How to plot the real time data in qwtplot while erasing fixed-width old data?

[lockdown]

Guess what you mean by this code snippet is that you draw each curve to its own pixmap first and the plot is done finally by drawing the pixmaps.
Then you would have only one drawPolyline call, when only one curve has changed.
Of course the code as being posted is not even close to what needs to be done in reality - but the idea is understood.

So your code will have a positive effect, when having several curves, that are updated on different intervals - but it won't help for a situation with one curve and many points.

Uwe

All curves plot in the only m_curvePixmap,I update the code snippet in the previous reply. All curves partial update at the same time ,update every 30ms.

The original code looks too complicated,I just want to try to modify it as simple as the qwt.

[Uwe]

All curves plot in the only m_curvePixmap,....

Then your code does not make any sense.

Uwe

[lockdown]

Then your code does not make any sense.

Uwe

I have renewed.Thanks for the quick reply!

What I don’t understand is why these my two methods are so different, why the CPU consumption is so different? Looks like they are doing the same partial refresh.

Maybe as you said:QWidget does not offer to "erase" something - all you can do is to redraw from scratch.

[Uwe]

What I don’t understand is why these my two methods are so different, why the CPU consumption is so different?

Hard to say what your code is doing as the code snippet does not show the real code.

But the plot canvas is actually using a backing store ( a QPixmap ), that gets updated on certain operations like when the size of the canvas is changing or replot has been called.
Beside that the content of the widget is always restored from this backing store only.

So when calling QWidget::update( region ) only all what happens is, that the rectangles from region will be copied from the backing store of the canvas to another backing store - the default one every widgets has.

Uwe

[Mazzin]

@lockdown Have you found a way to achieve this? because I am running in circles about the same issue and that would be really great help.