QwtSpline man page

QwtSpline —

A class for spline interpolation.


#include <qwt_spline.h>

Public Types

enum SplineType { Natural, Periodic }

Public Member Functions

const QwtArray< double > & coefficientsA () const

const QwtArray< double > & coefficientsB () const

const QwtArray< double > & coefficientsC () const

bool isValid () const

QwtSpline & operator= (const QwtSpline &)

QPolygonF points () const

QwtSpline ()

QwtSpline (const QwtSpline &)

void reset ()

bool setPoints (const QPolygonF &points)

void setSplineType (SplineType)

SplineType splineType () const

double value (double x) const

~QwtSpline ()

Protected Member Functions

bool buildNaturalSpline (const QPolygonF &)

bool buildPeriodicSpline (const QPolygonF &)

Protected Attributes

PrivateData * d_data

Detailed Description

A class for spline interpolation.

The QwtSpline class is used for cubical spline interpolation. Two types of splines, natural and periodic, are supported.


First call setPoints() to determine the spline coefficients for a tabulated function y(x).
After the coefficients have been set up, the interpolated function value for an argument x can be determined by calling QwtSpline::value().


#include <qwt_spline.h>

QPolygonF interpolate(const QPolygonF& points, int numValues)
    QwtSpline spline;
    if ( !spline.setPoints(points) ) 
        return points;

    QPolygonF interpolatedPoints(numValues);

    const double delta = 
        (points[numPoints - 1].x() - points[0].x()) / (points.size() - 1);
    for(i = 0; i < points.size(); i++)  / interpolate
        const double x = points[0].x() + i * delta;
    return interpolatedPoints;

Member Enumeration Documentation

enum QwtSpline::SplineType

Spline type.

Constructor & Destructor Documentation

QwtSpline::QwtSpline ()


QwtSpline::QwtSpline (const QwtSpline &other) Copy constructor


other Spline used for initilization

QwtSpline::~QwtSpline ()


Member Function Documentation

bool QwtSpline::buildNaturalSpline (const QPolygonF &points) [protected]

Determines the coefficients for a natural spline. Returns:

true if successful

bool QwtSpline::buildPeriodicSpline (const QPolygonF &points) [protected]

Determines the coefficients for a periodic spline. Returns:

true if successful

const QwtArray< double > & QwtSpline::coefficientsA () const Returns

A coefficients

const QwtArray< double > & QwtSpline::coefficientsB () const Returns

B coefficients

const QwtArray< double > & QwtSpline::coefficientsC () const Returns

C coefficients

bool QwtSpline::isValid () const

True if valid.

QwtSpline & QwtSpline::operator= (const QwtSpline &other) Assignment operator


other Spline used for initilization

QPolygonF QwtSpline::points () const Return points passed by setPoints

void QwtSpline::reset ()

Free allocated memory and set size to 0.

bool QwtSpline::setPoints (const QPolygonF &points)

Calculate the spline coefficients. Depending on the value of periodic, this function will determine the coefficients for a natural or a periodic spline and store them internally.


points Points


true if successful


The sequence of x (but not y) values has to be strictly monotone increasing, which means points[i].x() < points[i+1].x(). If this is not the case, the function will return false

void QwtSpline::setSplineType (SplineTypesplineType) Select the algorithm used for calculating the spline


splineType Spline type

See also:


QwtSpline::SplineType QwtSpline::splineType () const Returns

the spline type

See also:


double QwtSpline::value (doublex) const Calculate the interpolated function value corresponding to a given argument x.


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Qwt User's Guide Version 5.2.2 Mon Aug 1 2011