Qwt User's Guide: QwtWeedingCurveFitter Class Reference

Qwt User's Guide 6.1.5

Public Member Functions | List of all members
QwtWeedingCurveFitter Class Reference

A curve fitter implementing Douglas and Peucker algorithm. More...

`#include <qwt_curve_fitter.h>`

Inheritance diagram for QwtWeedingCurveFitter:
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## Public Member Functions
| QwtWeedingCurveFitter (double tolerance=1.0)
virtual | ~QwtWeedingCurveFitter ()
| Destructor.
void | setTolerance (double)
double | tolerance () const
void | setChunkSize (uint)
uint | chunkSize () const
virtual QPolygonF | fitCurve (const QPolygonF &) const
Public Member Functions inherited from QwtCurveFitter
virtual | ~QwtCurveFitter ()
| Destructor.

## Additional Inherited Members
Protected Member Functions inherited from QwtCurveFitter
| QwtCurveFitter ()
| Constructor.

## Detailed Description

A curve fitter implementing Douglas and Peucker algorithm.

The purpose of the Douglas and Peucker algorithm is that given a 'curve' composed of line segments to find a curve not too dissimilar but that has fewer points. The algorithm defines 'too dissimilar' based on the maximum distance (tolerance) between the original curve and the smoothed curve.

The runtime of the algorithm increases non linear ( worst case O( n*n ) ) and might be very slow for huge polygons. To avoid performance issues it might be useful to split the polygon ( setChunkSize() ) and to run the algorithm for these smaller parts. The disadvantage of having no interpolation at the borders is for most use cases irrelevant.

The smoothed curve consists of a subset of the points that defined the original curve.

In opposite to QwtSplineCurveFitter the Douglas and Peucker algorithm reduces the number of points. By adjusting the tolerance parameter according to the axis scales QwtSplineCurveFitter can be used to implement different level of details to speed up painting of curves of many points.

## Constructor & Destructor Documentation

## ◆ QwtWeedingCurveFitter()

QwtWeedingCurveFitter::QwtWeedingCurveFitter | ( | double | tolerance = `1.0` | ) |

Constructor

Parameters

  tolerance | Tolerance

See also
  setTolerance(), tolerance()

## Member Function Documentation

## ◆ chunkSize()

uint QwtWeedingCurveFitter::chunkSize | ( |  | ) | const

Returns
  Maximum for the number of points passed to a run of the algorithm - or 0, when unlimited

See also
  setChunkSize()

## ◆ fitCurve()

QPolygonF QwtWeedingCurveFitter::fitCurve | ( | const QPolygonF & | points | ) | const
| virtual

Parameters

  points | Series of data points

Returns
  Curve points

Implements QwtCurveFitter.

## ◆ setChunkSize()

void QwtWeedingCurveFitter::setChunkSize | ( | uint | numPoints | ) |

Limit the number of points passed to a run of the algorithm

The runtime of the Douglas Peucker algorithm increases non linear with the number of points. For a chunk size > 0 the polygon is split into pieces passed to the algorithm one by one.

Parameters

  numPoints | Maximum for the number of points passed to the algorithm

See also
  chunkSize()

## ◆ setTolerance()

void QwtWeedingCurveFitter::setTolerance | ( | double | tolerance | ) |

Assign the tolerance

The tolerance is the maximum distance, that is acceptable between the original curve and the smoothed curve.

Increasing the tolerance will reduce the number of the resulting points.

Parameters

  tolerance | Tolerance

See also
  tolerance()

## ◆ tolerance()

double QwtWeedingCurveFitter::tolerance | ( |  | ) | const

Returns
  Tolerance

See also
  setTolerance()