CPU usage due to curves with many points

[Uwe]

I need to handle both the case in which there's only one trace that "eats" its own tail after reaching the right end of the screen...

Well this one should be doable with using QwtDirectPainter. All you need to do is to call replot manually once, when the curve restarts - like it is done in the oscilloscope example.

and the case in which the last N traces are kept on the monitor (at the moment I have N QwtPlotCurve which I update circularly with new traces with setRawSamples method).

In this case all previous points have a different position, what means you always have to replot. Then it depends on how many points you have and of course on the specific attributes being used for drawing the curve.

For reducing the number of points I again recommend to have a look at the QwtPlotCurve::FilterPointsAggressive flag. It is useful for line plots being monotonic in x or y direction. The effect is, that it limits the number of points to be rendered in the worst case to 4 * the canvas width ( or height ).

Concerning the attributes: antialiasing or a pen width > 1 slow down rendering. You could also try a OpenGL canvas ( better use trunk then ) - or Qt4/X11 ( graphicssystem=native ), what is the best choice for hardware acceleration. Guess you don't use symbols - as this doesn't make too much sense for your type of plot.

And of course always decouple the refresh rate of the plot from the sample rate. 10 fps should be by far good enough for oscilloscopes as only few pixels on screen are changing, but often less frequent updates are fine as well.

Uwe

[fcona]

Well this one should be doable with using QwtDirectPainter. All you need to do is to call replot manually once, when the curve restarts - like it is done in the oscilloscope example.

The oscilloscope example is a bit different because it completely clears the old trace, while in my case it'd need to disappear gradually as the new trace is plotted over it (For some odd reason which I haven't figured out yet in my case calling the replot when the curve restarts as you suggest doesn't even clear the plot, but that's most probably due to something stupid I did...)
Maybe these images can clear what I mean:
oscilloscope:
oscilloscope.png
my application:
my application.jpg
This is my application with the 10 traces overlapped (the more saturated the color the more recent the trace):
10 traces.jpg

Not sure what you mean by

In this case all previous points have a different position

For reducing the number of points I again recommend to have a look at the QwtPlotCurve::FilterPointsAggressive flag.

I checked it and it is very interesting because I already do something similar to reduce the amount of points when there are just too many to plot compared to the available widht in pixels, so I might switch to using QwtPlotCurve::FilterPointsAggressive that seems much more optimized!

Concerning the attributes: antialiasing or a pen width > 1 slow down rendering

When I inherited the software it had a pen width of 2 and actually it was much more demanding. Fortunately I read on a forum that it could be a good idea to use thin curves and corrected them.

You could also try a OpenGL canvas

Is there any example of how I can use OpenGL canvas?

10 fps should be by far good enough for oscilloscopes

That's exactly my refresh rate

Thanks again for the great support!

Filippo