Qwt User's Guide: QwtBezier Class Reference

Qwt User's Guide 6.3.0

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Public Member Functions | Static Public Member Functions | List of all members
QwtBezier Class Reference

An implementation of the de Casteljau’s Algorithm for interpolating Bézier curves. More...

`#include <qwt_bezier.h>`

## Public Member Functions
| QwtBezier (double tolerance=0.5)
| Constructor.
| ~QwtBezier ()
| Destructor.
void | setTolerance (double tolerance)
double | tolerance () const
QPolygonF | toPolygon (const QPointF &p1, const QPointF &cp1, const QPointF &cp2, const QPointF &p2) const
| Interpolate a Bézier curve by a polygon.
void | appendToPolygon (const QPointF &p1, const QPointF &cp1, const QPointF &cp2, const QPointF &p2, QPolygonF &polygon) const
| Interpolate a Bézier curve by a polygon.

## Static Public Member Functions
static QPointF | pointAt (const QPointF &p1, const QPointF &cp1, const QPointF &cp2, const QPointF &p2, double t)

## Detailed Description

An implementation of the de Casteljau’s Algorithm for interpolating Bézier curves.

The flatness criterion for terminating the subdivision is based on "Piecewise Linear Approximation of Bézier Curves" by Roger Willcocks ( http://www.rops.org )

This article explains the maths behind in a very nice way: https://jeremykun.com/2013/05/11/bezier-curves-and-picasso

Definition at line 29 of file qwt_bezier.h.

## Constructor & Destructor Documentation

## ◆ QwtBezier()

QwtBezier::QwtBezier | ( | double | tolerance = 0.5 | ) |

Constructor.

Parameters

  tolerance | Termination criterion for the subdivision

See also
  setTolerance()

Definition at line 116 of file qwt_bezier.cpp.

## ◆ ~QwtBezier()

QwtBezier::~QwtBezier | ( |  | ) |

Destructor.

Definition at line 123 of file qwt_bezier.cpp.

## Member Function Documentation

## ◆ appendToPolygon()

void QwtBezier::appendToPolygon | ( | const QPointF & | p1,
|  | const QPointF & | cp1,
|  | const QPointF & | cp2,
|  | const QPointF & | p2,
|  | QPolygonF & | polygon ) const

Interpolate a Bézier curve by a polygon.

appendToPolygon() is tailored for cumulating points from a sequence of bezier curves like being created by a spline interpolation.

Parameters

  p1 | Start point
  cp1 | First control point
  cp2 | Second control point
  p2 | End point
  polygon | Polygon, where the interpolating points are added

Note
  If the last point of the incoming polygon matches p1 it won't be inserted a second time.

Definition at line 186 of file qwt_bezier.cpp.

## ◆ pointAt()

QPointF QwtBezier::pointAt | ( | const QPointF & | p1,
|  | const QPointF & | cp1,
|  | const QPointF & | cp2,
|  | const QPointF & | p2,
|  | double | t )
| static

Find a point on a Bézier Curve

Parameters

  p1 | Start point
  cp1 | First control point
  cp2 | Second control point
  p2 | End point
  t | Parameter value, something between [0,1]

Returns
  Point on the curve

Definition at line 239 of file qwt_bezier.cpp.

## ◆ setTolerance()

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

Set the tolerance

The tolerance is a measurement for the flatness of a curve. A curve with a flatness below the tolerance is considered as being flat terminating the subdivision algorithm.

When interpolating a Bezier curve to render it as a sequence of lines to some sort of raster ( f.e to screen ) a value of 0.5 of the pixel size is a good value for the tolerance.

Parameters

  tolerance | Termination criterion for the subdivision

See also
  tolerance()

Definition at line 141 of file qwt_bezier.cpp.

## ◆ tolerance()

double QwtBezier::tolerance | ( |  | ) | const
| inline

Returns
  Tolerance, that is used as criterion for the subdivision

See also
  setTolerance()

Definition at line 56 of file qwt_bezier.h.

## ◆ toPolygon()

QPolygonF QwtBezier::toPolygon | ( | const QPointF & | p1,
|  | const QPointF & | cp1,
|  | const QPointF & | cp2,
|  | const QPointF & | p2 ) const

Interpolate a Bézier curve by a polygon.

Parameters

  p1 | Start point
  cp1 | First control point
  cp2 | Second control point
  p2 | End point

Returns
  Interpolating polygon

Definition at line 157 of file qwt_bezier.cpp.