Qwt spectrogram with dashed contour

[Pierrick]

Hi,

I'm trying to display contour line of a qwt spectrogramm as dashed line.
Thus I sligthly modified the spectrogram exemple

void Plot::showContour( bool on )
{
    d_spectrogram->setDisplayMode( QwtPlotSpectrogram::ContourMode, on );
    d_spectrogram->setDefaultContourPen(QColor(Qt::yellow),3, Qt::DashLine);
    replot();
}
 
void Plot::showSpectrogram( bool on )
{
    d_spectrogram->setDisplayMode( QwtPlotSpectrogram::ImageMode, on );
    //d_spectrogram->setDefaultContourPen(
     //   on ? QPen( Qt::black, 0 ) : QPen( Qt::NoPen ) );
 
    replot();
}

void Plot::showContour( bool on )
{
    d_spectrogram->setDisplayMode( QwtPlotSpectrogram::ContourMode, on );
    d_spectrogram->setDefaultContourPen(QColor(Qt::yellow),3, Qt::DashLine);
    replot();
}

void Plot::showSpectrogram( bool on )
{
    d_spectrogram->setDisplayMode( QwtPlotSpectrogram::ImageMode, on );
    //d_spectrogram->setDefaultContourPen(
     //   on ? QPen( Qt::black, 0 ) : QPen( Qt::NoPen ) );

    replot();
}

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and here's the result:
spectro.jpg

any idea on how I can handle this?

thanks for your attention

regards

[Uwe]

The CONREC algo returns many, many small lines that look like a being connected.

There is another algo that calculates real polygons ( what could be interesting option for drawing a spectrogram in a different way - being composed of vector graphics ) but I never implemented this one.

Uwe

[Pierrick]

Thanks a lot for your answer Uwe,

is there a simple way to get access to the contour lines? In that way I could perhaps decimate them to archieve the desired effect.

Pierrick

[Uwe]

You could overload QwtRasterData::contourLines() or QwtPlotSpectrogram::renderContourLines and store the result somewhere.

For creating vector shapes or bezier paths you could have a look at: http://apptree.net/conrec.htm. If you want to go this direction ( or you have any other vectorizer in mind ) I would be very happy if you would contribute some code.

Uwe

[Pierrick]

I overloaded the QwtRasterData::contourLines() to see the result and now (thanks to you Uwe) I can see the problem which is the number of polylines.

I have no time yet to resolve that but I think this link can help : http://bost.ocks.org/mike/simplify/ it shows the use of the Visvalingam’s algorithm which can simplify shapes.

I'll try to do something as soon as I got times

Pierrick

[Uwe]

it shows the use of the Visvalingam’s algorithm which can simplify shapes.

This is an alternative for Douglas Peucker ( already implemented as QwtWeedingCurveFitter ) what is about reducing the number of points of a polygon. What you need is to build a polygon from an "unordered" set of single lines - what is something very different.

But nevertheless I will check the algo as it seems to be a "less quality but faster" thing. But I'm not sure as by reducing the chunk size you already have an easy way to speed to Douglas Peucker heavily.

Uwe