Qwt question

I have some data in my app using Qwt Plot and i want to measure some delta values using my mouse in my plot.

eg:

Quote:

---

given:
point 1 (1,3)
point 2 (2,4)
point 3 (3,1)

now if i point my mouse to point 1 (click) then point 2 (click)

then i should show:

delta x = 1 and delta y = 1

---

how can i implement this in Qwt? please enlightened me

baray98

Have a look at the CanvasPicker class in the event_filter example. It shows how to find a point from a mouse click. But note, that the implementation is slow for curves with many points.

But if I had to implement a distance measurement I would use a tailored QwtPlotPicker object, because of its tracker text.

Uwe

Hi uwe,

I did follow your advise and pop up a widget showing my delta at the end of my selction

I connected signal QwtPlotPicker::selected(const QwtDoubleRect &) to trigger my class to display the delta.

I wanted to modify this ,instead of showing delta at the end of my selecetion I want to show the delta while the user is dragging a selection , its like overiding the the tracker text how can i do this? i tried re-implementing the trackerText but did not work .

baray98

I found some old code on my disk, maybe it helps:

Code:

---

class DistancePicker : public QwtPlotPicker
{
public:
    DistancePicker(QwtPlotCanvas *);
 
protected:
    virtual void drawRubberBand(QPainter *) const;
    virtual QwtText trackerText(const QwtDoublePoint &) const;
};
 
DistancePicker::DistancePicker(QwtPlotCanvas* canvas):
   QwtPlotPicker(canvas)
{
   /*
      We don't have a picker for a line, but the rectangle
      selection is also a selection of 2 points. So all we
      have to do is to paint a line rubberband.
    */
   setSelectionFlags(QwtPicker::RectSelection);
   setRubberBand(PolygonRubberBand);
   setTrackerMode(QwtPicker::ActiveOnly);
 
   // Disable keyboard handling
   setKeyPattern(QwtEventPattern::KeySelect1, Qt::Key_unknown);
   setKeyPattern(QwtEventPattern::KeySelect2, Qt::Key_unknown);
}
 
void DistancePicker::drawRubberBand(QPainter *painter) const
{
   painter->drawPolygon(selection());
}
 
QwtText DistancePicker::trackerText(const QwtDoublePoint &) const
{
    const QwtPolygon &polygon = selection();
    if ( polygon.size() != 2 )
        return QwtText();
 
    const QLineF line(invTransform(polygon[0]), invTransform(polygon[1]));
 
    QwtText text( QString::number(line.length()) );
 
    QColor bg(Qt::white);
    bg.setAlpha(180);
    text.setBackgroundBrush(QBrush(bg));
 
    return text;
}

---

Uwe