Qwt realtime plotting using QwtPlotDirectPainter

[Uwe]

II may have missed your point.
If you worry about performance you can subclass curve to cache itself when it's plotted.
Then next time you replot the plot curve should check what have changed since last time, use the part of cache that's the same and draw only extra points ,then cache new pixmap again.

Drawing a pixmap is usually too slow for "realtime" conditions. Using such a cache might make sense for very few situations ( stable scales and a fixed size of the plot canvas ).

I would recommend to study the realtime example first as it is much simpler than the oscilloscope. It simply appends points to the end of the curve data and then uses QwtPlotDirectPointer to paint only these points on top of the existing plot. Guess you could use the code of the IncrementalPlot class like it is - beside some modifications, when you want to draw lines and not only symbols:

void IncrementalPlot::appendPoint( const QPointF &point )
{
    CurveData *data = static_cast<CurveData *>( d_curve->data() );
    data->append( point );
 
    int from = qMin( data->size() - 2, 0 );
    d_directPainter->drawSeries( d_curve, from, data->size() - 1 );
}

void IncrementalPlot::appendPoint( const QPointF &point )
{
    CurveData *data = static_cast<CurveData *>( d_curve->data() );
    data->append( point );

    int from = qMin( data->size() - 2, 0 );
    d_directPainter->drawSeries( d_curve, from, data->size() - 1 );
}

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Uwe

[Jean-Luc]

I'll look into the Realtime example some more. I assume you meant QwtPlotDirectPainter. I was actually trying to do what you did in your code, in the code that I attached; however, I see that I'm trying to paint an object that does not yet exist when I run my code. Does this mean that I cannot use Qt Designer for the plot?

[Uwe]

Important is when the code is executed - this has nothing to do with the tool you are using to write your code.
The designer is no compiler and won't execute any C++ code at all.

Uwe

[Jean-Luc]

Thank you for your help. I have gotten it to work.

[cheuwleeyeo]

Hi,

I am a newbie to Qt/Qwt; but find it very useful; I am having similar question as Jean-Luc; I have been studying Qwt examples: Oscilloscope, Refreshtest, and Curvedemo2.

Oscilloscope does a 'replot" so the plot is restarted at the beginning of the plot window, Refreshtest does a "scroll" to the data; I would like to do a "erase bar" to the wave; so very similar to Oscilloscope but does NOT clear the plot window; I can stop the refresh by not doing "replot" after the interval; but I can't seem to create the "simple" erase bar; ie. deleting the previous plot(eg. 5 points in the future) and replacing them with the new points(as currently done in the examples); I am thinking of doing the "brute force method" of using similar brush as background color; but thought, there maybe a better solution or even a simpler solution.

Appreciate any suggestion to do this.

Thanks.

Lee

[Uwe]

In earlier Qt releases ( Qt 3 ) it was possible to erase something by drawing in XOR mode, but this feature is lost since Qt 4.0. So the answer is simple, you can't erase something from a widget ( by Qt design ).

Uwe