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

[Momergil]

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:

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 );
}

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 );
}

To copy to clipboard, switch view to plain text mode 

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

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

void QwtPlotCanvas::replot()
{
    invalidateBackingStore();

    if ( testPaintAttribute( QwtPlotCanvas::ImmediatePaint ) )
        repaint( contentsRect() );
    else
        update( contentsRect() );
}

To copy to clipboard, switch view to plain text mode 

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

[Uwe]

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:

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() );
}

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() );
}

To copy to clipboard, switch view to plain text mode 

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

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 );
}

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 );
}

To copy to clipboard, switch view to plain text mode 

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