qwt_plot_curve.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 <qpainter.h>
#include <qpixmap.h>
#include <qbitarray.h>
#include "qwt_global.h"
#include "qwt_legend.h"
#include "qwt_legend_item.h"
#include "qwt_data.h"
#include "qwt_scale_map.h"
#include "qwt_double_rect.h"
#include "qwt_math.h"
#include "qwt_clipper.h"
#include "qwt_painter.h"
#include "qwt_plot.h"
#include "qwt_plot_canvas.h"
#include "qwt_curve_fitter.h"
#include "qwt_symbol.h"
#include "qwt_plot_curve.h"

#if QT_VERSION < 0x040000
#include <qguardedptr.h>
#else
#include <qpointer.h>
#endif

#if QT_VERSION >= 0x040000

#include <qevent.h>
#include <qpaintengine.h>

class QwtPlotCurvePaintHelper: public QObject
{
public:
    QwtPlotCurvePaintHelper(const QwtPlotCurve *curve, int from, int to):
        _curve(curve),
        _from(from),
        _to(to)
    {
    }

    virtual bool eventFilter(QObject *, QEvent *event)
    {
        if ( event->type() == QEvent::Paint )
        {
            _curve->draw(_from, _to);
            return true;
        }
        return false;
    }
private:
    const QwtPlotCurve *_curve;
    int _from;
    int _to;
};

#endif // QT_VERSION >= 0x040000

// Creating and initializing a QPainter is an
// expensive operation. So we keep an painter
// open for situations, where we paint outside
// of paint events. This improves the performance
// of incremental painting like in the realtime
// example a lot.
// But it is not possible to have more than
// one QPainter open at the same time. So we
// need to close it before regular paint events
// are processed.

class QwtGuardedPainter: public QObject
{
public:
    ~QwtGuardedPainter()
    {
        end();
    }

    QPainter *begin(QwtPlotCanvas *canvas)
    {
        _canvas = canvas;

        QMap<QwtPlotCanvas *, QPainter *>::iterator it = _map.find(_canvas);
        if ( it == _map.end() )
        {
            QPainter *painter = new QPainter(_canvas);
            painter->setClipping(true);
            painter->setClipRect(_canvas->contentsRect());

            it = _map.insert(_canvas, painter);
            _canvas->installEventFilter(this);
        }
#if QT_VERSION < 0x040000
        return it.data();
#else
        return it.value();
#endif
    }

    void end()
    {
        if ( _canvas )
        {
            QMap<QwtPlotCanvas *, QPainter *>::iterator it = _map.find(_canvas);
            if ( it != _map.end() )
            {
                _canvas->removeEventFilter(this);

#if QT_VERSION < 0x040000
                delete it.data();
#else
                delete it.value();
#endif
                _map.erase(it);
            }
        }
    }

    virtual bool eventFilter(QObject *, QEvent *event)
    {
        if ( event->type() == QEvent::Paint )
            end();

        return false;
    }

private:
#if QT_VERSION < 0x040000 
    QGuardedPtr<QwtPlotCanvas> _canvas;
#else
    QPointer<QwtPlotCanvas> _canvas;
#endif
    static QMap<QwtPlotCanvas *, QPainter *> _map;
};

QMap<QwtPlotCanvas *, QPainter *> QwtGuardedPainter::_map;

static int verifyRange(int size, int &i1, int &i2)
{
    if (size < 1) 
        return 0;

    i1 = qwtLim(i1, 0, size-1);
    i2 = qwtLim(i2, 0, size-1);

    if ( i1 > i2 )
        qSwap(i1, i2);

    return (i2 - i1 + 1);
}

class QwtPlotCurve::PrivateData
{
public:
    class PixelMatrix: private QBitArray
    {
    public:
        PixelMatrix(const QRect& rect):
            QBitArray(rect.width() * rect.height()),
            _rect(rect)
        {
            fill(false);
        }

        inline bool testPixel(const QPoint& pos)
        {
            if ( !_rect.contains(pos) )
                return false;

            const int idx = _rect.width() * (pos.y() - _rect.y()) + 
                (pos.x() - _rect.x());

            const bool marked = testBit(idx);
            if ( !marked )
                setBit(idx, true);

            return !marked;
        }

    private:
        QRect _rect;
    };

