[Wayland-bugs] [Bug 85715] New: wl_fixed is not precise enough for high dpi mice

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=85715

            Bug ID: 85715
           Summary: wl_fixed is not precise enough for high dpi mice
           Product: Wayland
           Version: unspecified
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: weston
          Assignee: wayland-bugs at lists.freedesktop.org
          Reporter: derekf at osg.samsung.com

(Related to bug 85632)

I have a mouse here (a Razer Abyssus) with a 3500DPI sensor.

There is discussion about having libinput normalize mouse input to a 400dpi
standard - some patches have landed in libinput, but there isn't yet a way to
change from the default 400dpi setting.

Once that works, this 3500 dpi mouse will have its deltas multiplied by
400/3500 before being fed to the pointer acceleration filter. On the other end
of the accel filter this slow motion becomes even slower, and the deltas end up
below the threshold for wl_fixed.  It's entirely possible to slowly move your
mouse along indefinitely without any motion occurring.

I have confirmed that this is not just really slow motion (that will eventually
accumulate to on screen movement) - motion is being rounded to zero during the
wl_fixed_to_double() conversion in weston's libinput-device.c


To reproduce:
Get a ridiculous mouse.  The more blue LEDs the better. (3000+ DPI can show the
problem easily, 2000 will do in a pinch)

Use libinput master branch and change device->dpi = DEFAULT_MOUSE_DPI; in
src/evdev.c to whatever is appropriate

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