Why does setRange of QDateTimeAxis take so much time?

tovax

I used QChart to display real-time curves, and found that setRange() of QDateTimeAxis take so much time, about 30ms.
Here is a link to the demo code: RealTimeCurve

        QElapsedTimer t;
        t.start();
        beginTime = QDateTime::currentDateTime();
        endTime = beginTime.addMSecs(MaxSize * static_cast<uint32_t>(timer->interval()));
        axisX->setMin(beginTime);
        axisX->setMax(endTime); // TODO: waste about 30ms
        // axisX->setRange(beginTime, endTime);
        qDebug() << "QElapsedTimer" << t.elapsed();

Could you help me, please?
Best Regards!

JonB

@tovax
You asked "Why does setRange of QDateTimeAxis take so much time?". I am suggesting it is because it already has a lot of points. I suggested you try without (many) points to see if this is the case. Did you try that? If that is not the reason you should discover that first.

I recall a long time ago the same question/problem being asked in this forum. I'm afraid I don't recall what/whether there was a good resolution. There is a (possibly old?) source code at https://www.advsofteng.com/doc/cdcppdoc/realtimezoomscrollqt.htm which you might look at to see if it helps you any? There is QChart::scroll(). Maybe Google for "qchart real time`, something like https://stackoverflow.com/questions/50853387/qchart-realtime-performance or other hits may help you?

JonB

@tovax
I might be wrong(!), but doesn't changing the range require complete rescaling/redrawing of the entire chart? If you already have a lot of points plotted(?), that could be quite a bit of work? Is there anyway you can set the range before you plot the points (or at least test that)?

tovax

@JonB
Hi,
Thanks very much for your reply.
I want to achieve a display like this:real-time-curve-gif
So I have to reset the range when the data is updated.

JonB

@tovax
You asked "Why does setRange of QDateTimeAxis take so much time?". I am suggesting it is because it already has a lot of points. I suggested you try without (many) points to see if this is the case. Did you try that? If that is not the reason you should discover that first.

I recall a long time ago the same question/problem being asked in this forum. I'm afraid I don't recall what/whether there was a good resolution. There is a (possibly old?) source code at https://www.advsofteng.com/doc/cdcppdoc/realtimezoomscrollqt.htm which you might look at to see if it helps you any? There is QChart::scroll(). Maybe Google for "qchart real time`, something like https://stackoverflow.com/questions/50853387/qchart-realtime-performance or other hits may help you?

tovax

@JonB
This demo only has 10 points now.
I will read these posts, thanks again.

tovax

Test Results:
1.
"test code" in function initialize():
Debug output: QElapsedTimer "1" 0 0
Consume 0ms.
2.
"test code" in timer slot onTimer():
Debug output: QElapsedTimer "2" 53 46
Consume 53ms.

test code as follows:

void JCDemoRealTimeCurve::test(uint32_t num)
{
    QElapsedTimer t;
    t.start();
    beginTime = QDateTime::currentDateTime();
    endTime = beginTime.addMSecs(MaxSize * static_cast<uint32_t>(timer->interval()));
    axisX->setMin(beginTime);
    axisX->setMax(endTime); // TODO: waste about 30ms
    // axisX->setRange(beginTime, endTime);
    qDebug() << "QElapsedTimer" << QString::number(num) << t.elapsed() << beginTime.msecsTo(QDateTime::currentDateTime());
}

tovax

@JonB
Hi，
This is a very good website! Thank you very much!
https://www.advsofteng.com/
Best Regards!