    PrivateData():
        curveType(Yfx),
        style(QwtPlotCurve::Lines),
        reference(0.0),
        attributes(0),
        paintAttributes(0)
    {
        symbol = new QwtSymbol();
        pen = QPen(Qt::black);
        curveFitter = new QwtSplineCurveFitter;
    }

    ~PrivateData()
    {
        delete symbol;
        delete curveFitter;
    }

    QwtPlotCurve::CurveType curveType;
    QwtPlotCurve::CurveStyle style;
    double reference;

    QwtSymbol *symbol;
    QwtCurveFitter *curveFitter;

    QPen pen;
    QBrush brush;

    int attributes;
    int paintAttributes;

    QwtGuardedPainter guardedPainter;
};

QwtPlotCurve::QwtPlotCurve():
    QwtPlotItem(QwtText())
{
    init();
}

QwtPlotCurve::QwtPlotCurve(const QwtText &title):
    QwtPlotItem(title)
{
    init();
}

QwtPlotCurve::QwtPlotCurve(const QString &title):
    QwtPlotItem(QwtText(title))
{
    init();
}

QwtPlotCurve::~QwtPlotCurve()
{
    delete d_xy;
    delete d_data;
}

void QwtPlotCurve::init()
{
    setItemAttribute(QwtPlotItem::Legend);
    setItemAttribute(QwtPlotItem::AutoScale);

    d_data = new PrivateData;
    d_xy = new QwtPolygonFData(QwtArray<QwtDoublePoint>());

    setZ(20.0);
}

int QwtPlotCurve::rtti() const
{
    return QwtPlotItem::Rtti_PlotCurve;
}

void QwtPlotCurve::setPaintAttribute(PaintAttribute attribute, bool on)
{
    if ( on )
        d_data->paintAttributes |= attribute;
    else
        d_data->paintAttributes &= ~attribute;
}

bool QwtPlotCurve::testPaintAttribute(PaintAttribute attribute) const
{
    return (d_data->paintAttributes & attribute);
}

void QwtPlotCurve::setStyle(CurveStyle style)
{
    if ( style != d_data->style )
    {
        d_data->style = style;
        itemChanged();
    }
}

QwtPlotCurve::CurveStyle QwtPlotCurve::style() const 
{ 
    return d_data->style; 
}

void QwtPlotCurve::setSymbol(const QwtSymbol &symbol )
{
    delete d_data->symbol;
    d_data->symbol = symbol.clone();
    itemChanged();
}

const QwtSymbol &QwtPlotCurve::symbol() const 
{ 
    return *d_data->symbol; 
}

void QwtPlotCurve::setPen(const QPen &pen)
{
    if ( pen != d_data->pen )
    {
        d_data->pen = pen;
        itemChanged();
    }
}

const QPen& QwtPlotCurve::pen() const 
{ 
    return d_data->pen; 
}

void QwtPlotCurve::setBrush(const QBrush &brush)
{
    if ( brush != d_data->brush )
    {
        d_data->brush = brush;
        itemChanged();
    }
}

const QBrush& QwtPlotCurve::brush() const 
{
    return d_data->brush;
}


void QwtPlotCurve::setData(const double *xData, const double *yData, int size)
{
    delete d_xy;
    d_xy = new QwtArrayData(xData, yData, size);
    itemChanged();
}

void QwtPlotCurve::setData(const QwtArray<double> &xData, 
    const QwtArray<double> &yData)
{
    delete d_xy;
    d_xy = new QwtArrayData(xData, yData);
    itemChanged();
}

#if QT_VERSION < 0x040000
void QwtPlotCurve::setData(const QwtArray<QwtDoublePoint> &data)
#else
void QwtPlotCurve::setData(const QPolygonF &data)
#endif
{
    delete d_xy;
    d_xy = new QwtPolygonFData(data);
    itemChanged();
}

void QwtPlotCurve::setData(const QwtData &data)
{
    delete d_xy;
    d_xy = data.copy();
    itemChanged();
}

void QwtPlotCurve::setRawData(const double *xData, const double *yData, int size)
{
    delete d_xy;
    d_xy = new QwtCPointerData(xData, yData, size);
    itemChanged();
}

