Re: How to debug emacs repaint problems?

[joakim]

joakim@verona.se writes:

screenshot_20231014_215725.png
Description: PNG image

> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> From: joakim@verona.se
>>> Date: Sat, 14 Oct 2023 20:09:31 +0200
>>> 
>>> Hello,
>>> 
>>> I have a long standing problem where it appears that my build of emacs
>>> seems to invalidate a lot of regions on screen, which leads to flicker
>>> in my setup of vnc. 
>>> 
>>> This flicker problem doesnt appear so much in the emacs that is
>>> installed by my distro, which is fedora, emacs 28.
>>> 
>>> It would be easier to debug this if emacs could be persuaded to give
>>> some visual hints of which regions its invalidating. Is this possible?
>>
>> Not AFAIU, since Emacs doesn't invalidate any regions.  It decides by
>> itself which parts of the screen need to be redrawn, and then does
>> that.
>
> If so, it appears to me that my build of emacs redraws a lot of regions
> unecessarily. So the my question is if its possible to make Emacs show
> some hints of which areas its redrawing. Then I would know if the
> problem is in emacs, or elsewhere.
>
> The emacs 28 in my distro doesnt flicker, nor does apps such as gedit.
>
>>
>>> I use these settings in turbovnc:
>>> Tight+Low quality jpeg(WAN)
>>> /opt/TurboVNC/bin/tvncconfig -set ALR=1
>>> /opt/TurboVNC/bin/tvncconfig -set ALRAll=true
>>> 
>>> The idea is that a low quality codec should be used to refresh
>>> invalidated rects, and then after a while be losslessly refreshed.
>>> This works well in applications that dont invalidate the entire window,
>>> when only small parts of the window needs to be changed.
>>
>> I don't know anything about turbovnc, but what you say is not how
>> Emacs display works.
>>
>> Btw, does the flickering happen with text-mode frames or GUI frames?
>> And which part(s) of the Emacs display flicker?

I managed to make a screenshot that shows the problem. In the image I
made an emacs window with some black text on red background.

If I move the cursor just one line, you can see that 2 large regions
become blurred. Since I just moved the cursor one line down, its rather
confusing why these 2 seemingly unrelated regions should be redrawn.



>
> - If I access the same emacs instance in a tty, it does not flicker.
>
> - only gui frames flicker
>
> - the text flickers in a region around the cursor when moving it, it
>   seems not consistent.
>
> To be a little bit more elaborate:
>
> - I open a file or an emacs application which has a lot of colors when
>   font locked. The colors matter, because when using low quality jpeg
>   encoding for vnc, red colors in text is more obviously "bleeding".
>
> - if i wait a little bit, a lossless refresh occurs, and all text is now
>   crisp regardless of color. so far so good.
>
> - if I know move the cursor to a new line the line will need to be sent
>   by vnc to the client, because there was a cursor drawn there. Therefore
>   the line will flicker, and this is expected.
>
> - However, I dont expect other regions in the emacs frame to be needed
>   to be sent again, because nothing happened there. But in my case 
>   random parts of the emacs window seems to be redrawn, and thus
>   flicker. And again, this doesnt seem to happen with the distro emacs.
-- 
Joakim Verona
joakim@verona.se