frame rate and QTimer accuracy

[JohannesMunk]

Hi High_flyer!

You are absolutely right! The setting for the high resolution timer is done only by the timeBeginPeriod call.

And the privilege adjustment is not required for it! I didn't realize! It was only in context of an NTP-Clock sync, that this made sense.

But as I mentioned none of it is required to have a 1 ms QTimer work correctly. Only if you want QThread::sleep to work accurately you will need to set the timer resolution yourself.
I just mentioned it, because I had trouble with it in context of high precision timing.

Joh

[high_flyer]

But as I mentioned none of it is required to have a 1 ms QTimer work correctly. Only if you want QThread::sleep to work accurately you will need to set the timer resolution yourself.

So just to see if I understand what you are saying is that for having QThread::sleep() work accurately, one must set the windows timer resolution (per process) via timeBeginPeriod() + timeEndPeriod(), is that correct?

[JohannesMunk]

Yes, this is correct for Windows. I experienced that on WinXPSP3,Win7-32 and 64bit. The timer resolution equals the kernels schedulers granularity so unless you set it to 1ms you can't get accurate sleep intervals.

To prove it, I hacked together a small test application:

It runs msleep(1) 100x times. As the default timer resolution of windows is 10ms, you get about 1sec for 100 calls to msleep(1):

999 
998
BeginPeriod!
110
94
110
EndPeriod!
998
1014

999 
998
BeginPeriod!
110
94
110
EndPeriod!
998
1014

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Before you run it, make sure all other processes that might request a higher timer resolution are closed. This includes winamp, your browser (flash plugin!), ..
You can then check the current timer resolution with sysinternals clockres.

Here is the sourcecode:

main.h
#ifndef MAIN_H
#define MAIN_H
 
#include <QtGui>
 
#ifdef Q_OS_WIN
#include "windows.h"
#endif
 
class SleeperThread : public QThread
{    Q_OBJECT
public:
    SleeperThread(int interval,QObject* parent = 0) : QThread(parent), m_interval(interval) {}
protected:
    virtual void run()
    {
        QElapsedTimer timer;
        timer.start();
        for (int i=0;i<100;++i)
        {
            msleep(m_interval);
        }
        QMetaObject::invokeMethod(parent(),"showResult",Q_ARG(int, timer.elapsed()));
    }
private:
    int m_interval;
};
 
class MainWindow : public QMainWindow
{    Q_OBJECT
public:
    MainWindow()
    {
        QPushButton* start_pb = new QPushButton("Test Sleep");
        connect(start_pb,SIGNAL(clicked()),this,SLOT(testSleep()));
 
        QPushButton* beginPeriod_pb = new QPushButton("Begin Period");
        connect(beginPeriod_pb,SIGNAL(clicked()),this,SLOT(beginPeriod()));
        QPushButton* endPeriod_pb = new QPushButton("End Period");
        connect(endPeriod_pb,SIGNAL(clicked()),this,SLOT(endPeriod()));
 
        QHBoxLayout* hl = new QHBoxLayout();
        hl->addWidget(beginPeriod_pb);
        hl->addWidget(start_pb);
        hl->addWidget(endPeriod_pb);
 
        output_te = new QPlainTextEdit();
        output_te->setReadOnly(true);
 
        QVBoxLayout* vl = new QVBoxLayout();
        vl->addLayout(hl);
        vl->addWidget(output_te);
 
 
        QWidget* w = new QWidget();
        w->setLayout(vl);
 
        setCentralWidget(w);
    }
public slots:
    void testSleep()
    {
        SleeperThread* st = new SleeperThread(1,this);
        connect(st,SIGNAL(result(int)),this,SLOT(showResult(int)));
        connect(st,SIGNAL(finished()),st,SLOT(deleteLater()));
        st->start();
    }
 
    void beginPeriod()
    {
        output_te->appendPlainText("BeginPeriod!");
        #ifdef Q_OS_WIN
            if (timeBeginPeriod(1) == TIMERR_NOCANDO)
                qWarning() << "Could not start the time period!";
        #endif
    }
 
    void endPeriod()
    {
        output_te->appendPlainText("EndPeriod!");
        #ifdef Q_OS_WIN
            timeEndPeriod(1);
        #endif
    }
 
    void showResult(int elapsed)
    {
        output_te->appendPlainText(QString::number(elapsed));
    }
private:
    QPlainTextEdit* output_te;
};
 
#endif // MAIN_H
 
main.cpp:
 
#include <QApplication>
#include <QtGui>
 
#include "main.h"
 
int main(int argc, char** argv)
{
    QApplication app(argc,argv) ;
 
    MainWindow mw;
    mw.show();
 
    return app.exec();
}

main.h
#ifndef MAIN_H
#define MAIN_H

#include <QtGui>

#ifdef Q_OS_WIN
#include "windows.h"
#endif

class SleeperThread : public QThread
{    Q_OBJECT
public:
    SleeperThread(int interval,QObject* parent = 0) : QThread(parent), m_interval(interval) {}
protected:
    virtual void run()
    {
        QElapsedTimer timer;
        timer.start();
        for (int i=0;i<100;++i)
        {
            msleep(m_interval);
        }
        QMetaObject::invokeMethod(parent(),"showResult",Q_ARG(int, timer.elapsed()));
    }
private:
    int m_interval;
};

class MainWindow : public QMainWindow
{    Q_OBJECT
public:
    MainWindow()
    {
        QPushButton* start_pb = new QPushButton("Test Sleep");
        connect(start_pb,SIGNAL(clicked()),this,SLOT(testSleep()));