QwtDoubleRect QwtPlotCurve::boundingRect() const
{
    if ( d_xy == NULL )
        return QwtDoubleRect(1.0, 1.0, -2.0, -2.0); // invalid

    return d_xy->boundingRect();
}

void QwtPlotCurve::draw(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap,
    const QRect &) const
{
    draw(painter, xMap, yMap, 0, -1);
}

void QwtPlotCurve::draw(int from, int to) const
{
    if ( !plot() )
        return;

    QwtPlotCanvas *canvas = plot()->canvas();

#if QT_VERSION >= 0x040000
#if 0
    if ( canvas->paintEngine()->type() == QPaintEngine::OpenGL )
    {
        /*
            OpenGL alway repaint the complete widget.
            So for this operation OpenGL is one of the slowest
            environments.
         */
        canvas->repaint();
        return;
    }
#endif

    if ( !canvas->testAttribute(Qt::WA_WState_InPaintEvent) &&
        !canvas->testAttribute(Qt::WA_PaintOutsidePaintEvent) )
    {
        /*
          We save curve and range in helper and call repaint.
          The helper filters the Paint event, to repeat
          the QwtPlotCurve::draw, but now from inside the paint
          event.
         */

        QwtPlotCurvePaintHelper helper(this, from, to);
        canvas->installEventFilter(&helper);

        const bool noSystemBackground =
            canvas->testAttribute(Qt::WA_NoSystemBackground);
        canvas->setAttribute(Qt::WA_NoSystemBackground, true);
        canvas->repaint();
        canvas->setAttribute(Qt::WA_NoSystemBackground, noSystemBackground);

        return;
    }
#endif

    const QwtScaleMap xMap = plot()->canvasMap(xAxis());
    const QwtScaleMap yMap = plot()->canvasMap(yAxis());

    if ( canvas->testPaintAttribute(QwtPlotCanvas::PaintCached) &&
        canvas->paintCache() && !canvas->paintCache()->isNull() )
    {
        QPainter cachePainter((QPixmap *)canvas->paintCache());
        cachePainter.translate(-canvas->contentsRect().x(),
            -canvas->contentsRect().y());

        draw(&cachePainter, xMap, yMap, from, to);
    }

#if QT_VERSION >= 0x040000
    if ( canvas->testAttribute(Qt::WA_WState_InPaintEvent) )
    {
        QPainter painter(canvas);

        painter.setClipping(true);
        painter.setClipRect(canvas->contentsRect());

        draw(&painter, xMap, yMap, from, to);
    }
    else
#endif
    {
        QPainter *painter = d_data->guardedPainter.begin(canvas);
        draw(painter, xMap, yMap, from, to);
    }
}

void QwtPlotCurve::draw(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    if ( !painter || dataSize() <= 0 )
        return;

    if (to < 0)
        to = dataSize() - 1;

    if ( verifyRange(dataSize(), from, to) > 0 )
    {
        painter->save();
        painter->setPen(QwtPainter::scaledPen(d_data->pen));

        /*
          Qt 4.0.0 is slow when drawing lines, but it's even 
          slower when the painter has a brush. So we don't
          set the brush before we really need it.
         */

        drawCurve(painter, d_data->style, xMap, yMap, from, to);
        painter->restore();

        if (d_data->symbol->style() != QwtSymbol::NoSymbol)
        {
            painter->save();
            drawSymbols(painter, *d_data->symbol, xMap, yMap, from, to);
            painter->restore();
        }
    }
}

void QwtPlotCurve::drawCurve(QPainter *painter, int style,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    switch (style)
    {
        case Lines:
            if ( testCurveAttribute(Fitted) )
            {
                // we always need the complete 
                // curve for fitting
                from = 0;
                to = dataSize() - 1;
            }
            drawLines(painter, xMap, yMap, from, to);
            break;
        case Sticks:
            drawSticks(painter, xMap, yMap, from, to);
            break;
        case Steps:
            drawSteps(painter, xMap, yMap, from, to);
            break;
        case Dots:
            drawDots(painter, xMap, yMap, from, to);
            break;
        case NoCurve:
        default:
            break;
    }
}

void QwtPlotCurve::drawLines(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    int size = to - from + 1;
    if ( size <= 0 )
        return;

    QwtPolygon polyline;
    if ( ( d_data->attributes & Fitted ) && d_data->curveFitter )
    {
        // Transform x and y values to window coordinates
        // to avoid a distinction between linear and
        // logarithmic scales.

