QT/QWT: add axis scale to Qpainter using QwtPlotScaleItem

Hello everybody,

i have a large Qt (4.8.4 / linux, opensuse) code which displays some data. I need to extend the main display window by adding axis scale. This window has a zoom (through mouse wheel), so the scale must adapt depending on the zoom. I read qwt (6.1) is appropriate for this task. I am performing some simple tests with qwt on a simplified code, but i am not able to display this scale using QwtPlotScaleItem, although I am successful at using qwt to display some curves !.

Can you tell me what I am doing wrong with the scale ?

Here is a section of the code ...

Code:

---

//Set scene, pixmap and get an image of the data
            scene = new QGraphicsScene(this);
            pixmapItem = new QGraphicsPixmapItem();
            scene->addItem(pixmapItem);
 
            view = new QGraphicsView();
            view->setScene(scene);
 
            setCentralWidget(view);
 
            imageSrc = new ImageSource();
            pixmapItem->setPixmap(*imageSrc->nextImage());
 
            QImage* imagerectangle;
            imagerectangle = new QImage(27500,250,QImage::Format_ARGB32);
            imagerectangle->fill(qRgba(0, 0, 0, 0));
 
            // Set a Qpainter
            QPainter* rectangle;
            rectangle = new QPainter(imagerectangle);
 
            //Pen properties
            QPen lepen;
            QRectF rect(5000,100,3000,50);
            lepen.setColor(Qt::red);
            lepen.setWidth(10);
 
 
            // TEST QWT: OK work -> a curve is displayed *****************************
            QwtPlotCurve d_curves;
            QwtScaleMap xMap;
            QwtScaleMap yMap;
 
            xMap.setScaleInterval(0, 10000);
            yMap.setScaleInterval(0, 250);
 
            // Calculate values
            double xval[10000];
            double yval[10000];
            for(int i=0; i<10000;i++)
            {
                xval[i] = i;
                yval[i] = qSin(xval[i]) * qCos(2.0 * xval[i]) * 100 +101;
            }
 
            d_curves.setPen(QColor(Qt::yellow));
            d_curves.setStyle(QwtPlotCurve::Lines);
            d_curves.setRenderHint(QwtPlotItem::RenderAntialiased);
            d_curves.setCurveAttribute(QwtPlotCurve::Fitted);
 
            d_curves.setRawSamples(xval, yval, 10000);
 
            const QRect &r = contentsRect();
 
            xMap.setPaintInterval(r.left(), r.right());
            yMap.setPaintInterval(r.top(), r.bottom());
 
            d_curves.draw(rectangle, xMap, yMap, r);
            d_curves.show();
            // So far this is good ! ************************************************
 
 
            // add a scale to the plot. DOES NOT WORK nothing is displayed ????????????
            QwtPlotScaleItem* echelle;
            echelle = new QwtPlotScaleItem();
            echelle->setAlignment(QwtScaleDraw::TopScale);
            echelle->setBorderDistance(0);
            echelle->scaleDraw()->setTickLength(QwtScaleDiv::MajorTick,100);
            echelle->scaleDraw()->setTickLength(QwtScaleDiv::MinorTick,50);
            echelle->scaleDraw()->enableComponent(QwtAbstractScaleDraw::Labels,true);
            rectangle->setRenderHint(QPainter::Antialiasing,
                        echelle->testRenderHint(QwtPlotItem::RenderAntialiased) );
            echelle->setVisible(TRUE);
            echelle->draw(rectangle,xMap,yMap,r);
            echelle->show();
            // what is wrong here ??????????????????????
 
 
           // add to scene
           scene->addPixmap(QPixmap::fromImage(*imagerectangle));

---

Thank you very much for your help !

Val

Well this code has not much to do with Qwt beside that it uses a plot curve to draw to a QGraphicsScene.

IMO this only makes sense, when you need to display something else together with a plot - otherwise I would remove the QGV stuff and use QwtPlot as view. But if there is really no way to get rid of the QGV code I would create a hidden QwtPlot widget creating the pixmap using QwtPlotRenderer.

Uwe

Thank you very much for your feedback ...

yes, keeping QGV is required for other features of the complete code.

I try the following without success ...

Code:

---

//Same as before
            scene = new QGraphicsScene(this);
            view = new QGraphicsView();
            view->setScene(scene);
            setCentralWidget(view);
 
 
// Create a QWTPLOT
QwtPlotCurve* m_curve= new QwtPlotCurve;
 
QwtPlot* testhidden = new QwtPlot;
testhidden->setHidden(TRUE);                          //Hide the plot
testhidden->setTitle("QwtPlotWidget");
testhidden->setAxisTitle(QwtPlot::yLeft,"Y");
testhidden->setAxisTitle(QwtPlot::xBottom,"X");
 
// Calculate some fake curve
double xxval[10000];
double yyval[10000];
for(int i=0; i<10000;i++)
{
    xxval[i] = i;
    yyval[i] = qSin(xxval[i]) * qCos(2.0 * xxval[i]) * 100 +101;
}
 
//
// attach
m_curve->setRawSamples(xxval, yyval,250);
m_curve->attach(testhidden);
 
testhidden->show(); // Not sure if needed
 
 
            //QT pixmap and rendering
            pixmapItem = new QGraphicsPixmapItem();
            scene->addItem(pixmapItem);
 
            QPixmap tmp;
            QwtPlotRenderer* plotrend = new QwtPlotRenderer;
            plotrend->renderTo(testhidden,tmp);
 
            //show the pixmap
            pixmapItem->setPixmap(tmp);
            // It compiles but fails to show the plot 8blank window) ...

---

Is it what you meant ? or did I miss something ?

Val

Sure because tmp has no size - but in general yes.

Uwe

I apologize for this mistake. I set up Qpixmap tmp(width,height).

The hidden QwtPlot is always right.

The QGV outcome is rather surprising. It shows a white rectangle framed by black borders. In the border I have got in gray color the axis names, the axis and plot title, but no curve, no axis labels.

I explored the color setup of the QwtPlot. If I setup QwtPlot to transparent -> QGV shows all black but for the axis name,axis and plot title. If I setup the background color of QwtPlot to red -> QGV shows a red rectangle with the black frame. No curve. So the background is displayed but not the curve.

I tries m_curve->show(); and m_curve->setVisible(TRUE); but to no avail.

Obviously, I am missing something with respect to the curve. I am carrying on to explore and to understand what is going on : this a good way to learn Qwt! I'll let you know if I find out a solution but at the same time I am open to any suggestion ...

Val

You need to fill your pixmap initially with some background - otherwise you might see some random trash from the graphics memory.

Uwe

I filled the QPixmap with some background color but that did not make it.

Reading through other examples, I have tried to "replot" the QwtPlot just before to "renderTo" the QwtPlotRenderer and it worked -> the curve and the axis label are shown correctly.

So my small example code is working and I can go ahead with the real code now.

Thank you very much Uwe. I have greatly appreciated your support.

Val