QChart slows down in QT application

Question

So im writing a QT application that will read values from a serial port and display them in a graph during runtime. I managed to update my QChart during runtime with just a random generated value to try out updating in real time and it all works fine.

But my application slows down the more and more i append until it gets completely unusable.

I do understand that the list containing my points grows, but after a 100 points or so it really really slows down, thats really fast, it feels like i have some sort of memory leak?

I know the usual answer is "Don't use QCharts" but im a beginner at both C++ and QT so this is what i'm using for simplicity.

MainWindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QtCharts/QChartView>
#include <QtCharts/QLineSeries>
#include <QGridLayout>

#include <QLabel>
#include <QDebug>

QT_CHARTS_USE_NAMESPACE
MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    series = new QLineSeries();

    chart = new QChart();
    chart->legend()->hide();
    chart->addSeries(series);
    chart->createDefaultAxes();
    chart->axes(Qt::Vertical).back()->setRange(-10, 10);
    chart->axes(Qt::Horizontal).back()->setRange(0, 100);

    chart->setContentsMargins(0, 0, 0, 0);
    chart->setBackgroundRoundness(0);

    QChartView *chartView = new QChartView(chart);
    chartView->setRenderHint(QPainter::Antialiasing);
    ui->setupUi(this);

    QLabel *label = new QLabel();
    label->setText("Hello World");

    QGridLayout *layout = new QGridLayout;
    QWidget * central = new QWidget();
    setCentralWidget(central);
    centralWidget()->setLayout(layout);

    layout->addWidget(chartView, 0, 0);

    clock = 0;

    SerialPortReader *reader = new SerialPortReader(this);

    connect(reader, SIGNAL(onReadValue(int)), this, SLOT(onReadValue(int)));

    reader->run();
}

MainWindow::~MainWindow()
{
    delete ui;
}

void MainWindow::onReadValue(int value){
    ++clock;    
    series->append(clock + 30, value);
    chart->axes(Qt::Horizontal).back()->setRange(0 + clock, 100 + clock);    
}

SerialPortReader.cpp

#include "serialportreader.h"
#include "mainwindow.h"
#include <QtCore>
#include <QRandomGenerator>
#include <QDebug>

SerialPortReader::SerialPortReader(QObject *parent) : QThread(parent)
{
    this->parent = parent;
    this->randomGenerator = QRandomGenerator::global();
}

void SerialPortReader::run() {

    QTimer *timer = new QTimer(this);
    timer->start(100);
    connect(timer, SIGNAL(timeout()), this, SLOT(readValue()));
}

void SerialPortReader::readValue() {
    int value = randomGenerator->bounded(-10, 10);
    emit onReadValue(value);
}

I was just wondering if anyone has any suggestion to what could be wrong? or if there is a anything i can do, except for changing chart-lib.

Answer 1

After tinkering around i found out that the culprit wasn't actually a memory leak it was the:

chartView->setRenderHint(QPainter::Antialiasing);

As more and more data was presented the slower it got to do all the Antialiasing.

When i removed this everything suddenly went very smooth.