Curve tracking example is very slowly

Hi all.

I have faced with the problem of very slow movements of "rubber band" lines when more than 3-curves are attached.
I have implemented the code from the "qwt-6.1.3\playground\curvetracker\" example:

Code:

---

struct compareX
{
    inline bool operator()(const double x, const QPointF &pos) const
    {
        return (x < pos.x());
    }
};
 
CurveTracker::CurveTracker(QWidget *canvas)
    : QwtPlotPicker(canvas)
{
}
 
QRect CurveTracker::trackerRect(const QFont &font) const
{
    QRect r = QwtPlotPicker::trackerRect(font);
    // Always show the info on left-top of window.
    r.moveTo(0, 0);
    return r;
}
 
QwtText CurveTracker::trackerTextF(const QPointF &pos) const
{
    // Inverse iterations over all attached curves.
    QString info;
    const auto curves = plot()->itemList(QwtPlotItem::Rtti_PlotCurve);
    for (auto curveIt = curves.crbegin(); curveIt != curves.crend(); ++curveIt) {
 
        // Show info only for visible curves.
        if (!(*curveIt)->isVisible())
            continue;
 
        const QString curveInfo = curveInfoAt(static_cast<const QwtPlotCurve *>(*curveIt), pos);
        if (!curveInfo.isEmpty()) {
            if (!info.isEmpty())
                info += tr("<br>");
            info += curveInfo;
        }
    }
    return info;
}
 
QString CurveTracker::curveInfoAt(const QwtPlotCurve *qwtcurve, const QPointF &pos) const
{
    const QLineF line = curveLineAt(qwtcurve, pos.x());
    if (line.isNull())
        return QString();
 
    const double y = line.pointAt((pos.x() - line.p1().x()) / line.dx()).y();
    QString info(tr("<font color=""%1"">%2</font>"));
    return info.arg(qwtcurve->pen().color().name()).arg(y);
}
 
QLineF CurveTracker::curveLineAt(const QwtPlotCurve *qwtcurve, double x) const
{
    QLineF line;
    const auto samplesCount = qwtcurve->dataSize();
    if (samplesCount >= 2) {
        const QRectF boundingRect = qwtcurve->boundingRect();
        if ((boundingRect.width() > 0)
                && (x >= boundingRect.left())
                && (x <= boundingRect.right())) {
 
            auto lessfunc = [](const double x, const QPointF &pos) { return (x < pos.x()); };
            int sampleIndex = qwtUpperSampleIndex<QPointF>(*qwtcurve->data(), x, lessfunc);
 
            if (sampleIndex == -1) {
                const auto lastSample = qwtcurve->sample(samplesCount - 1);
                if (x == lastSample.x())
                    sampleIndex = samplesCount - 1;
            }
            if (sampleIndex > 0) {
                line.setP1(qwtcurve->sample(sampleIndex - 1));
                line.setP2(qwtcurve->sample(sampleIndex));
            }
        }
    }
    return line;
}

---

And I see that if I use 3 and more curves, then the rubber band's crosshair isn't in time behind movements of the
mouse cursor, it lags behind on speed. And with the 15-curves or more, the delay of a sight is stronger.

I see this delay even with "pure" QwtPlotPicker...

So, is any tricks to solve it?

PS: Qwt6.3-multiaxes, Qt 5.7, MSVC2015x86.

BR,
Denis

---

Added after 13 minutes:

I found, that even this code brings to lags:

Code:

---

QwtText CurveTracker::trackerTextF(const QPointF &pos) const
{
    QString info;
    for (int i = 0; i < 15; ++i) {
        info += tr("<br>");
        info += QString::number(qrand() % 1000);
    }
    return info;
}

---

---

Added after 11 minutes:

UPD: Seems, that source of issue in HTML tags:

Code:

---

QString info(tr("<font color=""%1"">%2</font>"));

---

So... seems, we need to avoid to use the trackerTextF() method.

Seems, we need to draw the tracker's rect with all contents manually, directly in QwtPicker::drawTracker.. :(

Or, I am mistaken ?

Using rich text is slow and there is nothing you can do about it, but it is not that slow to explain the effects you describe.
The only obvious thing I can see in your code is, that you are running the text through tr ( what doesn't make any sense to me ) - maybe it does something in your context.

I increased the curvetracker example to 10 curves and I can't see any difference.

Code:

---

insertCurve( "Curve 3", Qt::darkCyan, points2 );
    insertCurve( "Curve 4", Qt::darkYellow, points2 );
    insertCurve( "Curve 5", Qt::darkRed, points2 );
    insertCurve( "Curve 6", Qt::darkRed, points2 );
    insertCurve( "Curve 7", Qt::darkRed, points2 );
    insertCurve( "Curve 8", Qt::darkRed, points2 );
    insertCurve( "Curve 9", Qt::darkRed, points2 );
    insertCurve( "Curve 1o", Qt::darkRed, points2 );

---

• Are you running on a slow piece of hardware - like the raspi ?
• Did you build Qt in debug mode ?
• Anything else what explains, why you are that much slower than my desktop system ( Linux, OpenSuse 12.2, bogomips : 4988.86 )

Uwe

Hi Uwe,

thanks for your answer.

Now I too use the curvetracker with this simplification:

Code:

---

Plot::Plot( QWidget *parent ):
    QwtPlot( parent)
{
    // curves
    for (int i = 0; i < 15; ++i) {
        QPolygonF points(2000);
        for (int p = 0; p < 2000; ++p) {
            points.append({ double(p), double(qrand() % 100) });
        }
 
        insertCurve( tr("Curve %1").arg(i), qrand() % 255, points );
    }
 
    CurveTracker* tracker = new CurveTracker( this->canvas() );
 
    // for the demo we want the tracker to be active without
    // having to click on the canvas
    tracker->setStateMachine( new QwtPickerTrackerMachine() );
    tracker->setRubberBandPen( QPen( "MediumOrchid" ) );
}
 
void Plot::insertCurve( const QString &title, 
    const QColor &color, const QPolygonF &points )
{
    QwtPlotCurve *curve = new QwtPlotCurve();
    curve->setTitle( title );
    curve->setPen(color),
    curve->setRenderHint( QwtPlotItem::RenderAntialiased, true );
    curve->setSamples( points );
    curve->attach( this );
}

---

and I got same result. When I expand the plot to the whole Display width, I got lags of a crosshair from the mouse position (this depends on mouse moving speed).

Quote:

---

Are you running on a slow piece of hardware - like the raspi ?

---

No, I use the Desktop PC with Windows 10 (64bit) + Qwt 6.3 (yes? multiaxes) + Qt 5.7.0 + MSVC2015x86

Quote:

---

Did you build Qt in debug mode ?

---

Yes, in debug mode.

Quote:

---

Anything else what explains, why you are that much slower than my desktop system ( Linux, OpenSuse 12.2, bogomips : 4988.86 )

---

I have Desktop i5-3470@3.2GHzx8GB RAM .

Quote:

---

Originally Posted by kuzulis

Yes, in debug mode.

---

When Qt has been built in debug mode lots of assertions and extra tests are compiled in. AFAIR this has a significant slow down effect on text rendering.
So the first thing I would do is to try a release version.

Uwe

Yes, you are right. Using the Release mode improves the performance.
But, anyway, If I move the mouse quickly, then crosshair lags behind, but to a lesser extent.
But, OK, many thanks, it is enougth for me. :)