the replot of a qwtPlot is too slow

[gaia86]

Hi There,
I'm Using Qwt 6.0.1 and Qt 4.8.0.

I have a QwtPlot and attach on it one qwtPlotCurve with 100000 points and a qwtPlotItems.
I want to redraw the qwtPlotItems based on mouse movement. After updating of the qwtPlotItems position is necessary make a replot the qwtPlot to show the new qwtPlotItems position.
For each mouse movement the entire qwtPlot is redraw therefore this passage is very slow, consequently the movement of the qwtPlotItems results not-allingned with the mouse movement. In short terms the updating of User Inteface isn't coherent with mouse movements.

Is there a way to make more efficient this operation? For example is there a way for not redraws the entire qwtPlot, as in case of replot operation.

[Spitfire]

You can do it in many ways.

You can inherit from the QwtPlot and replot only a 'dirty rect' instead of whole plot.

You could reduce your data as 100k poits is too high resolution for the screen no mater what.

You could use a QWidget instead of QwtPlotItem when you move your mouse and just move the QwtPlotItem once when you're done with it.

Or you could do all of the above.

For example: in last app I've used Qwt I have curves made from between 2M and 5M points (up to 5 curves on the plot) and markers tracking each of the curves in real time. Reploting the plot in worst case takes around 200ms and markers are always smooth regardles of how many items there's on the plot.
So, depending on your requirements - it can be done.

ps. All above is true for qwt 5.x.x. For qwt 6.x.x there are few flags you can set to speed up things, see this thread for more details.

[Uwe]

ps. All above is true for qwt 5.x.x. For qwt 6.x.x there are few flags ...

Only true for Qwt 6.0.x - in Qwt 6.1.x ( = SVN trunk ) you have even more flags - none of them will help for the specific problems, that replotting a curve of many points is always too slow for mouse movements !

For situations like these I'm always using an overlay widget, so that the repaints of the plot canvas only mean blitting the pixmap from the backing store of the canvas. You could also introduce a pixmap cache for plotitems up to a specific layer ( QwtPlotItem::z() ), but the advantage of using an overlay widget is, that the update regions for the plot canvas are calculated from the mask of the overlay widget by Qt.

F.e. the rubberband and the tracker text of QwtPlotPicker are using overlay widgets.

Uwe

[gaia86]

thanks for the quick response. the link is very helpful, I'm sorry for posting more times the same question but it was not a voluntary thing