qt5agg image quality changes when window is out of focus #13012

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1. WeatherGod commented on Dec 17, 2018

   

   WeatherGod

   on Dec 17, 2018 via email

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   Does this happen with any other backend? I wonder if this might be something Ubuntu's window manager might be doing. You wouldn't happen to be using something custom like Mint or Cinnamon? Which desktop manager are you using?

   …

   On Mon, Dec 17, 2018 at 5:02 PM Deepak Cherian ***@***.***> wrote: Bug report *Bug summary* Quality of qt rendered figures is drastically reduced when window is moved out of focus and then back in focus. *Code for reproduction* import matplotlib.pypot as plt plt.plot([1, 2, 3, 4], [5, 5.5, 6.5, 8.5], alpha=0.5) *Actual outcome* [image: mpl-before] <https://user-images.githubusercontent.com/2448579/50118171-c4b71100-0203-11e9-9bbf-1cc763aa1525.png> Switch to different window and back: [image: mpl-after] <https://user-images.githubusercontent.com/2448579/50118177-c7b20180-0203-11e9-9e72-f26dbbfed192.png> At this stage, calling plt.draw() fixes it till the window goes out of focus. Zooming in on the figure also returns it to the earlier quality. Note that transparency is lost. *Expected outcome* Rendering doesn't change. *Matplotlib version* - Operating system: Ubuntu 16.04 - Matplotlib version: 3.0.2 - Matplotlib backend (print(matplotlib.get_backend())): Qt5Agg - Python version: 3.6.7 - Jupyter version (if applicable): console 6.0.0 - Other libraries: pyqt 5.6.0 py36h8210e8a_7 conda-forge everything installed from conda-forge using conda. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#13012>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AARy-AIeOjNuRgj2ww9jke3Nh45WBVwsks5u6BRtgaJpZM4ZXNbe> .
2. dcherian commented on Dec 17, 2018

   

   dcherian

   on Dec 17, 2018

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   I'm using Xfce and the Tk backend works fine.
3. tacaswell commented on Dec 18, 2018

   

   tacaswell

   on Dec 18, 2018

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   Note that transparency is lost.

   Do you have other transparency than the alpha on the line?
4. dcherian commented on Dec 18, 2018

   

   dcherian

   on Dec 18, 2018

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   No, but axes.facecolor=None and figure.facecolor=None. Does that count?
5. tacaswell commented on Dec 18, 2018

   

   tacaswell

   on Dec 18, 2018

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   Yes, that is likely the critical detail! If you repeatedly focus / defocus does it get worse?

   I think what is going on is that for GUI backends we use Agg to render and RGBA buffer and then use the GUI frameworks drawImage or equivalent to paint the figure onto your screen (see https://github.com/matplotlib/matplotlib/blob/master/lib/matplotlib/backends/backend_qt5agg.py#L23) for details. If you set the Figure and Axes color to None the alpha on most of the rendered figure is 0. Unfortunately, we only clear the Qt painter right after triggering a draw (

   matplotlib/lib/matplotlib/backends/backend_qt5agg.py

   Lines 41 to 43 in 529410f

   	if self._erase_before_paint:
   	painter.eraseRect(self.rect())
   	self._erase_before_paint = False

   ). On a repaint where we did re-draw we just re-use the same buffer and draw to the screen. If you have an opaque background, this is OK as when Qt does the compositing the opaque layer is the only thing that survives, however all of your pixels have non-zero alpha so we are repeatedly compositing it with its self.

   If you delete the 3 lines above does the problem go away?
6. tacaswell

   added this to the v3.1 milestone on Dec 18, 2018
7. tacaswell

   added

   GUI: Qt

   on Dec 18, 2018
8. dcherian commented on Dec 18, 2018

   

   dcherian

   on Dec 18, 2018

   Last edited by dcherian

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   If you repeatedly focus / defocus does it get worse?

   Yes!

   When I remove those three lines my figure background is black.
9. tacaswell commented on Dec 18, 2018

   

   tacaswell

   on Dec 18, 2018

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   How about leaving just painter.eraseRect(self.rect()) ?
10. dcherian commented on Dec 19, 2018

    

    dcherian

    on Dec 19, 2018

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    Yup, commenting out this line fixes things:
    

    matplotlib/lib/matplotlib/backends/backend_qt5agg.py

    Line 43 in 529410f

    self._erase_before_paint = False
11. tacaswell

    added 2 commits that reference this issue on Dec 25, 2018

    FIX: always eraseRect in Qt widget

    ...

    This issue will close once commit 79ed0ee is merged into the 'main' branch.

    79ed0ee

    FIX: always eraseRect in Qt widget

    ...

    This issue will close once commit f4948aa is merged into the 'main' branch.

    f4948aa
12. timhoffm

    closed this as completedin #13050on Dec 30, 2018
13. timhoffm

    added a commit that references this issue on Dec 30, 2018

    FIX: always eraseRect in Qt widget (#13050)

    ...

    This issue will close once commit 879b7f0 is merged into the 'main' branch.

    879b7f0