#if QT_VERSION < 0x040000
        QwtArray<QwtDoublePoint> points(size);
#else
        QPolygonF points(size);
#endif
        for (int i = from; i <= to; i++)
        {
            QwtDoublePoint &p = points[i];
            p.setX( xMap.xTransform(x(i)) );
            p.setY( yMap.xTransform(y(i)) );
        }

        points = d_data->curveFitter->fitCurve(points);
        size = points.size();

        if ( size == 0 )
            return;

        // Round QwtDoublePoints to QPoints
        // When Qwt support for Qt3 has been dropped (Qwt 6.x)
        // we will use a doubles for painting and the following
        // step will be obsolete.

        polyline.resize(size);

        const QwtDoublePoint *p = points.data();
        QPoint *pl = polyline.data();
        if ( d_data->paintAttributes & PaintFiltered )
        {

            QPoint pp(qRound(p[0].x()), qRound(p[0].y()));
            pl[0] = pp;

            int count = 1;
            for (int i = 1; i < size; i++)
            {
                const QPoint pi(qRound(p[i].x()), qRound(p[i].y()));
                if ( pi != pp )
                {
                    pl[count++] = pi;
                    pp = pi;
                }
            }
            if ( count != size )
                polyline.resize(count);
        }
        else
        {
            for ( int i = 0; i < size; i++ )
            {
                pl[i].setX( qRound(p[i].x()) );
                pl[i].setY( qRound(p[i].y()) );
            }
        }
    }
    else
    {
        polyline.resize(size);

        if ( d_data->paintAttributes & PaintFiltered )
        {
            QPoint pp( xMap.transform(x(from)), yMap.transform(y(from)) );
            polyline.setPoint(0, pp);

            int count = 1;
            for (int i = from + 1; i <= to; i++)
            {
                const QPoint pi(xMap.transform(x(i)), yMap.transform(y(i)));
                if ( pi != pp )
                {
                    polyline.setPoint(count, pi);
                    count++;

                    pp = pi;
                }
            }
            if ( count != size )
                polyline.resize(count);
        }
        else
        {
            for (int i = from; i <= to; i++)
            {
                int xi = xMap.transform(x(i));
                int yi = yMap.transform(y(i));

                polyline.setPoint(i - from, xi, yi);
            }
        }
    }

    if ( d_data->paintAttributes & ClipPolygons )
        polyline = QwtClipper::clipPolygon(painter->window(), polyline);

    QwtPainter::drawPolyline(painter, polyline);

    if ( d_data->brush.style() != Qt::NoBrush )
        fillCurve(painter, xMap, yMap, polyline);
}

void QwtPlotCurve::drawSticks(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    int x0 = xMap.transform(d_data->reference);
    int y0 = yMap.transform(d_data->reference);

    for (int i = from; i <= to; i++)
    {
        const int xi = xMap.transform(x(i));
        const int yi = yMap.transform(y(i));

        if (d_data->curveType == Xfy)
            QwtPainter::drawLine(painter, x0, yi, xi, yi);
        else
            QwtPainter::drawLine(painter, xi, y0, xi, yi);
    }
}

void QwtPlotCurve::drawDots(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    const QRect window = painter->window();
    if ( window.isEmpty() )
        return;

    const bool doFill = d_data->brush.style() != Qt::NoBrush;

    QwtPolygon polyline;
    if ( doFill )
        polyline.resize(to - from + 1);

    if ( to > from && d_data->paintAttributes & PaintFiltered )
    {
        if ( doFill )   
        {
            QPoint pp( xMap.transform(x(from)), yMap.transform(y(from)) );

            QwtPainter::drawPoint(painter, pp.x(), pp.y());
            polyline.setPoint(0, pp);

            int count = 1;
            for (int i = from + 1; i <= to; i++)
            {
                const QPoint pi(xMap.transform(x(i)), yMap.transform(y(i)));
                if ( pi != pp )
                {
                    QwtPainter::drawPoint(painter, pi.x(), pi.y());

                    polyline.setPoint(count, pi);
                    count++;

                    pp = pi;
                }
            }
            if ( int(polyline.size()) != count )
                polyline.resize(count);
        }
        else
        {
            // if we don't need to fill, we can sort out
            // duplicates independent from the order

