Qwt dynamic x axis labels

[K4ELO]

I am making a spectrum display based on the RtFFT code.
My problem is how to set the x axis scale labels dynamically.
Looked at the cpuplot example and they are set dynamically but in an incremental fashion.
Mine need to be recalculated on the fly. The setAxisScale is done using an Xmin value and the set sample rate, so those are the axis labels that show up on the display. The center of the display will correspond to a radio frequency value that is known.
I know how to calculate the correct label values, but how to apply them? QwtScaleDraw? But I don't see any function to set the label values.

Thanks for any help.

[Uwe]

I am making a spectrum display based on the RtFFT code.
My problem is how to set the x axis scale labels dynamically.

The labels are always for the major ticks. Is your question about how to set the positions of the major ticks - or how to modify the tick labels ?
.

Looked at the cpuplot example and they are set dynamically but in an incremental fashion..

The cpuplot example sets the range of the axis scale dynamically - it doesn't deal with ticks at all. The tick labels display a date time string instead of a number..

If all you want to do is to set the range and the positions of the ticks manually QwtPlot::setAxisScaleDiv() is what you are looking for.

If true note that with Qwt 6.0 you can't use the default constructor of QwtScaleDiv, you have to use one of the other constructors ( as this issue is gone with Qwt 6.1 I don't go into detail ). Otherwise - please repeat your question somehow related to the terminology of my answer.

Uwe

[K4ELO]

Thanks Uwe. I just need to change the labels. For example, if the radio is tuned to a frequency of 7.190 mHz, then the x axis label in the center of the display should be 7.190 The x min would display 7.190 - sample rate/2 and the x max would display 7.190 + sample rate/2

I tried the following code:

class NewScaleDraw : public QwtScaleDraw
{
public:
    NewScaleDraw()
    {
    }
 
    virtual QwtText label(double value) const
    {
        float freqLabel = (::centerFreq-(value/2000000));
        QString strFreqLabel = QString("%1").arg(freqLabel);
        return strFreqLabel;
    }
};
 
void RtControl::newScale(QString freq)
{
    d_.view->setAxisScaleDraw(QwtPlot::xTop, new NewScaleDraw());
}

class NewScaleDraw : public QwtScaleDraw
{
public:
    NewScaleDraw()
    {
    }

    virtual QwtText label(double value) const
    {
        float freqLabel = (::centerFreq-(value/2000000));
        QString strFreqLabel = QString("%1").arg(freqLabel);
        return strFreqLabel;
    }
};

void RtControl::newScale(QString freq)
{
    d_.view->setAxisScaleDraw(QwtPlot::xTop, new NewScaleDraw());
}

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Where newScale is called each time the radio changes frequency, but the labels are not changing.
Is this the correct approach?

[Uwe]

You have to do a replot() - or at least do a QwtPlot::updateAxes() to make your changes happen.

Creating and assigning a new NewScaleDraw object each time is o.k. - but it would be enough to use one object, where you invalidate its cache:

class NewScaleDraw : public QwtScaleDraw
{
public:
   ...
   void reset()
   {
       invalidateCache();
   }
};
 
void RtControl::newScale(QString freq)
{
   dynamic_cast<NewScaleDraw>(d_view->axisScaleDraw())->reset(); 
   d_view->updateAxes();
}

class NewScaleDraw : public QwtScaleDraw
{
public:
   ...
   void reset()
   {
       invalidateCache();
   }
};

void RtControl::newScale(QString freq)
{
   dynamic_cast<NewScaleDraw>(d_view->axisScaleDraw())->reset(); 
   d_view->updateAxes();
}

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Uwe

[K4ELO]

Thanks Uwe - still not working but I'm not giving up. My NewScaleDraw gets executed on app startup but not when the frequency changes which is whats needed. So the axis labels are still fixed at the original values based on the sample rate.

Correction: when this line of code executes
d_.view->setAxisScaleDraw(QwtPlot::xTop, new NewScaleDraw);

The NewScaleDraw class constructor is executed, but not the
virtual QwtText label(double value) const