Re: Flicker during replotting in QwtPlot
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Re: Flicker during replotting in QwtPlot
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From: Jussi R. <jus...@ko...> - 2014-09-30 13:55:41
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<div class="moz-cite-prefix">Hi Uwe,<br>
<br>
Here are some additional comments/tests:<br>
- it seems that detachItems() redraws the plot, even if autoreplot
is set to false. I believe this is the cause for flickering as the
plot is first cleared and drawn (legend erased) and then soon
drawn again (with the legend).<br>
- when autoreplot is false, in same cases the plot is still
redrawn without the replot command. For example, consider the
following:<br>
<pre wrap=""> curve1->setSamples(x,y);
curve1->attach(qwtPlot);
qwtPlot->replot();
curve2->setSamples(x,y2);
curve2->attach(qwtPlot);
I would expect to see only one curve (the situation at replot()), but my test program actually shows both curves.
Best regards,
Jussi
</pre>
On 29.9.2014 12:39, Jussi Rahola wrote:<br>
</div>
<blockquote cite="mid:542...@ko..." type="cite">Hi
Uwe,
<br>
<br>
Thanks for your quick answer. Here are some comments.
<br>
- 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.
<br>
- 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
<br>
- the breakpoint in qwtplot.cpp, line 643, was never reached
<br>
- during the slider movement, updatelayout and updatelegenditems
are called twice, first for qwtplot::detachItems() and second time
for qwtplot::replot().
<br>
- in the attached code, on line 59, you can select three modes:
<br>
mode 0: original, one plot (now with 10000 samples)
<br>
mode 1: your proposed workaround
<br>
mode 2: two plot curves
<br>
<br>
Please let me know if I can run additional tests.
<br>
<br>
Best regards,
<br>
<br>
Jussi
<br>
<br>
On 26.9.2014 19:06, Uwe Rathmann wrote:
<br>
<blockquote type="cite">Hi Jussi,
<br>
<br>
<blockquote type="cite">I'm seeing some flickering while
updating plots rapidly, e.g. during a
<br>
slider movement.
<br>
</blockquote>
The flickering in the screen capture is because the legend
<br>
dis/re-appears. An easy workaround might be to change the
implementation
<br>
of your plot() method - like at the end of this answer.
<br>
<br>
But on my linux box I can't confirm any flickering with your
demo and to
<br>
be honest I'm wondering, how it is possible, that events are
processed (
<br>
resize + paint ) in a state without having a curve attached.
<br>
<br>
Could you please help me out with debugging the issue on your
box with
<br>
having a look for calls of QwtPlot::updateLayout(). As resizing
of the
<br>
plot canvas only happens according to recalculations of the plot
layout,
<br>
there has to be a call in a situation, where the legend gets
hidden,
<br>
because it is empty.
<br>
<br>
Please set a breakpoint in qwtplot.cpp, line 643 and post the
call stack.
<br>
<br>
Uwe
<br>
<br>
--
<br>
<br>
void MainWindow::plot() {
<br>
<br>
const QwtPlotItemList items = qwtPlot->itemList(
<br>
QwtPlotItem::Rtti_PlotCurve );
<br>
<br>
QwtPlotCurve* curve;
<br>
if ( items.isEmpty() )
<br>
{
<br>
curve = new QwtPlotCurve("Sine");
<br>
curve1->attach(qwtPlot);
<br>
}
<br>
else
<br>
{
<br>
curve = items.first();
<br>
}
<br>
<br>
QVector<double> x, y;
<br>
double coeff=slider->value();
<br>
const double pi = 4 * atan(1.0);
<br>
const int nsamples=100;
<br>
for (int k=0; k <= nsamples; ++k) {
<br>
double xx=((double)k)/nsamples*2*pi;
<br>
x.append(xx);
<br>
y.append(sin(xx*coeff));
<br>
}
<br>
curve1->setSamples(x,y);
<br>
<br>
qwtPlot->replot();
<br>
}
<br>
<br>
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