QUOTE]
Show us the code.

It's been established that timers are not the way to go, but I'll paste what I have:

Flickerer::Flickerer(int timerInterval, QWidget *parent) {
        m_timer = new QTimer( this );
        connect( m_timer, SIGNAL(timeout()), this, SLOT(timeOutSlot()) );
        m_timer->start( timerInterval );
}
void Flickerer::timeOutSlot()
{
    showingG1 = !showingG1;
    updateGL();
}
void Flickerer::paintGL()
{
    if(showingG1) /* paints gradient 1 */ else  /* paints gradient 2 */
}

Flickerer::Flickerer(int timerInterval, QWidget *parent) {
        m_timer = new QTimer( this );
        connect( m_timer, SIGNAL(timeout()), this, SLOT(timeOutSlot()) );
        m_timer->start( timerInterval );
}
void Flickerer::timeOutSlot()
{
    showingG1 = !showingG1;
    updateGL();
}
void Flickerer::paintGL()
{
    if(showingG1) /* paints gradient 1 */ else  /* paints gradient 2 */
}

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QTimer can be set in whole milliseconds only. Approximating a 1/60 seconds rate with 16 msecs will lead a whole frame difference in time after 24 or 25 frames even if the timer events were perfectly spaced. A 17 msec approximation will extend the sync out to around 48-50 frames. QTimer events are processed when the program returns to the event loop after the time has expired. Depending on what else the program is doing, like rendering your gradient, this might be a significant delay. The combination of the two will lead to a mismatch that is variable with activity.

MythTV can use timers to maintain video refresh sync, but I believe it also has extensive frame counting over relatively long periods to determine the average frame rate and drops/duplicates frames to realign things periodically. The preferred method in MythTV is to use the OpenGL vsync. I don't know how this can be accessed.

I agree- I need to use OpenGL vsync but I don't know how to get a signal from the display so I'd know when to update.



Thanks you.

[tbscope]

You do not need to get the signal from the display. OpenGl has a function that does that for you.

What I would try to do:
Create a graphicsview that contains the user interface. Make it so that everything inside the graphicsview is painted via OpenGl
There are lots of tutorials in the Qt labs and documentation.
And then use the information from the blog post about blocking the eventloop and synchronising the eventloop to the refresh rate of the monitor.

If you use a graphicsview and opengl, the effects possible with your user interface are infinite, although limited to the contents of the graphicsview.

Edit:
Here's a nice example:
http://labs.qt.nokia.com/2008/06/27/...s-with-opengl/

Edit 2:
You need this function:
http://doc.qt.nokia.com/latest/qglfo...etSwapInterval
Set it to 1

Tip:
QML does this out of the box. It's worth studying the code.