Need to resize the window to make QwtPlotSpectrogram shows correctly

[embeddedmz]

Any news concerning this subject ? Sorry of insisting, but I want to adopt Qwt as the plotting library in our products (and tools) and some of the issues it brings (unlike QCP) are really frustrating.

Thanks.

[Uwe]

Any news concerning this subject ?

I had a quick look at your demo and the problem is related to the implementation of QWidget::updateGeometry().

When looking into the implementation you can see, that it blocks posting the LayoutRequest events, when the parent is not visible. In your situation updateGeometry is called from the scale widget and the parent is the plot widget being invisible. Not sure, why this is implemented like this in qwidget.cpp as updates are processed for QLayouts - so I'm tempted to call it a Qt bug.

A possible workaround for your demo would be:

QObject::connect(pushButton, &QPushButton::pressed, [this]()
{
   d_plot->d_spectrogram->setData( new SpectrogramData() );
   d_plot->setAxisScale( QwtPlot::yRight, 100000, 900000 );
 
   if ( !d_plot->isVisible() )
       d_plot->updateLayout(); // does not cause expensive operation, when being done before the replot
 
   d_plot->replot();
 });

QObject::connect(pushButton, &QPushButton::pressed, [this]()
{
   d_plot->d_spectrogram->setData( new SpectrogramData() );
   d_plot->setAxisScale( QwtPlot::yRight, 100000, 900000 );

   if ( !d_plot->isVisible() )
       d_plot->updateLayout(); // does not cause expensive operation, when being done before the replot

   d_plot->replot();
 });

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A possible workaround in qwt_scale_widget.cpp would be:

void QwtScaleWidget::layoutScale( bool update_geometry )
{
    ....
 
    if ( update_geometry )
    {
        updateGeometry();
        if ( auto w = parentWidget() )
        {
            if ( !w->isVisible() )
                QApplication::postEvent( w, new QEvent(QEvent::LayoutRequest ) );
        }
        update();
    }
}

void QwtScaleWidget::layoutScale( bool update_geometry )
{
    ....

    if ( update_geometry )
    {
        updateGeometry();
        if ( auto w = parentWidget() )
        {
            if ( !w->isVisible() )
                QApplication::postEvent( w, new QEvent(QEvent::LayoutRequest ) );
        }
        update();
    }
}

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Sorry of insisting, but I want to adopt Qwt as the plotting library in our products (and tools) and some of the issues it brings (unlike QCP) are really frustrating.

You would have a similar amount of problems when migrating the other way round. But QCP is a commercial product and you can pay for what you expect from me to do for free.

Uwe

[embeddedmz]

Hello Uwe,

First of all, thank you very much for your reply, that's very kind of you.

This workaround won't work. I have already tried it : calling updateLayout before replot won't fix the replot issue. However, I will use the condition to avoid updating the layout after the replot uselessly. I will try the second workaround another time (soon).

if ( !d_plot->isVisible() )
       d_plot->updateLayout(); // does not cause expensive operation, when being done before the replot

if ( !d_plot->isVisible() )
       d_plot->updateLayout(); // does not cause expensive operation, when being done before the replot

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Otherwise (in the same subject), I have an issue reploting the QwtPlotSpectrogram with a slider (screenshot in my second post) : I use the slider to rescale the data range and I am doing this :

// dLower, dUpper are double variables (new data range)
      m_plot->setAxisScale(QwtPlot::yRight, dLower, dUpper);
 
      QwtScaleWidget* axis = m_plot->axisWidget( QwtPlot::yRight );
      axis->setColorMap(QwtInterval(dLower, dUpper), new LinearColorMapRGB()); // I need to do this, otherwise, the color bar will change
      //axis->setColorBarEnabled(true); // already done elsewhere
 
      RawPlotData* spectroData = static_cast<RawPlotData*>(m_spectrogram.data());
      if (spectroData)
      {
         spectroData->SetZRange(dLower, dUpper); // here I'm just calling setInterval(Qt::ZAxis, QwtInterval(dLower, dUpper) on the QwtMatrixRasterData object
      }
 
      m_plot->replot(); // I tried also to call repaint after it etc...

// dLower, dUpper are double variables (new data range)
      m_plot->setAxisScale(QwtPlot::yRight, dLower, dUpper);

      QwtScaleWidget* axis = m_plot->axisWidget( QwtPlot::yRight );
      axis->setColorMap(QwtInterval(dLower, dUpper), new LinearColorMapRGB()); // I need to do this, otherwise, the color bar will change
      //axis->setColorBarEnabled(true); // already done elsewhere

      RawPlotData* spectroData = static_cast<RawPlotData*>(m_spectrogram.data());
      if (spectroData)
      {
         spectroData->SetZRange(dLower, dUpper); // here I'm just calling setInterval(Qt::ZAxis, QwtInterval(dLower, dUpper) on the QwtMatrixRasterData object
      }

      m_plot->replot(); // I tried also to call repaint after it etc...

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But the replot with or without updateLayout and/or repaint, doesn't update the shown result. Only resizing the window, replots the data with the new data range.

[Uwe]

This workaround won't work. I have already tried it : calling updateLayout before replot won't fix the replot issue.

