How to plot the real time data in qwtplot while erasing fixed-width old data?

[lockdown]

QWidget does not offer to "erase" something - all you can do is to redraw from scratch. This is why QwtPlotDirectPainter is useful - it at least allows to draw on top of something existing.

Uwe

Another way to achieve the effect I want,and in this way, the CPU usage is also very small,but need to write a lot of code,use the most primitive QPainter , not as concise as the qwt.The code snippet is as follows:

void Mywidget::paintEvent(QPaintEvent* event)
{
    QPainter painter(this);
    painter.drawPixmap(0,0,m_gridPixmap);//paint grid backgroup pixmap
    painter.drawPixmap(0,0,m_curvePixmap);//paint curve pixmap on the top, and set QPainter::CompositionMode_Source mode when paint m_curvePixmap
    painter.end();
}

void Mywidget::paintEvent(QPaintEvent* event)
{
    QPainter painter(this);
    painter.drawPixmap(0,0,m_gridPixmap);//paint grid backgroup pixmap
    painter.drawPixmap(0,0,m_curvePixmap);//paint curve pixmap on the top, and set QPainter::CompositionMode_Source mode when paint m_curvePixmap
    painter.end();
}

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void Mywidget::updateCurvePixmap()
{
    QPainter painter;
    painter.begin(&m_curvePixmap);
    painter.setCompositionMode(QPainter::CompositionMode_Source);
 
    //Calculate the current  12 refreshed area first,and fill transparent
    for(int i = 0; i < 12;i++)
    {
        QRect rect;//The calculation process is omitted here!!!
        painter.fillRect(rect,Qt::transparent);
 
       //Plot newly added data points
        QPolygonF points;//The process of filling points is omitted!!!
        painter.drawPoints(points);
    }
    painter.end();
 
//At this point, the new data point is successfully partial updated on the m_curvePixmap,Partial refresh widget 
    QRegion region;//save all need updated area
    for(int i = 0; i < 12;i++)
    {
        QRect rect;//The calculation process is omitted here!!!
        region += rect;
    }
    update(region );
}

void Mywidget::updateCurvePixmap()
{
    QPainter painter;
    painter.begin(&m_curvePixmap);
    painter.setCompositionMode(QPainter::CompositionMode_Source);

    //Calculate the current  12 refreshed area first,and fill transparent
    for(int i = 0; i < 12;i++)
    {
        QRect rect;//The calculation process is omitted here!!!
        painter.fillRect(rect,Qt::transparent);

       //Plot newly added data points
        QPolygonF points;//The process of filling points is omitted!!!
        painter.drawPoints(points);
    }
    painter.end();

//At this point, the new data point is successfully partial updated on the m_curvePixmap,Partial refresh widget 
    QRegion region;//save all need updated area
    for(int i = 0; i < 12;i++)
    {
        QRect rect;//The calculation process is omitted here!!!
        region += rect;
    }
    update(region );
}

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Refer to this example:https://doc.qt.io/qt-5/qtwidgets-pai...n-example.html,
Used this class method:QPainter::setCompositionMode.

In order to achieve the effect I want, is this the only way?

[Uwe]

Guess what you mean by this code snippet is that you draw each curve to its own pixmap first and the plot is done finally by drawing the pixmaps.
Then you would have only one drawPolyline call, when only one curve has changed.
Of course the code as being posted is not even close to what needs to be done in reality - but the idea is understood.

So your code will have a positive effect, when having several curves, that are updated on different intervals - but it won't help for a situation with one curve and many points.

Uwe

[lockdown]

Guess what you mean by this code snippet is that you draw each curve to its own pixmap first and the plot is done finally by drawing the pixmaps.
Then you would have only one drawPolyline call, when only one curve has changed.
Of course the code as being posted is not even close to what needs to be done in reality - but the idea is understood.

So your code will have a positive effect, when having several curves, that are updated on different intervals - but it won't help for a situation with one curve and many points.

Uwe

All curves plot in the only m_curvePixmap,I update the code snippet in the previous reply. All curves partial update at the same time ,update every 30ms.

The original code looks too complicated,I just want to try to modify it as simple as the qwt.

[Uwe]

All curves plot in the only m_curvePixmap,....

Then your code does not make any sense.

Uwe

[lockdown]

Then your code does not make any sense.

Uwe

I have renewed.Thanks for the quick reply!

What I don’t understand is why these my two methods are so different, why the CPU consumption is so different? Looks like they are doing the same partial refresh.

Maybe as you said:QWidget does not offer to "erase" something - all you can do is to redraw from scratch.

[Uwe]

What I don’t understand is why these my two methods are so different, why the CPU consumption is so different?

Hard to say what your code is doing as the code snippet does not show the real code.

But the plot canvas is actually using a backing store ( a QPixmap ), that gets updated on certain operations like when the size of the canvas is changing or replot has been called.
Beside that the content of the widget is always restored from this backing store only.

So when calling QWidget::update( region ) only all what happens is, that the rectangles from region will be copied from the backing store of the canvas to another backing store - the default one every widgets has.

Uwe

[Mazzin]

@lockdown Have you found a way to achieve this? because I am running in circles about the same issue and that would be really great help.