libinput - touchpad acceleration - piecewise linear profile

I have implemented a piecewise linear acceleration profile for touchpad in a fork of libinput.

The mapping function is defined by an array of points: (speed, factor), which must be sorted in order of increasing speed. Between the points, factor is determined by linear interpolation.

A single point gives a fixed acceleration factor, like the libinput flat profile.

Two points give a single, linear slope, clipped at the upper and lower input speeds. But the clipping is irrelevant if the speeds of the two points are sufficiently low (e.g. 0) and high respectively, in which case all inputs will be at speeds between the points.

Factor
^
|                   _________
|                 /
|                /
|               /
|-----------/
|
+-----------------------------------------------> Speed

More complex curves can be approximated by more points. With enough points, any of the profiles in the old X.org server can be approximated.

At the moment, configuration parameters are hard coded, because I haven't figured out how to add configuration parameters and because using new parameters would require changing code outside libinput (e.g. the X or Wayland libinput drivers).

One of the limitations of libinput is that the only means of configuration is via the API. As a result, to add new configuration parameters requires modification of both the libinput library and whatever software (e.g. Wayland compositor, xf86-input-libinput, etc.) calls it. I may add configuration via environment variable to work around this limitation but in the meantime, as you will have to compile and install this version anyway, it is almost as easy to edit the source as a separate configuration file or environment variable.

See the touchpad-pl branch of this fork of libinput.

To try this profile for your touchpad:

$ git clone https://github.com/ig3/libinput.git
$ cd libinput
$ git checkout touchpad-pl
$ meson --prefix=/usr builddir/
$ ninja -C builddir/
$ sudu ninja -C builddir/ install

This is working for me on Debian 11 with xfce desktop. I haven't tried a system running Wayland.

To change the profile, edit src/filter-touchpad-pl.c. The parameters are in function touchpad_accel_profile().

To test:

$ sudu libinput debug-events