The problem is about a missing layout update - not replotting. And on my box the workaround solved it in the demo code ( note, that updateLayout needs to be alled after setAxisScale and before replot ). So maybe you are talking about a different situation/problem now ?

Otherwise (in the same subject), I have an issue reploting the QwtPlotSpectrogram with a slider ...

Your code is changing attributes behind the back of the spectrogram item and therefore its internal cache is not invalidated. It is not wrong to do things this way, but then you have to help the spectrogram item by calling:

m_spectrogram->invalidateCache().

m_spectrogram->invalidateCache().

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HTH,
Uwe

[embeddedmz]

The problem is about a missing layout update - not replotting. And on my box the workaround solved it in the demo code ( note, that updateLayout needs to be alled after setAxisScale and before replot ). So maybe you are talking about a different situation/problem now ?

I called updateLayout just before replot, so after any eventual setAxisScale call. I even copied/pasted your code with the workaround in the demo and nothing changed !

My Qt version is 5.12.3 by the way and actually working under Ubuntu 18.04 (but will also run the program on Centos 7 later).

Like for the yRight axis's color bar issue (see the comment above), reassigning a new color map to the spectrogram object fixed my issue ! It's not pretty but it doesn't matter

m_spectrogram.setColorMap(new LinearColorMapRGB());

m_spectrogram.setColorMap(new LinearColorMapRGB());

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Invalidating the cache worked perfectly ! Awesome, thank you !

[Uwe]

I even copied/pasted your code with the workaround in the demo and nothing changed !!

Ah yes I only checked the other workaround. Calling updateLayout does not work because the missing LayoutRequest is from the autoscaler for the x axis, that happens in replot.

But this one should work:

QObject::connect(pushButton, &QPushButton::pressed, [this]()
    {
       d_plot->d_spectrogram->setData( new SpectrogramData() );
       d_plot->setAxisScale( QwtPlot::yRight, 100000, 900000 );
       QApplication::postEvent( d_plot, new QEvent(QEvent::LayoutRequest ) );
       d_plot->replot();
    });

QObject::connect(pushButton, &QPushButton::pressed, [this]()
    {
       d_plot->d_spectrogram->setData( new SpectrogramData() );
       d_plot->setAxisScale( QwtPlot::yRight, 100000, 900000 );
       QApplication::postEvent( d_plot, new QEvent(QEvent::LayoutRequest ) );
       d_plot->replot();
    });

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The situations is kind of specific as you need to do changes for an invisible plot, that has been shown before at least once. Sending the LayoutRequest manually is of course stupid and leads to a recalulation of the layout, that is not always necessary. But it doesn't result in recreating the image of your spectrogram, what is the heavy operation, that needs to be avoided.

The other workaround in qwt_scale_widget is a better one, as it avoids any pointless operation. I will add it as a fix to the Qwt code - something like this:

if ( update_geometry )
 {
     updateGeometry();
 
     if ( QWidget* w = parentWidget() )
     {
         if ( !w->isVisible() && w->layout() == NULL )
         {
             if ( w->testAttribute( Qt::WA_WState_Polished ) )
                QApplication::postEvent( w, new QEvent( QEvent::LayoutRequest ) );
         }
     }
 
     update();
 }

if ( update_geometry )
 {
     updateGeometry();

     if ( QWidget* w = parentWidget() )
     {
         if ( !w->isVisible() && w->layout() == NULL )
         {
             if ( w->testAttribute( Qt::WA_WState_Polished ) )
                QApplication::postEvent( w, new QEvent( QEvent::LayoutRequest ) );
         }
     }

     update();
 }

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HTH,
Uwe

[embeddedmz]

Neat !

I have two more questions (I'm really curious) :
- What does "Qt::WA_WState_Polished" mean ? is this made to handle the case when a plot has already been shown but was reploted when it's hidden ?
- As mentionned somewhere in this thread : what do you think about the fact that I need to set an expanding vertical and horizontal size policy for the QwtPlot which is a child of another QWidget so that the QwtPlot takes all the available space (like in the screenshots) and not like this : small_spectrogram.jpg

I have also a suggestion : when changing the range of data of a spectrogram, I need also to rescale the color bar of the yRight axis :

axis->setColorMap(QwtInterval(dLower, dUpper), new LinearColorMapRGB());

axis->setColorMap(QwtInterval(dLower, dUpper), new LinearColorMapRGB());

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The problem is that I need to make a memory allocation each time I need to rescale, whereas I only need to change the interval (first parameter). There's indeed this const method, but it returns a copy :

QwtInterval colorBarInterval() const;

QwtInterval colorBarInterval() const;

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It would be great if there's a non const method that returns a reference to the internal QwtInterval, so we can modify it without reallocating a new color map !

[Uwe]

What does "Qt::WA_WState_Polished" mean ?

It is an attribute, that is set once the widget has received a QEvent::Polish event - see https://doc.qt.io/qt-5/qwidget.html#ensurePolished.

