A problem when using QwtPlotPanner and QwtPlotDirectpaint

Hi,

I encounter a problem when using QwtPlotPannerand QwtPlotDirectPainter. It is described as following.

I make a plot widget by QwtPlot, and implement panner, picker and magnifier by inheriting QwtPlotPanner, QwtPlotPickand QwtPlotMagnifier.

The symbol selection is implemented by connect signal http://doc.qt.io/qt-4.8/QwtPlotPick::selected(QPointF) with a select(QPointF) function. I also write a highlight function for selected symbol, using QwtPlotDirectPainter, similar to the example of event_filter with qwt package. The code show as follow

The plot:

Code:

---

class QwtFoilPlot: public QwtPlot
{
	Q_OBJECT
public:
	QwtFoilPlot(QWidget *parent = NULL);
....
}
 
QwtFoilPlot::QwtFoilPlot( QWidget *parent /*= NULL*/ ):
	curve_np(200)
{
   ......
	// panner 
	p_panner = new CanvasPanner(this);
	// picker
	p_picker = new CanvasPicker(canvas());
	p_picker->setStateMachine(new QwtPickerClickPointMachine);
	p_picker->setRubberBand(QwtPicker::NoRubberBand);
   ......
}

---

The plot picker:

Code:

---

class CanvasPicker: public QObject
{
     QOBJECT
public:
    CanvasPicker(QwtPlotCanvas* p);
private:
    void select(const QPoint &);
.......
    void showCursor(bool enable);
.......
    QwtPlotCurve *d_selectedCurve;
    int d_selectedPoint;
};

---

The method definition for this picker

Code:

---

void CanvasPicker::select( const QPointF& p )
{
	QPoint pos = transform(p);
	QwtPlotCurve *curve = NULL;
	double dist = 10e10;
	int index = -1;
 
	const QwtPlotItemList& itmList = plot()->itemList();
	for ( QwtPlotItemIterator it = itmList.begin();
		it != itmList.end(); ++it )
	{
		if ( (*it)->rtti() == QwtPlotItem::Rtti_PlotCurve )
		{
			QwtPlotCurve *c = (QwtPlotCurve*)(*it);
			double d;
			int idx = c->closestPoint(pos, &d);
			if ( d < dist )
			{
				curve = c;
				index = idx;
				dist = d;
			} 
		}
	}
 
	showCursor(false);
	p_selectedCurve = NULL;
	p_selectedPoint = -1;
 
	if ( curve && dist < 10 ) // 10 pixels tolerance
	{
		p_selectedCurve = curve;
		p_selectedPoint = index;
		showCursor(true);
	}
}
 
void CanvasPicker::showCursor(bool showIt)
{
	if ( !p_selectedCurve )
		return;
 
	QwtSymbol *symbol = const_cast<QwtSymbol *>( p_selectedCurve->symbol() );
 
	const QBrush brush = symbol->brush();
	if ( showIt )
		symbol->setBrush(symbol->brush().color().dark(180));
 
	QwtPlotDirectPainter directPainter;
	directPainter.drawSeries(p_selectedCurve, p_selectedPoint, p_selectedPoint);
 
	if ( showIt )
		symbol->setBrush(brush); // reset brush
}

---

The panner

Code:

---

class CanvasPanner: public QwtPlotPanner
{
	Q_OBJECT
public:
	CanvasPanner(QwtFoilPlot *plot);
 
private:
 
 
};
 
CanvasPanner::CanvasPanner( QwtFoilPlot *plot ):
	QwtPlotPanner(plot->canvas())
{
	setMouseButton(Qt::MidButton);
}

---

The problem appear when I pan the canvas after a selection. When I pick a symbol on a curve by click, the symbol will be highlighted normally(1.png). Then I start to pan the canvas without releasing the mid button, the highlighted symbol will offset a little to upper left(2.png and 3.png). Then I release the button, the offset disappeared.

Did anyone see this problem before? Is there any simple way to solve this problem?
Thank you
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