qwt_bezier.cpp

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/* -*- mode: C++ ; c-file-style: "stroustrup" -*- *****************************
 * Qwt Widget Library
 * Copyright (C) 1997   Josef Wilgen
 * Copyright (C) 2002   Uwe Rathmann
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the Qwt License, Version 1.0
 *****************************************************************************/

#include "qwt_bezier.h"
#include <qstack.h>

namespace 
{
    class BezierData
    {
    public:
        inline BezierData()
        {
            // default constructor with unitialized points
        }

        inline BezierData( const QPointF &p1, const QPointF &cp1,
                const QPointF &cp2, const QPointF &p2 ):
            d_x1( p1.x() ),
            d_y1( p1.y() ),
            d_cx1( cp1.x() ),
            d_cy1( cp1.y() ),
            d_cx2( cp2.x() ),
            d_cy2( cp2.y() ),
            d_x2( p2.x() ),
            d_y2( p2.y() )
        {
        }

        static inline double minFlatness( double tolerance )
        {
            // we can make simplify the tolerance criterion check in
            // the subdivison loop cheaper, by precalculating some
            // flatness value.

            return 16 * ( tolerance * tolerance );
        }

        inline double flatness() const
        {
            // algo by Roger Willcocks ( http://www.rops.org )

            const double ux = 3.0 * d_cx1 - 2.0 * d_x1 - d_x2;
            const double uy = 3.0 * d_cy1 - 2.0 * d_y1 - d_y2;
            const double vx = 3.0 * d_cx2 - 2.0 * d_x2 - d_x1;
            const double vy = 3.0 * d_cy2 - 2.0 * d_y2 - d_y1;

            const double ux2 = ux * ux;
            const double uy2 = uy * uy;

            const double vx2 = vx * vx;
            const double vy2 = vy * vy;

            return qMax( ux2, vx2 ) + qMax( uy2, vy2 );
        }

        inline BezierData subdivided()
        {
            BezierData bz;

            const double c1 = midValue( d_cx1, d_cx2 );

            bz.d_cx1 = midValue( d_x1, d_cx1 );
            d_cx2 = midValue( d_cx2, d_x2 );
            bz.d_x1 = d_x1;
            bz.d_cx2 = midValue( bz.d_cx1, c1 );
            d_cx1 = midValue( c1, d_cx2 );
            bz.d_x2 = d_x1 = midValue( bz.d_cx2, d_cx1 );

            const double c2 = midValue( d_cy1, d_cy2 );

            bz.d_cy1 = midValue( d_y1, d_cy1 );
            d_cy2 = midValue( d_cy2, d_y2 );
            bz.d_y1 = d_y1;
            bz.d_cy2 = midValue( bz.d_cy1, c2 );
            d_cy1 = midValue( d_cy2, c2 );
            bz.d_y2 = d_y1 = midValue( bz.d_cy2, d_cy1 );

            return bz;
        }

        inline QPointF p2() const
        {
            return QPointF( d_x2, d_y2 );
        }

    private:
        inline double midValue( double v1, double v2 )
        {
            return 0.5 * ( v1 + v2 );
        }

        double d_x1, d_y1;
        double d_cx1, d_cy1;
        double d_cx2, d_cy2;
        double d_x2, d_y2;
    };
}

QwtBezier::QwtBezier( double tolerance ):
    m_tolerance( qMax( tolerance, 0.0 ) ),
    m_flatness( BezierData::minFlatness( m_tolerance ) )
{
}

QwtBezier::~QwtBezier()
{
}

void QwtBezier::setTolerance( double tolerance )
{
    m_tolerance = qMax( tolerance, 0.0 );
    m_flatness = BezierData::minFlatness( m_tolerance );
}

QPolygonF QwtBezier::toPolygon( const QPointF &p1,
    const QPointF &cp1, const QPointF &cp2, const QPointF &p2 ) const
{
    QPolygonF polygon;

    if ( m_flatness > 0.0 )
    {
        // a flatness of 0.0 is not achievable
        appendToPolygon( p1, cp1, cp2, p2, polygon );
    }

    return polygon;
}

void QwtBezier::appendToPolygon( const QPointF &p1, const QPointF &cp1,
    const QPointF &cp2, const QPointF &p2, QPolygonF &polygon ) const
{
    if ( m_flatness <= 0.0 )
    {
        // a flatness of 0.0 is not achievable
        return;
    }

    if ( polygon.isEmpty() || polygon.last() != p1 )
        polygon += p1;

    // to avoid deep stacks we convert the recursive algo
    // to something iterative, where the parameters of the
    // recursive class are pushed to a stack instead

    QStack<BezierData> stack;
    stack.push( BezierData( p1, cp1, cp2, p2 ) );

    while( true )
    {
        BezierData &bz = stack.top();

        if ( bz.flatness() < m_flatness )
        {
            if ( stack.size() == 1 )
            {
                polygon += p2;
                return;
            }

            polygon += bz.p2();
            stack.pop();
        }
        else
        {
            stack.push( bz.subdivided() );
        }
    }

}