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Avoid event accumulation in webagg backend. #27160

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raphaelquast
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PR summary

This PR is intended to address the problem of event-accumulation during image-processing in the webagg backend
which can result in an extreme lag that makes "fast updated" events like motion_notify_event etc. unusable at the moment.

Not 100% sure, but this might also improve problems concerning blitting in jupyter-notebooks (e.g. #19116) but would require sending "ack" messages in ipympl. The implementation is made in a way such that ipympl (or more generally backends that don't send "ack" messages) are not affected.

The following gifs show the outcome of this change: (left: current behavior, right: result of this PR)

🐍 code to run example 
import matplotlib as mpl
mpl.use("webagg")
mpl.rcParams['webagg.port']=8988
mpl.rcParams['webagg.open_in_browser']=True

import matplotlib.pyplot as plt

fig, ax = plt.subplots(figsize=(14, 8))
ax.plot([0, 1], [0, 1], 'r')
ln, = ax.plot([0, 1], [0, 0], 'g', animated=True)
ax.figure.canvas.draw()

fig._last_blit_bg = fig.canvas.copy_from_bbox(ax.bbox)

def on_resize(event):
    ax.figure.canvas.draw()
    fig._last_blit_bg = fig.canvas.copy_from_bbox(ax.bbox)

def on_move(event):
    ax.figure.canvas.restore_region(fig._last_blit_bg)
    ln.set_ydata([event.ydata, event.ydata])
    ax.draw_artist(ln)
    ax.figure.canvas.blit(ax.bbox)

fig.canvas.mpl_connect('motion_notify_event', on_move)
fig.canvas.mpl_connect('resize_event', on_resize)

plt.show()

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@raphaelquast raphaelquast mentioned this pull request Oct 20, 2023
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timhoffm
timhoffm reviewed Oct 20, 2023
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@raphaelquast raphaelquast requested a review from timhoffm October 23, 2023 07:40
@raphaelquast raphaelquast mentioned this pull request Oct 31, 2023
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@tacaswell
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Comments from call:

  • add ability to opt-in to this behavior with a string control (so we can support other stratgies)
  • look into de-bouncing in blit as well

attn @ianhi

@raphaelquast raphaelquast mentioned this pull request Dec 7, 2023
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@ghost
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ghost commented Sep 25, 2024

I am using an interactive ipympl widget in an application, that would profit a lot from blitting, so i have been waiting for this PR to be merged. Thank you for the promising work done.
I just wanted to ask now, if we can expect this to be merged eventually, or if development has stopped, or is happening somewhere else, or ... ?

@raphaelquast
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Hey, just a quick comment from my side:
I would be really happy if this PR is merged sometime soon...
Unfortunately however, I have absolutely no time right now to continue working on the remaining issues that need to be resolved and it will stay this way for a few months more...

I am happy if someone takes over in the meantime, otherwise I'll try to continue working on it as soon as i find some time (best guess is sometime early next year).

@sjmarwitz you could also monkey-patch the fix in the meantime to check if it does the job for you... I implemented a workaround for EOmaps ( see here)

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ghost commented Sep 26, 2024 •
edited by ghost
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@raphaelquast Thanks for your quick response and for pointing me to your monkey-path fix. I tried it and immediately hit the problem with ipympl, that was already mentioned in the PR summary. Actually, when using ipympl, after the initial display of a figure, it won't update the canvas/figure, not even zooming or panning is possible. It seems, no 'ack' messages are being received (and sent?).

I would have liked to contribute a fix somehow, given the limited amount of time i have, but looking into the ipympl code, i got lost, as i don't "speak" js and the like and are not familiar with web technologies.

QuLogic
QuLogic reviewed Oct 4, 2024
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lib/matplotlib/backends/backend_webagg_core.py

# Keep track of the number of received ack messages.
if self._num_received_images is None:
self._num_received_images = 0 # To support no "ack" messages.
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@ianhi ianhi mentioned this pull request Oct 17, 2024
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@raphaelquast @QuLogic
Update lib/matplotlib/backends/backend_webagg_core.py
9110f10 
Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>
raphaelquast and others added 2 commits March 5, 2025 09:39
@raphaelquast @QuLogic
Update lib/matplotlib/backends/backend_webagg_core.py
8a91f7b 
Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>
@raphaelquast @QuLogic
Update lib/matplotlib/backends/backend_webagg_core.py
7551b47 
Co-authored-by: Elliott Sales de Andrade <quantum.analyst@gmail.com>
This was referenced Mar 6, 2025
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