Slow plotting of spectrogram

[Uwe]

The very first thing I recommend to do is - again - enable DEBUG_RENDER in qwt_plot_spectrogram.cpp and check the time needed for your image composition.
What type of values have been reported ( of course in release mode ) ?

Uwe

[Anton1]

I set enable DEBUG_RENDER in qwt_plot_spectrogram.cpp.
If the spectrogram takes nearly full screen, there is the following result - renderImage QSize(1818, 909) 179
If the spectrogram takes small part of screen, there is the following result - renderImage QSize(705, 80) 11
Initial screen resolution is 1920x1080. The data size exceeds the size of the spectrogram.
Can you say what should be done?
Thank you in advance.

[Uwe]

QSize(1818, 909) 179
QSize(705, 80) 11

With the spectrogram example I can see on my box ( i7-3770T CPU @ 2.50GHz with 4 cores ) values like:

• QSize(484, 343) 3
• QSize(1804, 1075) 27

As my system is far away from being a burner I guess we are talking about some slow embedded device on your side ?
To compare the systems: could you check the numbers of the spectrogram example on your box ?

--

Concerning the "Incremental image composition":

class YourSpectrogram: public QwtPlotSpectrogram
{
    ....
 
protected:
    virtual QImage renderImage(
        const QwtScaleMap &xMap, const QwtScaleMap &yMap,
        const QRectF &area, const QSize &imageSize ) const override
    {
        if ( m_image.isNull() )
        {
            m_image = QwtPlotSpectrogram::renderImage(
                xMap, yMap, area, imageSize );
        }
        else
        {
            // find the new area, not being part of the previous image
            // and the width/height in pixels
 
            subArea = ...;
            subImageSize = ...
 
            QImage subImage = QwtPlotSpectrogram::renderImage(
                xMap, yMap, subArea, subImageSize );
 
            // shift the pixels in m_image and copy in those from subImage
            m_image = ...
        }
 
        return m_image;
    }
 
private:
    QImage m_image;
};

class YourSpectrogram: public QwtPlotSpectrogram
{
    ....

protected:
    virtual QImage renderImage(
        const QwtScaleMap &xMap, const QwtScaleMap &yMap,
        const QRectF &area, const QSize &imageSize ) const override
    {
        if ( m_image.isNull() )
        {
            m_image = QwtPlotSpectrogram::renderImage(
                xMap, yMap, area, imageSize );
        }
        else
        {
            // find the new area, not being part of the previous image
            // and the width/height in pixels

            subArea = ...;
            subImageSize = ...

            QImage subImage = QwtPlotSpectrogram::renderImage(
                xMap, yMap, subArea, subImageSize );

            // shift the pixels in m_image and copy in those from subImage
            m_image = ...
        }

        return m_image;
    }

private:
    QImage m_image;
};

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HTH,
Uwe

PS: if you have a good example for a waterfall plot that could be used as an example ( no huge data files ) you could send me your slow implementation. I would do the optimized implementation then.

[Anton1]

Hello Uwe!
In the spectrogram example I see values like: renderImage QSize(1745, 910) 36.
My computer have parameters i7-3770 CPU @ 3.40GHz with 4 cores.
I'm working on a model for a waterfall plot, I think that I can send it to you early next week.
Thank you in advance.

[Anton1]

Good day, Uwe.
I've done program that could be an example after some changes.
I give a link to the archive: https://cloud.mail.ru/public/LPcx/a7VbvrFGx
Now the program is running slowly, but I hope that it is possible to optimize.
Thank you in advance.

[Uwe]

Not sure what this tarball is supposed to be - looks like some downstripped version of your application, but without doing anything. To be honest I was interested in some simple data factory, that can be used for a waterfall plot - and this part is unfortunately missing.

But at least it shows the implementation of your SpectrogramData::value(), that explains why you have a worse image composition performance than the spectrogram example. Regardless of any further optimization I recommend to speed this one up. As it is called for every pixel it is worth to spend some time on writing it in a performant way.

Uwe