QCustomPlot: Performance of plot degrades massively when increasing pen width

skebanga

I have a simple example app here which creates some random data (50 samples) and draws a line plot which has dragging enabled (full source code below)

I have a single constant, PEN_WIDTH, which I change from a value of 1 to 2:

const int PEN_WIDTH = 2;

This constant is used in the call to QCPGraph::setPen

QCPGraph* graph = plot.addGraph();
graph->setPen(QPen(Qt::red, PEN_WIDTH));

When using a PEN_WIDTH of 1, dragging is near instantaneous.
When using a PEN_WIDTH of 2, dragging lags drastically, like several seconds.

I can drag this with no perceptible delay:
PEN_WIDTH=1
Dragging this has a several second delay lag:
PEN_WIDTH=2

System specs:

In case it has some bearing on the cause/solution, I am running this on Ubuntu 14.04 with Gnome 3 desktop. I have a 4-core Core i7-4790, am using onboard Xeon E3 graphics, and my system has 8GB RAM.

Question:

How can I have a larger pen width without sacrificing performance?

Full example below:

#include <QApplication>
#include <QMainWindow>
#include <qcustomplot/qcustomplot.h>
#include <random>

const int PEN_WIDTH   = 2;
const int NUM_SAMPLES = 50;

class Plot
{
public:
    Plot(int& argc, char** argv)
        : app(argc, argv)
    {
        window.setCentralWidget(&plot);

        plot.setInteractions(QCP::iRangeDrag);
        plot.axisRect()->setRangeDrag(Qt::Horizontal | Qt::Vertical);

        plot.xAxis->setTickLabelType(QCPAxis::ltDateTime);
        plot.xAxis->setDateTimeFormat("hh:mm:ss");
    }

    int exec()
    {
        std::random_device rd;
        std::mt19937 mt(rd());
        std::uniform_real_distribution<double> dist(0, 10);

        QVector<double> x,y;
        double i = 0;
        while (i < NUM_SAMPLES)
        {
            x.push_back(i);
            y.push_back(i * dist(mt));
            i += .1;
        }

        QCPGraph* graph = plot.addGraph();
        graph->setPen(QPen(Qt::red, PEN_WIDTH));
        graph->addData(x, y);

        plot.rescaleAxes();
        plot.replot();

        window.show();
        return app.exec();
    }
private:
    QApplication app;
    QMainWindow  window;
    QCustomPlot  plot;
};

int main(int argc, char** argv)
{
    return Plot(argc, argv).exec();
}

Paul Colby

Just a small note for anyone else interested: there's some further discussion on this over on the QCustomPlot forums too.

Cheers.