emit "smooth scrolling/pixel perfect" events

dustin dustin.bensing at googlemail.com
Mon Dec 29 11:07:31 PST 2014


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 

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
          2: -1.00 (-1.00)

EVENT type 15 (RawButtonPress)
    device: 2 (10)
    detail: 5
    flags: emulated

EVENT type 16 (RawButtonRelease)
    device: 2 (10)
    detail: 5
    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
          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.

best regards,
Dustin Bensing
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