QwtPlotCurve plots quite slowly with 5K points

[sexgirl]

Hi, I am using Qwt5 and Qt4.2.3 to plot curve with 5K points and it may take 20 seconds to plot every time.
I looked up some similar threads in the forum but still have no idea.
My code referred to Qwt Example realtime_plot:

class CustomData
{
public:
    CustomData();
    void append(double *x, double *y, int count);
 
    int count() const;
    int size() const;
    const double *x() const;
    const double *y() const;
private:
    int d_count;
    QwtArray<double> d_x;
    QwtArray<double> d_y;
};
 
//class CustomQwtPlot:public QwtPlot
void CustomQwtPlot::appendDataShow(double *x, double *y, int size, const QPen &curvePen)
{
    ......
 
    if ( d_data == NULL )
        d_data = new CustomData;
    if(cur == NULL)
    {
        cur = new QwtPlotCurve("curve");
        cur->setPaintAttribute(QwtPlotCurve::PaintFiltered);
        cur->setRenderHint(QwtPlotItem::RenderAntialiased);
        d_data->append(x, y, size);
        cur->setRawData(d_data->x(), d_data->y(), d_data->count());
        cur->setPen(curvePen);
        cur->attach(this);
    }
 
   ......
}
 
//CustomQwtPlot *energyplot;
void plot::updateEnergyplot(QList<QStringList> data, QStringList header)
{
    ......
 
    if (energyplot == NULL)
    {
        ......
        energyplot = new CustomQwtPlot;
        double *xData = new double[count];
        double *yData = new double[count];
 
        for (int i = 0; i < count; i++)
        {
            xData[i] = (double)i;
            yData[i] = data.at(0).at(i).toDouble(&ok);
        }
        energyplot->appendDataShow(xData, yData, count, c);
        ......
 
        delete []xData;
        delete []yData;
 
       ......
    }
}

class CustomData
{
public:
    CustomData();
    void append(double *x, double *y, int count);

    int count() const;
    int size() const;
    const double *x() const;
    const double *y() const;
private:
    int d_count;
    QwtArray<double> d_x;
    QwtArray<double> d_y;
};

//class CustomQwtPlot:public QwtPlot
void CustomQwtPlot::appendDataShow(double *x, double *y, int size, const QPen &curvePen)
{
    ......

    if ( d_data == NULL )
        d_data = new CustomData;
    if(cur == NULL)
    {
        cur = new QwtPlotCurve("curve");
        cur->setPaintAttribute(QwtPlotCurve::PaintFiltered);
        cur->setRenderHint(QwtPlotItem::RenderAntialiased);
        d_data->append(x, y, size);
        cur->setRawData(d_data->x(), d_data->y(), d_data->count());
        cur->setPen(curvePen);
        cur->attach(this);
    }

   ......
}

//CustomQwtPlot *energyplot;
void plot::updateEnergyplot(QList<QStringList> data, QStringList header)
{
    ......

    if (energyplot == NULL)
    {
        ......
        energyplot = new CustomQwtPlot;
        double *xData = new double[count];
        double *yData = new double[count];

        for (int i = 0; i < count; i++)
        {
            xData[i] = (double)i;
            yData[i] = data.at(0).at(i).toDouble(&ok);
        }
        energyplot->appendDataShow(xData, yData, count, c);
        ......

        delete []xData;
        delete []yData;

       ......
    }
}

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Hope someone can help me.
Thanks.
Regards,
sexgirl

[sexgirl]

Could anybody tell me where I make a mistake and how to fix the problem?
Maybe my discription is hard to understand and forgive my broken english.

[Spitfire]

Create small, compilable example of the issue and post it here.

Posting semi-relevant parts of the code isn't useful.

Also when creating the example there's high chance that you'll what was the cause of your problem.

[d_stranz]

Well, for one thing, if you read the documentation for QwtPlotCurve::setRawData(), the pointers to the x and y arrays must be valid for the entire life of the curve. In lines 47-48, you create new arrays, then pass them into the curve, then in 58-59, you delete them. Thus, QwtPlotCurve is trying to draw using pointers that have already been deleted. If you resize your plot or do anything to cause a replot after the first time it draws those points, the program will probably crash. You have been lucky so far because QwtPlot makes a cache of the plot, which is re-built only when something changes to require a replot.

Second, you are passing your data in as QList of QStringList of QString, then parsing 5000 points from deep down inside this structure from strings into doubles. Why? If all you are interested in is the y values buried down inside this complex data structure, find a way to pass them in as a QVector of doubles instead of converting them into strings and burying them.

The slowness isn't Qwt's fault at all, it is the way you have chosen to represent and access your data. You are making your PC do so much extra work, I am surprised your PC didn't burn up.

[d_stranz]

Well, for one thing, if you read the documentation for QwtPlotCurve::setRawData(), the pointers to the x and y arrays must be valid for the entire life of the curve. In lines 47-48, you create new arrays, then pass them into the curve, then in 58-59, you delete them. Thus, QwtPlotCurve is trying to draw using pointers that have already been deleted. If you resize your plot or do anything to cause a replot after the first time it draws those points, the program will probably crash. You have been lucky so far because QwtPlot makes a cache of the plot, which is re-built only when something changes to require a replot.

OK, when I look at your CustomData class again, it looks like maybe you are making a copy of the x-y arrays as a QwtArray inside od CustomData. You don't give the code for the CustomData::append() method, but if it is either copying or appending to the QwtArray, then this is OK.

So if you are making a copy of the data, there is no need to create new arrays on the heap inside of updateEnergyplot(); use stack-allocated memory instead

QwtArray< double > xData( count );
QwtArray< double > yData( count );
 
for (int i = 0; i < count; i++)
{
      xData[i] = (double)i;
      yData[i] = data.at(0).at(i).toDouble(&ok);
}
energyplot->appendDataShow( &(xData[0]), &(yData[0]), count, c);

QwtArray< double > xData( count );
QwtArray< double > yData( count );

for (int i = 0; i < count; i++)
{
      xData[i] = (double)i;
      yData[i] = data.at(0).at(i).toDouble(&ok);
}
energyplot->appendDataShow( &(xData[0]), &(yData[0]), count, c);

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Faster, and you don't fragment memory so much by repeatedly allocating and deleting the arrays.