avoid repaint when using a cursor

[Daniel Hrisca]

Hello,

for my plots I use an Infinite vertical line to simulate a Cursor. When the cursor is dragged by the user the instantaneous signal values at that timestamp are reported to the user in a different widget. 

My issue is that with a large signal count (let's say 4000 signals) moving the cursor triggers a complete repaint that is really slow to execute.

Is there some way to have like an overlay where the bottom layer contains the signal curves (and thus dos not require a repaint when the cursor is moved) and the top layer contains the Cursor?

Thanks! 

[Ognyan Moore]

Hi,

For this sort of thing, the trick is to get all the lines to do a repaint in one go instead of repainting one line at a time.

A while back, Kenneth Lyons did a proof of concept to make that work (note it requires altering some library code); but I would start with something like this first: 

https://github.com/pyqtgraph/pyqtgraph/issues/1039#issuecomment-533756769

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[Daniel Hrisca]

It's not clear to me how the changes made in PlotCurveItem could impact a PlotWidget with multiple curves. 

[Daniel Hrisca]

I wonder if this is possible: 

* have multiple layers of graphical items, each backed by a pixmap

* changing an graphical item would only trigger the repainting of the pixmap for the corresponding layer

* the final paint would be a composition of the pixmaps

This would allow for faster updates in the cases where only a few items are changed (shifted curves, cusor, etc.).

In my use case there is a single curve per viewbox, and the viewboxes are X-linked, so a Y zoom change would not affect the rest of the viewboxes. I don't know how the ideas above impact having multiple curves per viewbox. 

[Daniel Hrisca]

After days of tinkering and search I have come up with the following solution:

1. derive new classes for the Cursor (based on InfiniteLine) and Region (based on LinearRegion) were the paint method had an argument that prevents painting by default (see https://github.com/danielhrisca/asammdf/blob/1a4ee1bad931c4a338dd12db2577914797785d5b/asammdf/gui/widgets/cursor.py#L36)

2. let the default painting of the PlotWidget with the axes and curves, then grab the widget into a QPixmap (see https://github.com/danielhrisca/asammdf/blob/1a4ee1bad931c4a338dd12db2577914797785d5b/asammdf/gui/widgets/plot.py#L4251)

3. once the QPixmap is available the  PlotWidget painting will first paint the  QPixmap  and after that the Cursor or Region on top of it

With this approach I can get close to 60FPS (painting + other stuff related to the cursor movement) compared to ~5s / cursor movement event if the Plot need to be repainted from scratch (tested with 1000 curves).