emit "smooth scrolling/pixel perfect" events

[Peter Hutterer]

On Mon, Dec 29, 2014 at 08:07:31PM +0100, dustin wrote:
> Hi,
> 
> i recently started to play around with the input facilitys of X. My goal is
> to develop a remote control for my PC and for that purpose i want to control
> the mouse with an android tablet. (Maybe other embeded devices)
> 
> I found a few ways to do this namely xtest and uinput/evdev (and untested
> xlib/xcb). Both working out of the box but i think i'll stick with
> uinput/evdev cuz i think this approach should work with libinput(wayland) as
> well? One problem arises that i can't circumvent and this is the scrolling.
> my first implementation works by simply emitting button 4/5 events like what
> ordinary mouse wheels do (looking at xev xinput tools) but what i am rally
> looking for is emitting "smooth scroll events"/"pixel perfect scrolling"
> (whatever this means). I came across some articles from Peter Hutterer
> talking about this and that Gnome implemented this. One can see this in live
> action via touchpad and the epiphany webbrowser. What i discovered was, that
> a client program has to catch XI2 events (XIDeviceEvent)and watch for
> XI_Motion and its valuators. But i really don't know how to emit these? with
> uinput/evdev even if i am emiting EV_REL, REL_WHEEL events and not button
> 4/5 events i got ordinary scrolling. what is the "right way" to emit these
> XIDeviceEvent/XI_Motion events (if these are really the ones i am looking
> for)
> 
> what i was seeing from xinput --test-xi2 for ordinary mouse wheels was
> 
> EVENT type 17 (RawMotion)
>    device: 2 (10)
>    detail: 0
>    valuators:
>    flags:
>          2: -1.00 (-1.00)
> 
> EVENT type 15 (RawButtonPress)
>    device: 2 (10)
>    detail: 5
>    valuators:
>    flags: emulated
> 
> EVENT type 16 (RawButtonRelease)
>    device: 2 (10)
>    detail: 5
>    valuators:
>    flags: emulated
> 
> As i understand this right the first XI_RawMotion is the "now" the regular
> new event and the button presses are for compatibility and can be ignored
> cuz of the "emulated" flag
> 
> the valuator 2 means
> 
> Class originated from: 10. Type: XIValuatorClass
>    Detail for Valuator 2:
> 	Label: Rel Vert Wheel
> 	Range: -1.000000 - -1.000000
> 	Resolution: 1 units/m
> 	Mode: relative
> 
> if i scroll with my touchpad i get
> 
> EVENT type 17 (RawMotion)
>    device: 2 (13)
>    detail: 0
>    valuators:
>    flags:
>          3: 4.00 (4.00)
> 
> there i have a valuator 3 which i guess means Rel Vert Scroll regarding to
> this
> 
> Class originated from: 13. Type: XIValuatorClass
>    Detail for Valuator 3:
> 	Label: Rel Vert Scroll
> 	Range: 0.000000 - -1.000000
> 	Resolution: 0 units/m
> 	Mode: relative
> 
> So the difference must somehow be with the different valuators. If so i just
> don't know how i can influence this with uinput/evdev. Maybe its only doable
> with xlib/xcb/xtest etc. i don't know.
> 
> all i want is to simulate the "pixel perfect smooth scrolling" i get from
> regular laptop touchpads so that apps who respond to this will do from my
> application.

the output of xinput should also include a XIScrollClass. That contains
information whether it's vertical/horizontal and refers to the valuator
number. If such a class is present, the valuator listed is a scrolling
valuator and should be interpreted as such.

You don't have to do anything special in uinput, just send REL_WHEEL and the
rest of the xorg stack will do the right thing. Though since REL_WHEEL is
discreet, you won't get as much use out of smooth scrolling as you would
from a touchpad.

Cheers,
   Peter