[systemd-devel] [RFC] hwdb: add MOUSE_WHEEL_CLICK_ANGLE as property

[Peter Hutterer]

On Wed, Jan 14, 2015 at 01:07:40PM +0100, Zbigniew Jędrzejewski-Szmek wrote:
> On Wed, Jan 14, 2015 at 03:57:03PM +1000, Peter Hutterer wrote:
> > On Mon, Jan 12, 2015 at 09:48:55AM +1000, Peter Hutterer wrote:
> > > Most mice have a wheel click angle of 15 degrees, i.e. 24 clicks per full
> > > wheel rotation. Some mice, like the Logitech M325 have a larger angle. To
> > > allow userspace to make use of that knowledge, add a property to the hwdb.
> > > ---
> > > libinput patches to read this are here:
> > > http://lists.freedesktop.org/archives/wayland-devel/2015-January/019340.html
> > > 
> > > This is an RFC, unless we ack this for libinput there's no user for it so
> > > I'd like to get both out first for review. Please don't merge yet.
> > 
> > alrighty, we'll go ahead with that for libinput. If anyone has any
> > suggestions let me know, otherwise I'll push this over the next couple of
> > days.
> 
> Yeah, it looks fine. For the record, what it the mouse angle useful
> for? (Uniform scrolling?)

better predictive scrolling, yes. e.g. a mouse that has a smaller click
angle will scroll faster, with this value you can accommodate this where
needed. Likewise, using "half turn of the wheel" or "full turn of the wheel"
as a UI element becomes possible.

Not 100% sure on use-cases though. tbh the main driving factor was that we
have a ...get_value() function that returns subpixel-precision scrolling for
touchpads and other continuous scroll events but unpredictable magic numbers
for mice. We used to return a hardcoded multiplier (10 * click-count) but
that is at odds with the precision coming from the same call on other devices.

Having the degrees gives us a cleaner API. we now have a ..get_value()
return a continuous value range (pixels or degrees) and get_value_discrete()
return "steps", i.e. mouse clicks.  Callers can choose what they prefer
based on the device.

And someone may come up with a use-case for degrees on the mouse wheel
soon :)

Cheers,
   Peter