A widget always gets an initial resize event that leads in case of the plot widget to calling updateLayout and there is no reason to trigger extra updates before that. So checking for Qt::WA_WState_Polished is a minor optimization for this initial phase.

what do you think about the fact that I need to set an expanding vertical and horizontal size policy for the QwtPlot which is a child of another QWidget so that the QwtPlot takes all the available space

A widget doesn't take any space it is always the parent that gives its children their geometries. So obviously your parent widget does this according to the size policies of its children. The default size policy of the plot widget is QSizePolicy::MinimumExpanding in both directions - why your parent does what it does then is hard to say without knowing your code.

Uwe

[moccij]

Hi all, first post in the forum. I had the same resize problem as the OP.

I wanted to show a bar plot:

BarChart::BarChart(QWidget *parent) :
    QwtCustomPlot(parent)
{
    QwtColumnSymbol *columnSymbol;
 
    columnSymbol = new QwtColumnSymbol(QwtColumnSymbol::Box);
    columnSymbol->setLineWidth(0);
    columnSymbol->setFrameStyle(QwtColumnSymbol::NoFrame);
    columnSymbol->setPalette(QPalette("Red"));
 
    _barChart = new QwtPlotBarChart();
    _barChart->setLayoutPolicy(QwtPlotBarChart::AutoAdjustSamples);
    _barChart->setSpacing(2);
    _barChart->setMargin(0);
    _barChart->setSymbol(columnSymbol);
    _barChart->attach(this);
 
    setAxisAutoScale(QwtPlot::yLeft);
}

BarChart::BarChart(QWidget *parent) :
    QwtCustomPlot(parent)
{
    QwtColumnSymbol *columnSymbol;

    columnSymbol = new QwtColumnSymbol(QwtColumnSymbol::Box);
    columnSymbol->setLineWidth(0);
    columnSymbol->setFrameStyle(QwtColumnSymbol::NoFrame);
    columnSymbol->setPalette(QPalette("Red"));

    _barChart = new QwtPlotBarChart();
    _barChart->setLayoutPolicy(QwtPlotBarChart::AutoAdjustSamples);
    _barChart->setSpacing(2);
    _barChart->setMargin(0);
    _barChart->setSymbol(columnSymbol);
    _barChart->attach(this);

    setAxisAutoScale(QwtPlot::yLeft);
}

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inherited from the custom plotter

QwtCustomPlot::QwtCustomPlot(QWidget *parent) :
    QwtPlot(parent)
{
    ((QFrame *)canvas())->setLineWidth(0);
    ((QFrame *)canvas())->setFrameStyle(QFrame::NoFrame);
    ((QFrame *)canvas())->setPalette(QColor("White"));
 
    plotLayout()->setCanvasMargin(0);
 
    setAttribute(Qt::WA_Hover);
 
    setAutoFillBackground(true);
    enableAxis(QwtPlot::xBottom, false);
    enableAxis(QwtPlot::yLeft, false);
 
    QwtText axisText;
    axisText.setFont(QApplication::font());
 
    setAxisTitle(QwtPlot::xBottom, axisText);
    setAxisTitle(QwtPlot::yLeft, axisText);
    setAxisFont(QwtPlot::xBottom, QApplication::font());
    setAxisFont(QwtPlot::yLeft, QApplication::font());
}

QwtCustomPlot::QwtCustomPlot(QWidget *parent) :
    QwtPlot(parent)
{
    ((QFrame *)canvas())->setLineWidth(0);
    ((QFrame *)canvas())->setFrameStyle(QFrame::NoFrame);
    ((QFrame *)canvas())->setPalette(QColor("White"));

    plotLayout()->setCanvasMargin(0);

    setAttribute(Qt::WA_Hover);

    setAutoFillBackground(true);
    enableAxis(QwtPlot::xBottom, false);
    enableAxis(QwtPlot::yLeft, false);

    QwtText axisText;
    axisText.setFont(QApplication::font());

    setAxisTitle(QwtPlot::xBottom, axisText);
    setAxisTitle(QwtPlot::yLeft, axisText);
    setAxisFont(QwtPlot::xBottom, QApplication::font());
    setAxisFont(QwtPlot::yLeft, QApplication::font());
}

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which sets the canvas margin to 0, hence filling the canvas with the bar plot.
Since a bar is 1 unit wide, the x axis range of n bars goes from -0.5 to n-0.5. I scale it manually at each update:

void BarChart::setBars(const QVector<double> &values)
{
    if(values.isEmpty())
        return;
 
    setAxisScale(
                QwtPlot::xBottom,
                -0.5,
                values.size()-0.5);
 
    _barChart->setSamples(values);
}

void BarChart::setBars(const QVector<double> &values)
{
    if(values.isEmpty())
        return;

    setAxisScale(
                QwtPlot::xBottom,
                -0.5,
                values.size()-0.5);

    _barChart->setSamples(values);
}

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This works as long as I don't resize the plot, with the bar at the extremes adjacent to the widget border.
However, resizing the plot resets the canvas margins to 0.5 units to the extremes, hence changing the layout.

The solution was to force the canvas margins to follow the scale

plotLayout()->setAlignCanvasToScales(true);

plotLayout()->setAlignCanvasToScales(true);

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The resize now keeps the layout unaltered!