https://bugs.freedesktop.org/show_bug.cgi?id=101139
--- Comment #38 from Peter Hutterer <peter.hutte...@who-t.net> ---
For the archives, I've done a visual comparison to the synaptics driver,
basically by running Xorg with synaptics and libinput debug-gui with libinput
(without the patch above, just git master at a59ce1c) and looking at the two
cursors. Tests done with libinput acceleration 0 and synaptics' default
acceleration without any changed settings.
1) at really slow motion libinput applies deceleration, cursor speed is approx
the same for both. That's for things like slowly rolling the finger.
2) at slow motion libinput has a much higher acceleration factor than
synaptics, possibly twice as high. Thats for slowly moving the finger across
the touchpad.
3) at some medium speeds, libinput acceleration is still the same baseline but
synaptics starts to accelerate already. This is where synaptics gets faster
than libinput.
4) at high speeds, the acceleration rate is roughly the same but the maximum
acceleration differs. When jerking the finger between two positions, sometimes
the cursors move in parallel, sometimes the libinput cursor moves further than
the synaptics one.
Despite my best efforts so far, I have yet to figure out how to print the
synaptics acceleration curves for analysis. I've tried a few things but none of
them match the perceived movements. There are too many factors and they are
hard to calculate (e.g. screen resolution plays a part). Specifically, once the
cursor is at the edges the behaviour changes and separating that is difficult.
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