Make a partial replot using reimplemented QwtPlot::replot()

Hello!

Is there a way for doing a partial replot using a reimplemented QwtPlot::replot()? This partial replot would be essentially to tell which area of the graph I want to be reploted instead of the entire canvas.

I noticed the code for QwtPlot::replot() in 6.1.0 is:

Code:

---

void QwtPlot::replot()
{
    bool doAutoReplot = autoReplot();
    setAutoReplot( false );
 
    updateAxes();
 
    /*
      Maybe the layout needs to be updated, because of changed
      axes labels. We need to process them here before painting
      to avoid that scales and canvas get out of sync.
     */
    QApplication::sendPostedEvents( this, QEvent::LayoutRequest );
 
    if ( d_data->canvas )
    {
        const bool ok = QMetaObject::invokeMethod( 
            d_data->canvas, "replot", Qt::DirectConnection );
        if ( !ok )
        {
            // fallback, when canvas has no a replot method
            d_data->canvas->update( d_data->canvas->contentsRect() );
        }
    }
 
    setAutoReplot( doAutoReplot );
}

---

When I tried, the problems arised when considering the "replot" method of the canvas:

Code:

---

void QwtPlotCanvas::replot()
{
    invalidateBackingStore();
 
    if ( testPaintAttribute( QwtPlotCanvas::ImmediatePaint ) )
        repaint( contentsRect() );
    else
        update( contentsRect() );
}

---

If I understood correctly, I'll have to create a new QwtPlotCanvas for my custom QwtPlot with a reimplemented replot() method in which instead of using contentsRect(), I'll use a QRect with customized form previously set by a new method.

Is this all ok or did I miss something?


Thanks,

Momergil

Assuming that you have new curve points in the area [x1,x2] ( in plot coordinates ) and the backingstore ( QwtPlotCanvas::BackingStore ) of the canvas is disabled:

Code:

---

void updatePlot( QwtPlot* plot, double x1, double x2 )
{
    const QwtScaleMap xMap = plot->canvasMap( QwtPlot::xBottom );
 
    const int l = xMap.transform( x1 );
    const int r = xMap.transform( x2 );
    const QRect cr = plot->canvas()->contentsRect();
 
    plot->canvas()->repaint( l, cr.y(), r - l, cr,height() );
}

---

The rectangle you have passed to the paint event will end up as clip region of the painter and you could overload QwtPlot::drawItems():

Code:

---

virtual void YourPlot::drawItems( QPainter *painter, const QRectF &canvasRect,
        const QwtScaleMap maps[axisCnt] ) const
{
    const QRectF rect = painter->clipRegion().boundingRect();
 
    // rect should have been filled with the background of the canvas
    // before. When you have other plot items ( f.e. a grid ) you have to restore
    // them for rect() too.
 
    ...
 
    double x1 = maps[curve->xAxis()].invTransform( rect.x() );
    double x2 = maps[curve->xAxis()].invTransform( rect.x() + rect.width()  );
 
    // find the segment of your curve for [x1, x2]
    // f.e. using qwtUpperSampleIndex() ?
 
    int from = ...;
    int to = ...
 
    curve->drawSeries( painter, 
        maps[curve->xAxis()], maps[curve->yAxis()], canvasRect, from, to );
}

---

You could try to use QwtPlotGLCanvas, that offers hardware acceleration and its limitations should be no big issue for your type of application. For QwtPlotCanvas ( = anything derived from QWidget ) there is only Qt4/X11 ( native graphicssystem ) where you have hardware acceleration.

Uwe