QwtPolarSpectrogram: Plotting just the upper half circle

[QTim]

Hi Uwe, and everybody,

I'm using a QwtPolarSpectrogram as a RADAR or SONAR PPI,it works faily well and is a great tool, thanks for this, btw.!!

The thing is: I only need the "upper half" of the widged (0-180° of the polar circle). Is i t possiblie to only show the upper half? Any Ideas about this?


Thank you!

QTim

[Uwe]

class YourData: public QwtRasterData
{
public:
    virtual double value( double azimuth, double radius ) const
    {
        if ( azimuth > M_PI )
            return 0.0; // some value indicating invalid
 
        ....
    }
};

class YourData: public QwtRasterData
{
public:
    virtual double value( double azimuth, double radius ) const
    {
        if ( azimuth > M_PI )
            return 0.0; // some value indicating invalid

        ....
    }
};

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Then the rest is to use a color map where 0.0 is mapped to 0 ( = an rgb with an alpha of 0 )

Uwe

[QTim]

Thanks for your answer, Uwe, and sorry for bothering you again, but i really tried setting the value 0 to a certain color, but it didn't wok at all!
(see one try below)

Can you help me a little further?

PlotPolarSpectrogramColorMap::PlotPolarSpectrogramColorMap():
      QwtLinearColorMap( Qt::darkBlue, Qt::darkRed )
            {
               addColorStop( 0.2, Qt::blue );
               addColorStop( 0.4, Qt::cyan );
               addColorStop( 0.6, Qt::yellow );
               addColorStop( 0.8, Qt::red );
 
               rgb(QwtInterval(-0.01, 0.01), 200); // Just to try!
 
            }

PlotPolarSpectrogramColorMap::PlotPolarSpectrogramColorMap():
      QwtLinearColorMap( Qt::darkBlue, Qt::darkRed )
            {
               addColorStop( 0.2, Qt::blue );
               addColorStop( 0.4, Qt::cyan );
               addColorStop( 0.6, Qt::yellow );
               addColorStop( 0.8, Qt::red );

               rgb(QwtInterval(-0.01, 0.01), 200); // Just to try!

            }

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Thanks
QTim

[Uwe]

An even easier solution would be to return Q_SNAN as NaN values are mapped to an RGB value of 0u. Then you don't need to change your color map.

Uwe

[QTim]

NaN values are mapped to an RGB value of 0u. Then you don't need to change your color map.

But how do I map the NaN values? It doesn't seem to work. :/

Thanks
QTim

[Uwe]

But how do I map the NaN values?

You don't - it is implemented in QwtLinearColorMap ( when using Qwt 6.1 ).

Uwe

[QTim]

Hi Uwe,

okay, the only thing I changed now is the value function:

class YourData: public QwtRasterData
{
public:
    virtual double value( double azimuth, double radius ) const
    {
        if ( azimuth > M_PI )
            return Q_SNAN;
 
        ....
    }
};

class YourData: public QwtRasterData
{
public:
    virtual double value( double azimuth, double radius ) const
    {
        if ( azimuth > M_PI )
            return Q_SNAN;
 
        ....
    }
};

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but there is no transparency at all. It still looks like a standard polar spectrogram.
I am on QWT 6.1.

Thank you!!

QTim

[Uwe]

Add the same to the spectrogram example. If it works there compare what is different to your application.

Uwe

[QTim]

Add the same to the spectrogram example. If it works there compare what is different to your application.

Uwe

Uwe, please have a look at the picture. As you can se, I use the plain spectrogram example, exept the

if ( azimuth > M_PI )
   return Q_SNAN;

if ( azimuth > M_PI )
   return Q_SNAN;

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in the value function.


It doesn't work.

[Uwe]

I can see a picture, where the spectrogram covers the upper half - the lower half is empty ( the background of the canvas ).
So maybe I misunderstood your question - you want to hide the lower half of the complete plot - not the spectrogram inside the plot ?

This is not supported by QwtPolarPlot and you would have to solve it by giving the plot a geometry, where the lower half is below the bottom of its parent widget.

Uwe