[Wayland-bugs] [Bug 101139] Acceleration still too fast with slow and slow-medium finger movements

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=101139

--- Comment #26 from Peter Hutterer <peter.hutterer at who-t.net> ---
And here's the thing - libinput's acceleration works in physical distances. So
the hw shouldn't really matter because a speed of 10mm/s is always that,
regardless of the hardware. As long as touchpad-edge-detector and you agree on
the ranges/dimensions on the hardware, there shouldn't be much difference.

That acceleration in the attachment is actually a slowdown, we multiply the
calculated input speed by a magic constant (0.37) so that a physical delta of
10mm should end up in a delta of 3.7mm [1], give or take with the changing
screen resolution. Greg's patch just slows things down even further and a -0.75
should then make this even slower.

The question was directed at anyone btw, sorry, should've made that clear.
If you have a compositor that doesn't support hidpi, you'd see a slowdown
because with twice the pixel density (which libinput knows nothing about) your
delta is half as fast - but that's something the compositor has to take care of
(mutter already does).

It would be good if you could attach an evemu recording from a finger movement.
Close your eyes, put your finger down and move from left to right at about
20mm/s - hard to guess I know but it's basically a medium movement. Then I can
try to reproduce this here, maybe, hopefully.

For reference: here's a video of me moving the finger and the corresponding
cursor movement in the libinput debug-gui:
https://photos.app.goo.gl/BTED3UdQwYqw5iW22

[1] having said that, I'm now wondering what's going on there, because if I
move 10mm, it moves by ~10mm on the screen, not 3.7

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