            PrivateData::PixelMatrix pixelMatrix(window);

            for (int i = from; i <= to; i++)
            {
                const QPoint p( xMap.transform(x(i)),
                    yMap.transform(y(i)) );

                if ( pixelMatrix.testPixel(p) )
                    QwtPainter::drawPoint(painter, p.x(), p.y());
            }
        }
    }
    else
    {
        for (int i = from; i <= to; i++)
        {
            const int xi = xMap.transform(x(i));
            const int yi = yMap.transform(y(i));
            QwtPainter::drawPoint(painter, xi, yi);

            if ( doFill )
                polyline.setPoint(i - from, xi, yi);
        }
    }

    if ( doFill )
    {
        if ( d_data->paintAttributes & ClipPolygons )
            polyline = QwtClipper::clipPolygon(painter->window(), polyline);

        fillCurve(painter, xMap, yMap, polyline);
    }
}

void QwtPlotCurve::drawSteps(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    QwtPolygon polyline(2 * (to - from) + 1);

    bool inverted = d_data->curveType == Yfx;
    if ( d_data->attributes & Inverted )
        inverted = !inverted;

    int i,ip;
    for (i = from, ip = 0; i <= to; i++, ip += 2)
    {
        const int xi = xMap.transform(x(i));
        const int yi = yMap.transform(y(i));

        if ( ip > 0 )
        {
            if (inverted)
                polyline.setPoint(ip - 1, polyline[ip-2].x(), yi);
            else
                polyline.setPoint(ip - 1, xi, polyline[ip-2].y());
        }

        polyline.setPoint(ip, xi, yi);
    }

    if ( d_data->paintAttributes & ClipPolygons )
        polyline = QwtClipper::clipPolygon(painter->window(), polyline);

    QwtPainter::drawPolyline(painter, polyline);

    if ( d_data->brush.style() != Qt::NoBrush )
        fillCurve(painter, xMap, yMap, polyline);
}


void QwtPlotCurve::setCurveAttribute(CurveAttribute attribute, bool on)
{
    if ( bool(d_data->attributes & attribute) == on )
        return;

    if ( on )
        d_data->attributes |= attribute;
    else
        d_data->attributes &= ~attribute;

    itemChanged();
}

bool QwtPlotCurve::testCurveAttribute(CurveAttribute attribute) const 
{ 
    return d_data->attributes & attribute;
}

void QwtPlotCurve::setCurveType(CurveType curveType)
{
    if ( d_data->curveType != curveType )
    {
        d_data->curveType = curveType;
        itemChanged();
    }
}

QwtPlotCurve::CurveType QwtPlotCurve::curveType() const
{
    return d_data->curveType;
}

void QwtPlotCurve::setCurveFitter(QwtCurveFitter *curveFitter)
{
    delete d_data->curveFitter;
    d_data->curveFitter = curveFitter;

    itemChanged();
}

QwtCurveFitter *QwtPlotCurve::curveFitter() const
{
    return d_data->curveFitter;
}

void QwtPlotCurve::fillCurve(QPainter *painter,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap,
    QwtPolygon &pa) const
{
    if ( d_data->brush.style() == Qt::NoBrush )
        return;

    closePolyline(xMap, yMap, pa);
    if ( pa.count() <= 2 ) // a line can't be filled
        return;

    QBrush b = d_data->brush;
    if ( !b.color().isValid() )
        b.setColor(d_data->pen.color());

    painter->save();

    painter->setPen(QPen(Qt::NoPen));
    painter->setBrush(b);

    QwtPainter::drawPolygon(painter, pa);

    painter->restore();
}

void QwtPlotCurve::closePolyline(
    const QwtScaleMap &xMap, const QwtScaleMap &yMap,
    QwtPolygon &pa) const
{
    const int sz = pa.size();
    if ( sz < 2 )
        return;

    pa.resize(sz + 2);

    if ( d_data->curveType == QwtPlotCurve::Xfy )
    {
        pa.setPoint(sz,
            xMap.transform(d_data->reference), pa.point(sz - 1).y());
        pa.setPoint(sz + 1,
            xMap.transform(d_data->reference), pa.point(0).y());
    }
    else
    {
        pa.setPoint(sz,
            pa.point(sz - 1).x(), yMap.transform(d_data->reference));
        pa.setPoint(pa.size() - 1,
            pa.point(0).x(), yMap.transform(d_data->reference));
    }
}

void QwtPlotCurve::drawSymbols(QPainter *painter, const QwtSymbol &symbol,
    const QwtScaleMap &xMap, const QwtScaleMap &yMap, 
    int from, int to) const
{
    painter->setBrush(symbol.brush());
    painter->setPen(QwtPainter::scaledPen(symbol.pen()));

    const QwtMetricsMap &metricsMap = QwtPainter::metricsMap();

