Plot rendering in thread

[viridis]

Hi,

I just want to know if it is possible to make Qwt do the curve rendering in a thread ?
I have curves with a lot of point to render and thus, it is quite slow and freeze the user interface.

Regards,

[Ginsengelf]

Hi, you can do your calculations in a separate thread, but you cannot draw directly into the gui thread, instead send some kind of notification message if one calculation is complete.

Ginsengelf

[Uwe]

As long as you didn't disable QwtPlotCanvas::PaintCached the content of the canvas is rendered to a pixmap cache, before it is painted to the widget. So what you can try is to overload YourPlot::drawCanvas, where you somehow delegate QwtPlot::drawCanvas to a temporary pixmap into a different thread. When it is done you can replace the canvas paint cache with your pixmap.

But I guess it is easier to optimize the performance of your replot operations. QwtPlotCurve::PaintFiltered might help. If this is not enough you should implement something like level of details ( skipping of points, depending on the zoom level ).

Uwe

[viridis]

I noticed that the rendering occurs in a pixmap and that why I though of the thread possibility.
I will give it a try if I have enough time.

For the level of details, I though about it too. Indeed it was the second question of my previous post, but I removed it as I noticed what I need is a low pass filter with resampling... which is quite complicated !
I have tried to skip points, but it is really not appropriate for my data. I have tried to draw a point only if it is far enough of the previous one too. This last one works very well for some curves (~500 points instead of 120000 !) but not for all (~88000 points instead of 120000 which too big).

A related problem is that I retrieve my data directly from a QAbstractItemModel, which works with QVariants, and its seems really slow for this problem. I will try to implement a cache for my "double" values here to avoid the QVariant.

[Uwe]

A related problem is that I retrieve my data directly from a QAbstractItemModel, which works with QVariants, and its seems really slow for this problem. I will try to implement a cache for my "double" values here to avoid the QVariant.

You can access almost everything through the QAbstractItemModel API, but of course such a design can't be as effective as a array of points.

I recommend to store your values in something like a QVector<QPointF> and to implement/use a model ( for your Qt widgets ) + a QwtData object ( for QwtPlot ) on top of it.

Uwe