In case you're not following the discussion on the xorg-devel list, a short
summary from the thread here:
http://lists.freedesktop.org/archives/xorg-devel/2011-August/024394.html
and particulary the last one in this thread here:
http://lists.freedesktop.org/archives/xorg-devel/2011-August/024474.html
XI 2.1 will include a new feature called "smooth scrolling". In the past
scrolling in X was emulated through logical button clicks 4-7. with this
change, scrolling information is exported through valuators. Axes that
function as scrolling axes are marked as such. Thus, your average wheel
mouse will now have three axes:
Rel X
Rel Y
Rel Wheel (XIAxisVertScroll)
And if the wheel triggers an event by 3 units, you you get one motion event
with value 3 on Rel Wheel. This allows clients to provide a smoother
scrolling experience than right now. so far, so good.
Since we don't expect all clients to switch to smooth scrolling overnight,
we will continue to also send button events. These events are flagged with
XIPointerEmulated, so clients that support smooth scrolling can safely
ignore these events. For the example above, the wheel turn by 3 will thus
generate these events:
XI_Motion: valuator Rel Wheel: 3
XI_ButtonPress:5 (XIPointerEmulated)
XI_ButtonRelease: 5 (XIPointerEmulated)
XI_ButtonPress: 5 (XIPointerEmulated)
XI_ButtonRelease: 5 (XIPointerEmulated)
XI_ButtonPress: 5 (XIPointerEmulated)
XI_ButtonRelease: 5 (XIPointerEmulated)
So far, so good.
Doing this for the wacom driver i more complicated with the following issues
identified so far:
On a typical scroll wheel, a turn by three units is the equivalent of 3
scroll events. On the I4 scroll ring, 3 units are 1 scroll event. The
solution to this is to expose the step increment to the client so
they can calculate what comprises one unit of scrolling.
Problem two was the I4 scroll ring that wraps around and jumps from 71 to
0. That too could be solved by an extra axis flag XIAxisWrapAround. Bonus:
handy for clients to know this if they're dealing with the raw valuator
information.
The biggest issue was custom scroll configuration. On a wheel, down is down.
on our strips this isn't as clear-cut and left strip up may mean scroll
down. If the server emulates button events, it needs to know which buttons
to emulate for each valuator. This is solved by marking an axis as either
vertical or horizontal and allowing for a negative increment for axis
inversion.
For complicated mapping (Ctrl+, Ctrl-), we can simply unflag the axis as
scrolling axis so it isn't interpreted by the clients anymore.
I think this covers all cases but if you have more, please let me know.
Cheers,
Peter
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