    QRect rect;
    rect.setSize(metricsMap.screenToLayout(symbol.size()));

    if ( to > from && d_data->paintAttributes & PaintFiltered )
    {
        const QRect window = painter->window();
        if ( window.isEmpty() )
            return;

        PrivateData::PixelMatrix pixelMatrix(window);

        for (int i = from; i <= to; i++)
        {
            const QPoint pi( xMap.transform(x(i)),
                yMap.transform(y(i)) );

            if ( pixelMatrix.testPixel(pi) )
            {
                rect.moveCenter(pi);
                symbol.draw(painter, rect);
            }
        }
    }
    else
    {
        for (int i = from; i <= to; i++)
        {
            const int xi = xMap.transform(x(i));
            const int yi = yMap.transform(y(i));

            rect.moveCenter(QPoint(xi, yi));
            symbol.draw(painter, rect);
        }
    }
}

void QwtPlotCurve::setBaseline(double reference)
{
    if ( d_data->reference != reference )
    {
        d_data->reference = reference;
        itemChanged();
    }
}

double QwtPlotCurve::baseline() const 
{ 
    return d_data->reference; 
}

int QwtPlotCurve::dataSize() const
{
    return d_xy->size();
}

int QwtPlotCurve::closestPoint(const QPoint &pos, double *dist) const
{
    if ( plot() == NULL || dataSize() <= 0 )
        return -1;

    const QwtScaleMap xMap = plot()->canvasMap(xAxis());
    const QwtScaleMap yMap = plot()->canvasMap(yAxis());

    int index = -1;
    double dmin = 1.0e10;

    for (int i=0; i < dataSize(); i++)
    {
        const double cx = xMap.xTransform(x(i)) - pos.x();
        const double cy = yMap.xTransform(y(i)) - pos.y();

        const double f = qwtSqr(cx) + qwtSqr(cy);
        if (f < dmin)
        {
            index = i;
            dmin = f;
        }
    }
    if ( dist )
        *dist = sqrt(dmin);

    return index;
}

void QwtPlotCurve::updateLegend(QwtLegend *legend) const
{
    if ( !legend )
        return;

    QwtPlotItem::updateLegend(legend);

    QWidget *widget = legend->find(this);
    if ( !widget || !widget->inherits("QwtLegendItem") )
        return;

    QwtLegendItem *legendItem = (QwtLegendItem *)widget;

#if QT_VERSION < 0x040000
    const bool doUpdate = legendItem->isUpdatesEnabled();
#else
    const bool doUpdate = legendItem->updatesEnabled();
#endif
    legendItem->setUpdatesEnabled(false);

    const int policy = legend->displayPolicy();

    if (policy == QwtLegend::FixedIdentifier)
    {
        int mode = legend->identifierMode();

        if (mode & QwtLegendItem::ShowLine)
            legendItem->setCurvePen(pen());

        if (mode & QwtLegendItem::ShowSymbol)
            legendItem->setSymbol(symbol());

        if (mode & QwtLegendItem::ShowText)
            legendItem->setText(title());
        else
            legendItem->setText(QwtText());

        legendItem->setIdentifierMode(mode);
    }
    else if (policy == QwtLegend::AutoIdentifier)
    {
        int mode = 0;

        if (QwtPlotCurve::NoCurve != style())
        {
            legendItem->setCurvePen(pen());
            mode |= QwtLegendItem::ShowLine;
        }
        if (QwtSymbol::NoSymbol != symbol().style())
        {
            legendItem->setSymbol(symbol());
            mode |= QwtLegendItem::ShowSymbol;
        }
        if ( !title().isEmpty() )
        {
            legendItem->setText(title());
            mode |= QwtLegendItem::ShowText;
        }
        else
        {
            legendItem->setText(QwtText());
        }
        legendItem->setIdentifierMode(mode);
    }

    legendItem->setUpdatesEnabled(doUpdate);
    legendItem->update();
}