        QPushButton* beginPeriod_pb = new QPushButton("Begin Period");
        connect(beginPeriod_pb,SIGNAL(clicked()),this,SLOT(beginPeriod()));
        QPushButton* endPeriod_pb = new QPushButton("End Period");
        connect(endPeriod_pb,SIGNAL(clicked()),this,SLOT(endPeriod()));

        QHBoxLayout* hl = new QHBoxLayout();
        hl->addWidget(beginPeriod_pb);
        hl->addWidget(start_pb);
        hl->addWidget(endPeriod_pb);

        output_te = new QPlainTextEdit();
        output_te->setReadOnly(true);

        QVBoxLayout* vl = new QVBoxLayout();
        vl->addLayout(hl);
        vl->addWidget(output_te);


        QWidget* w = new QWidget();
        w->setLayout(vl);

        setCentralWidget(w);
    }
public slots:
    void testSleep()
    {
        SleeperThread* st = new SleeperThread(1,this);
        connect(st,SIGNAL(result(int)),this,SLOT(showResult(int)));
        connect(st,SIGNAL(finished()),st,SLOT(deleteLater()));
        st->start();
    }

    void beginPeriod()
    {
        output_te->appendPlainText("BeginPeriod!");
        #ifdef Q_OS_WIN
            if (timeBeginPeriod(1) == TIMERR_NOCANDO)
                qWarning() << "Could not start the time period!";
        #endif
    }

    void endPeriod()
    {
        output_te->appendPlainText("EndPeriod!");
        #ifdef Q_OS_WIN
            timeEndPeriod(1);
        #endif
    }

    void showResult(int elapsed)
    {
        output_te->appendPlainText(QString::number(elapsed));
    }
private:
    QPlainTextEdit* output_te;
};

#endif // MAIN_H

main.cpp:

#include <QApplication>
#include <QtGui>

#include "main.h"

int main(int argc, char** argv)
{
    QApplication app(argc,argv) ;

    MainWindow mw;
    mw.show();

    return app.exec();
}

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Johannes

[high_flyer]

for the sake if completeness, here is a good link I just found on the subject:
http://msdn.microsoft.com/en-us/magazine/cc163996.aspx

[JohannesMunk]

Actually the article is not about a timer that can periodically schedule or delay something, but about using the Performance Counter API to supplement the precision of the system time and thus interval measurement. They explain the lack of resolution by the lack of resolution of the system timer, though.

Joh

---

Added after 9 minutes:

To get back on the original topic, I just tested, that when you start a QTimer with an arbitrary number above 20ms it will not adapt the timer resolution. So that a timer of 33ms will fire either after 30ms or 40ms, because the system timer resolution is at 10ms.

That problem can be resolved as i suggested by using a 1ms timer and counting its timeouts or probably (I didn't check this) by using a 33ms timer and manually setting the windows timer resolution to 1ms with timeBeginPeriod!

Joh

[high_flyer]

They explain the lack of resolution by the lack of resolution of the system timer, though.

Yes, but also how to increase the timer resolution in the limits of a given system, which I think is what the OP was generally after.

[JohannesMunk]

When I look at the similar threads below this seems to be a rather common problem.

http://www.qtcentre.org/threads/2316...is-not-acurate

Maybe I should create a WIKI-Article about precision timing under windows sometime. I have spent quite a lot of time with performance counters and their unreliable frequency statements and with thread-switching times, due to a real time data acquisition suite i'm currently developing.

Joh

[high_flyer]

Maybe I should create a WIKI-Article about precision timing under windows sometime. I have spent quite a lot of time with performance counters and their unreliable frequency statements and with thread-switching times, due to a real time data acquisition suite i'm currently developing.

I am not going to stop you!

[totem]

The setting for the high resolution timer is done only by the timeBeginPeriod call.

And the privilege adjustment is not required for it! I didn't realize! It was only in context of an NTP-Clock sync, that this made sense.

But as I mentioned none of it is required to have a 1 ms QTimer work correctly. Only if you want QThread::sleep to work accurately you will need to set the timer resolution yourself.
I just mentioned it, because I had trouble with it in context of high precision timing.
Joh

So If I don't use QThread::sleep I don't need to include windows-specific code ?
I'd be glad because it makes me link against some MS lib (i got linker error with timeBeginPeriod call I did not even tried to resolve)

By the way I tried your solution and it seems to work for me, thanks again.
As I said I needed a frame interval of 33ms so I used a timer interval of 11ms, which implies an update every 3 timer events. Now animation duration seems consistent

[JohannesMunk]

Hi Totem!

Yes, below 20ms QTimer-interval, windows timer resolution is set to 1ms, without you doing anything.
Thats why your 11ms should be quite accurate.

Glad that your problem is resolved,

Happy coding,

Johannes