Update QChart in real time (QtWidgets)

[ivanicy]

Hello!!

I am trying to use an histogram associated to a video streaming so, I want to update the chart frame by frame.
I have a QPushButton associated to a signal wich shows the histogram with this slot:

void MainWindow::createHistogram(bool status) {
    if (status) {
        showHist = true;

        chartHist = new QChart();

        int histSize[1] = {256}; // number of bins
        float hranges[2] = {0.0, 255.0}; // min andax pixel value
        const float* ranges[1] = {hranges};
        int channels[1] = {0}; // only 1 channel used

        cv::MatND hist;

        cv::Mat ret;
        imageFunctions::qimage_to_mat(imageViewer->item->pixmap().toImage(), ret);

        cv::calcHist(&ret, 1, channels, cv::Mat(), hist, 1, histSize, ranges);

        series = new QtCharts::QBarSet("");

        for (int h = 0; h < histSize[0]; ++h) {
            float bin_value = hist.at<float>(h);
            series->append(bin_value);
            if (valueHistMax < bin_value) valueHistMax = bin_value;
        }

        QPen pen(0x09418B);
        pen.setWidth(3);
        series->setPen(pen);

        QHorizontalStackedBarSeries *barseries = new QHorizontalStackedBarSeries();
        barseries->append(series);

        chartHist->removeAllSeries();
        chartHist->addSeries(barseries);
        chartHist->setAnimationOptions(QChart::SeriesAnimations);
        chartHist->legend()->setVisible(false);
        chartHist->setMaximumWidth(100);
        chartHist->setMargins(QMargins(0,0,4,0));
        chartHist->setAcceptHoverEvents(true);
        chartHist->layout()->setContentsMargins(0,0,0,0);
        chartHist->setBackgroundRoundness(0);

        ui->histLayout->addWidget(chartViewHist);
    } else {
        delete series;
        delete chartHist;
        delete chartViewHist;
        showHist = false;
    }
}

And when I receive the next frame I call this function:

void MainWindow::updateHistogram(cv::Mat *image) {

    int histSize[1] = {256}; // number of bins
    float hranges[2] = {0.0, 255.0}; // min andax pixel value
    const float* ranges[1] = {hranges};
    int channels[1] = {0}; // only 1 channel used

    cv::MatND hist;
    cv::calcHist(image, 1, channels, cv::Mat(), hist, 1, histSize, ranges);

    //series = new QtCharts::QBarSet("");

    for (int h = 0; h < histSize[0]; ++h) {
        float bin_value = hist.at<float>(h);
        series->replace(h, bin_value);
        //series->append(bin_value);
        if (valueHistMax < bin_value) valueHistMax = bin_value;
    }

    QPen pen(0x09418B);
    pen.setWidth(3);
    series->setPen(pen);

    QHorizontalStackedBarSeries *barseries = new QHorizontalStackedBarSeries();
    barseries->append(series);

    chartHist->removeAllSeries();
    chartHist->addSeries(barseries);

    //clearLayout(ui->histLayout);
    //ui->histLayout->addWidget(chartViewHist);
}

But this gives me a memory access error. I don't know how to do it. What could be wrong here?

Thank you very much!!

[jsulm]

@ivanicy said in Update QChart in real time (QtWidgets):

memory access error

Where exactly? Did you try to debug? Did you check the stack trace when it crashes?

[ivanicy]

@jsulm I don't know where. Is one of these times that appears an hexadecimal window, Qt doesn't show where is it crashed:

I have this lines in the MainWindow constructor:

    chartHist = new QChart();
    chartViewHist = new QChartView(chartHist);
    chartViewHist->setRenderHint(QPainter::Antialiasing);
    chartViewHist->setMaximumWidth(100);

I am wondering that it is crashing when I do the instantiation of the objects but I don't know.

[jsulm]

@ivanicy Are you testing with a release build? Build in debug mode and try again.

[ivanicy]

@jsulm I test in debug mode, and appears that. I was debugging line by line and only works the first time. The next calls of the function don't work.