[GrafX2] #136: Jerky mouse movement when using SDL2 backend on Linux

[GrafX2]

#136: Jerky mouse movement when using SDL2 backend on Linux
----------------------------+------------------------
Reporter: chestertonic@… | Owner: pulkomandy
Type: defect | Status: new
Priority: major | Milestone:
Component: GrafX2 | Version: 2.6
Keywords: |
----------------------------+------------------------
It only seems to affect the cursor on the canvas as movement over the UI
is smooth. It's especially noticeable when using the brush grab tool and
also when using the magnifier.
I have a high polling rate mouse and attempted to remedy the effect by
increasing the merge movement option by 1 a couple times to no avail. The
X11 backend works fine and SDL 1.2 works if I increase merge movement to
2. It's probably a bug in SDL2 but I figured I'd say something anyway.

--
Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/136>
GrafX2 <http://pulkomandy.tk/projects/GrafX2>
The ultimate 256-color painting program

[GrafX2]

#136: Jerky mouse movement when using SDL2 backend on Linux

-----------------------------+------------------------

Reporter: chestertonic@… | Owner: pulkomandy
Type: defect | Status: new

Priority: major | Milestone: 2.9
Component: GrafX2 | Version: 2.6
Resolution: | Keywords:
-----------------------------+------------------------
Comment (by n.bruenggel@…):

Hi there,

I noticed a similar problem on my Mac (macOS 11.6.6 (20G624), SDL 2). The
cursor moves fine when using Apple's Magic Mouse 2, but with my Logitech G
mouse it becomes super jerky if I set it to higher speed. I believe the
problem is because it works at a higher frequency (see this
[thread](https://github.com/libsdl-org/SDL/issues/4376) where they
recommend to use call `SDL_PumpEvents` just once per frame.

I do not fully understand how the event loop works (the `do .. while` loop
in `Main_handler` is pretty big), but from what I read so far I think the
problem could be fixed by running it at a fixed frequency (for example
60Hz). To quickly validate this I added some code to measure the number of
times `SDL_PumpEvents` is called per second. I measured it in `engine.c`,
at line 769 where `Get_input` (which calls `SDL_PumpEvents` when using SDL
2) is called. It turns out that, when using the Logitech Mouse in fast
mode, it is sometimes called almost 300 times per second which seems to
cause the jerky movements.

I would be glad if a more experienced developer could comment on this so
the bug can eventually be fixed.
--
Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/136#comment:3>

[GrafX2]

#136: Jerky mouse movement when using SDL2 backend on Linux

-----------------------------+------------------------

Reporter: chestertonic@… | Owner: pulkomandy
Type: defect | Status: new

Priority: major | Milestone: 2.9
Component: GrafX2 | Version: 2.6
Resolution: | Keywords:
-----------------------------+------------------------

Comment (by PulkoMandy):

As mentionned in the ticket, there is a "merge movement" setting for that
in the settings file and it works for the other backends but apparently
not for SDL2.

The code is intentionally designed to capture all movements from the mouse
as fast as possible, and then process them even if it happens with a bit
of delay. This code was written this way for old, slow machines, back when
GrafX2 was running for DOS. If you are drawing with a rather large brush
or some complex effect, it's expected that the drawing will not happen in
realtime.

If you write code in a way that skips mouse events, it will draw faster,
sure, but the precision of your mouse gesture (or "brush stroke") will be
lost. Instead of a nice curve exactly matching the movement, you will now
have several straight line segments between the points that GrafX2
captured. We don't want that.

However, with high resolution mouses, we get a lot more movements than we
really need. Especially when working with high zoom factors where really a
movement of just a few pixels doesn't really matter. That's what the
"merge movement" code is supposed to solve, by allowing to manually tweak
this to match your personal setup (your mouse, your OS, your display size
and resolution, and the way you use grafx2).
--
Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/136#comment:4>

[GrafX2]

#136: Jerky mouse movement when using SDL2 backend on Linux

-----------------------------+------------------------

Reporter: chestertonic@… | Owner: pulkomandy
Type: defect | Status: new

Priority: major | Milestone: 2.9
Component: GrafX2 | Version: 2.6
Resolution: | Keywords:
-----------------------------+------------------------

Comment (by n.bruenggel@…):

I'm not suggesting to drop any events but to limit the number of calls to
`SDL_PumpEvents` to one per frame (at a fixed frame rate). After pumping
the events can be retrieved with `SDL_PeepEvents`.
--
Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/136#comment:5>

[GrafX2]

#136: Jerky mouse movement when using SDL2 backend on Linux

-----------------------------+------------------------

Reporter: chestertonic@… | Owner: pulkomandy
Type: defect | Status: new

Priority: major | Milestone: 2.9
Component: GrafX2 | Version: 2.6
Resolution: | Keywords:
-----------------------------+------------------------

Comment (by yrizoud@…):

If I remember correctly, the editor engine has two states:
1) "when you are drawing". This starts when you first click, and only ends
when the operation is completed (not necessarily the same mouse button
release, some drawing tools involves several clicks.
2) at rest

We wanted to preserve maximum mouse faithfullness during drawing, so while
the engine is in state 1, every input event is taken into account,
possibly filling the event queue. In state 1, "merge mouse movement" is
taken into account, and will skip-ahead 1 mouse movement per loop, or more
depending on the setting.

> when using the brush grab tool

I had never thought of it before, but the entire "Brush" tool should
behave like state 2. Brush stretcher, too. There are many cases where
there the last mouse position is the only useful one. We should probably
add flags for tools, or event sub-states of tools, where full mouse
"trajectory" is not needed
--
Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/136#comment:6>

[GrafX2]

#136: Jerky mouse movement when using SDL2 backend on Linux

-----------------------------+------------------------

Reporter: chestertonic@… | Owner: pulkomandy
Type: defect | Status: new

Priority: major | Milestone: 2.9
Component: GrafX2 | Version: 2.6
Resolution: | Keywords:
-----------------------------+------------------------

Comment (by jpavel@…):

I've submitted a merge request

https://gitlab.com/GrafX2/grafX2/-/merge_requests/355

that seems to fix this problem for me, using SDL2 on Linux. You could try
that out to see if it does the trick.
--
Ticket URL: <http://pulkomandy.tk/projects/GrafX2/ticket/136#comment:7>