Re: Flicker during replotting in QwtPlot

[Jussi R. <jus...@ko...>]

Hi Uwe,

Thanks for your quick answer. Here are some comments.
- your workaround indeed removes the flicker, but this is quite awkward 
to use in my code, as there are many plot curves used in the same plot 
and the number of curves is varying.
- this seems to be a bit of a timing issue. My original code gave only 
flicker in the debug mode but not in release mode. However, when I 
increased the number of plot points or used multiple curves, the 
flickering also occurred in release mode
- the breakpoint in qwtplot.cpp, line 643, was never reached
- during the slider movement, updatelayout and updatelegenditems are 
called twice, first for qwtplot::detachItems() and second time for 
qwtplot::replot().
- in the attached code, on line 59, you can select three modes:
mode 0: original, one plot (now with 10000 samples)
mode 1: your proposed workaround
mode 2: two plot curves

Please let me know if I can run additional tests.

Best regards,

Jussi

On 26.9.2014 19:06, Uwe Rathmann wrote:
> Hi Jussi,
>
>> I'm seeing some flickering while updating plots rapidly, e.g. during a
>> slider movement.
> The flickering in the screen capture is because the legend
> dis/re-appears. An easy workaround might be to change the implementation
> of your plot() method - like at the end of this answer.
>
> But on my linux box I can't confirm any flickering with your demo and to
> be honest I'm wondering, how it is possible, that events are processed (
> resize + paint ) in a state without having a curve attached.
>
> Could you please help me out with debugging the issue on your box with
> having a look for calls of QwtPlot::updateLayout(). As resizing of the
> plot canvas only happens according to recalculations of the plot layout,
> there has to be a call in a situation, where the legend gets hidden,
> because it is empty.
>
> Please set a breakpoint in qwtplot.cpp, line 643 and post the call stack.
>
> Uwe
>
> --
>
> void MainWindow::plot() {
>
>       const QwtPlotItemList items = qwtPlot->itemList(
>           QwtPlotItem::Rtti_PlotCurve );
>
>       QwtPlotCurve* curve;
>       if ( items.isEmpty() )
>       {
>           curve = new QwtPlotCurve("Sine");
>           curve1->attach(qwtPlot);
>       }
>       else
>       {
>           curve = items.first();
>       }
>
>       QVector<double> x, y;
>       double coeff=slider->value();
>       const double pi = 4 * atan(1.0);
>       const int nsamples=100;
>       for (int k=0; k <= nsamples; ++k) {
>           double xx=((double)k)/nsamples*2*pi;
>           x.append(xx);
>           y.append(sin(xx*coeff));
>       }
>       curve1->setSamples(x,y);
>
>       qwtPlot->replot();
> }
>
>
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