QWT- Help with plotting data from a vector.

Hi all,

I am a newbie in QWT. Just wish to ask how can I plot data from a vector. The code below shows what I have done so far. Basically, the data I am storing into the vector are pixel values of an image along the axis. After that, I proceed to plot them, but there is always an error. I am not sure how to exactly do them either:/ I been trying to plot for almost 2 weeks plus already. But to no avail. I am still confused over the curve as well, why is there another vector needed for the setSamples() of curve?

Code:

---

vector<byte> v_char; //values of pixels taken from y-axis //tried double and int as well.
 
	//store pixel values into v_char
	v_char.reserve(image.rows * image.cols);
 
	for (int i = 0; i < image.rows; i++)
	{
		  int segment_start = *(uchar*)image.data + i * image.step;
		  v_char.insert(v_char.end(), segment_start, segment_start + image.cols);
	}
 
	//plotting of v_char vector
	QwtPlot *plota=new QwtPlot(); //Create plot widget
    plota->setTitle( "Plot Analysis" ); //Name the plot
    plota->setCanvasBackground( Qt::black ); //Set the Background colour
    plota->setAxisScale( QwtPlot::yLeft, 0, 1000000 ); //Scale the y-axis
    plota->setAxisScale(QwtPlot::xBottom,0,image.rows); //Scale the x-axis
    plota->insertLegend(new QwtLegend()); //Insert a legend
 
	//attach curve to plota
	QwtPlotCurve *curvea = new QwtPlotCurve(); // Create a curve
   // curvea->setTitle("Count"); //Name the curve
    curvea->setPen( Qt::white, 2);//Set colour and thickness for drawing the curve 
    //Use Antialiasing to improve plot render quality
    curvea->setRenderHint( QwtPlotItem::RenderAntialiased, true );
 
	QPolygonF points;
    for( int h = 0; h < v_char.size(); ++h) 
	{
        float bin_value = v_char[h];
        points << QPointF((float)h, bin_value);
    }
    curvea->setSamples(points); //pass points to be drawn on the curve
    curvea->attach( plota ); // Attach curve to the plot 
 
	plota->resize( 600, 400 ); //Resize the plot
	//plot.replot();
    plota->show(); //Show plot

---

If anybody do know, please please do help, I already at my wits end. I can't seem to find an example or tutorial online for QWT plots. I already tried lots of things... Is there a problem with my code? Or my whole programming flow is wrong? Please help. Thanks.

If you have'nt built the examples in the examples directory of qwt, then do so. Next select a project from the examples directory and run it. This is a quick way to know if all is well and you are ready to go. Do this and give a feedback

Quote:

---

Originally Posted by Cah

If you have'nt built the examples in the examples directory of qwt, then do so. Next select a project from the examples directory and run it. This is a quick way to know if all is well and you are ready to go. Do this and give a feedback

---

@Cah, Hi, I have done them. So the configurations should be all well. I have used QWT with QT with success. I am now using QWT via Microsoft Studio 2010 C++, QT add in. I have also managed to plot a histogram via the QT add in so far with QWT. So the configuration should be fine. Just that, I having problems when it come to plotting from a vector. Stackoverflow did have somebody pose the same question too, but the solution posted was not sufficient for me to solve my problem :(

you have any idea how I can code it to successfully plot from the vector? Thanks(:

Try this ....

Code:

---

#include <QApplication>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
 
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QwtPlot plot;
 
    QPolygonF data;
    data << QPointF(0, 9) << QPointF(1, 4.5) << QPointF(2,4.5) << QPointF(3,0);
 
    QwtPlotCurve* curve = new QwtPlotCurve;
    curve->setSamples(data);
 
    curve->attach(&plot);
    plot.show();
 
    return a.exec();
}

---

Attachment 9953

Done. This is the result. Anyway, I made a quick change.

Instead of using QwtPlot plot;, I used QwtPlot *plot=new QwtPlot();

Quote:

---

Originally Posted by Cah

Try this ....

Code:

---

#include <QApplication>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
 
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QwtPlot plot;
 
    QPolygonF data;
    data << QPointF(0, 9) << QPointF(1, 4.5) << QPointF(2,4.5) << QPointF(3,0);
 
    QwtPlotCurve* curve = new QwtPlotCurve;
    curve->setSamples(data);
 
    curve->attach(&plot);
    plot.show();
 
    return a.exec();
}

---

---

Quote:

---

Originally Posted by rockinfresh

... why is there another vector needed for the setSamples() of curve?

---

You have the option to copy your samples into a type of buffer that can be handled by QwtPlotCurve out of the box - or you can tell the curve how to read from your vector<byte> buffer:
simply derive from QwtSeriesData<QPointF>: http://qwt.sourceforge.net/class_qwt_series_data.html

Uwe