[PATCH libinput 1/2] Change the scroll step distance to 15 and document it as degrees

[Peter Hutterer]

On Mon, Jan 19, 2015 at 05:49:36PM -0800, Bill Spitzak wrote:
> I really find it doubtful that users are thinking about how many times the
> scroll wheel has revolved about it's axis! Much more likely is the tangent
> distance that the edge of the scroll wheel has moved.
> 
> On a really typical mouse that I have it seems to be about 2.5mm per click.
> Looking at it from the side I think maybe I get 4 clicks for a 45 degree
> rotation, which means one click is 11.25 degrees.

you can measure that quite accurately: mark the wheel with a pen, spin it
until the spot marked is in the same position and then count the number of
REL_WHEEL events received.

> I also have a Kensington track ball with a big ring around it that you
> rotate for the scroll wheel, I suppose then somebody would think about
> angle. I counted and it is doing 42 steps for a full rotation, or 8.6
> degrees per click.

that would need a udev hwdb entry then.

> Windows api, which probably should be paid attention to just because of the
> existence of software, indicates that it mulitplies the clicks by 120 (from
> WM_MOUSEWHEEL docs):

Qt does the same, one unit is defined at 1/8 of a degree. which makes sense
if you have an API with ints but we already use doubles so it's pretty much
the same.

Cheers,
   Peter

> "The wheel rotation will be a multiple of WHEEL_DELTA, which is set at 120.
> This is the threshold for action to be taken, and one such action (for
> example, scrolling one increment) should occur for each delta.
> The delta was set to 120 to allow Microsoft or other vendors to build
> finer-resolution wheels (a freely-rotating wheel with no notches) to send
> more messages per rotation, but with a smaller value in each message. To use
> this feature, you can either add the incoming delta values until WHEEL_DELTA
> is reached (so for a delta-rotation you get the same response), or scroll
> partial lines in response to the more frequent messages. You can also choose
> your scroll granularity and accumulate deltas until it is reached."
> 
> I remember doing some stuff with this and pretty much had to assume each
> event was a wheel click, as the suggested summation did not really work and
> would quickly get out of sync with the clicks. Assuming other software did
> this as well, it would explain why such "finer-resolution wheels"
> disappeared from the market: event the tiniest motion would be treated as a
> full wheel click by this software and would make your window scroll far too
> fast.
> 
> All in all this change seems pretty questionable.
> 
> On 01/11/2015 03:12 PM, Peter Hutterer wrote:
> >Similar to the mouse resolution, let's make the scroll distance a sensible
> >predictable value. Most mice use a 15 degree angle per scroll click, so let's
> >change to that. This will alter behaviour in clients that expect 10 but it
> >shouldn't be too bad. We return doubles anyway for the axis value, so that
> >leaves the option of really fine-grained step sizes.
> >
> >We currently assume all mice have 15 degree angles. Like the DPI settings, it
> >will require a udev property to be set. Patch for that to follow.