Qwt spline fitter - it doesn't work as expected

[Uwe]

I tried your points with the following code:

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_curve_fitter.h>
#include <qwt_symbol.h>
 
int main(int argc, char **argv)
{
    const QColor color(35, 70, 106);
 
    QApplication a(argc, argv);
 
    QwtPlot plot;
    plot.setCanvasBackground(Qt::white);
    plot.resize(600,400);
    plot.show();
 
    const double x[] = { 0, 1, 2, 4, 5, 8 };
    const double y[] = { 17, 16.5, 8, 3, 12, 14.5 };
 
    // Insert new curves
    QwtPlotCurve *curve = new QwtPlotCurve();
    curve->setStyle(QwtPlotCurve::Lines);
    curve->setCurveAttribute(QwtPlotCurve::Fitted, true);
 
    QwtSymbol symbol;
    symbol.setStyle(QwtSymbol::Rect);
    symbol.setSize(10, 10);
    symbol.setBrush(color);
 
    curve->setSymbol(symbol);
 
    QwtSplineCurveFitter *fitter = new QwtSplineCurveFitter();
    fitter->setFitMode(QwtSplineCurveFitter::ParametricSpline);
    fitter->setSplineSize(100);
    curve->setCurveFitter(fitter);
 
    curve->setRenderHint(QwtPlotItem::RenderAntialiased);
    curve->setPen(QPen(color, 3));
 
    curve->setData(x, y, 6);
    curve->attach(&plot);
 
    return a.exec();
}

#include <qapplication.h>
#include <qwt_plot.h>
#include <qwt_plot_curve.h>
#include <qwt_curve_fitter.h>
#include <qwt_symbol.h>

int main(int argc, char **argv)
{
    const QColor color(35, 70, 106);

    QApplication a(argc, argv);

    QwtPlot plot;
    plot.setCanvasBackground(Qt::white);
    plot.resize(600,400);
    plot.show();

    const double x[] = { 0, 1, 2, 4, 5, 8 };
    const double y[] = { 17, 16.5, 8, 3, 12, 14.5 };

    // Insert new curves
    QwtPlotCurve *curve = new QwtPlotCurve();
    curve->setStyle(QwtPlotCurve::Lines);
    curve->setCurveAttribute(QwtPlotCurve::Fitted, true);

    QwtSymbol symbol;
    symbol.setStyle(QwtSymbol::Rect);
    symbol.setSize(10, 10);
    symbol.setBrush(color);

    curve->setSymbol(symbol);

    QwtSplineCurveFitter *fitter = new QwtSplineCurveFitter();
    fitter->setFitMode(QwtSplineCurveFitter::ParametricSpline);
    fitter->setSplineSize(100);
    curve->setCurveFitter(fitter);

    curve->setRenderHint(QwtPlotItem::RenderAntialiased);
    curve->setPen(QPen(color, 3));

    curve->setData(x, y, 6);
    curve->attach(&plot);

    return a.exec();
}

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The result is different compared to the OpenOffice curve, but what exactly makes you so unhappy with it ?

Uwe

PS: You can play with the spline size parameter. IMHO the curve looks better with a value of 50.