ubuntu-x-swat team mailing list archive

[Chase Douglas <chase.douglas@xxxxxxxxxxxxx>]

Some background information on XInput: XInput 1.x added "valuators"
which are axes that provide data beyond just X and Y. Unfortunately, in
the infinite wisdom of the XInput 1.x creators, the valuators were not
labeled. Thus, the Qt code has #defines like WAC_XTILT_I. Essentially,
in XI 1.x the order of valuators is part of the ABI. However, I think
only the Wacom X module has provided a stable valuator ordering, so it's
all just a big mess.

This means that the code only works if you are using the X Wacom input
module. I'm assuming you all are, but it would be good to double check
(check /var/log/Xorg.0.log).

Another quirk in XI 1.x is that it provides valuator ranges instead of a
bitmask of changed valuators. So, if the values in axes 3 and 5 changed,
XI 1.x must send data for the current values of axes 3, 4, and 5. In XI
2.x, the server sends only the new values of 3 and 5. It is possible
that there is an issue in how masked valuators in the internals of the X
server are being handled, but I have not seen any indication this is the
case. xinput test and xinput test-xi2 are great for testing this. You
can run the same recording through both and see if there are any
discrepencies. Note that natty and Oneiric are the first X servers that
actually use masked valuators internally in the server. Clients who
aren't fully compliant to the XI specs will only start to see issues now
:).

I just got to this paragraph to summarize things, and then I realized
what may be the issue. Notice how the xTilt and yTilt variables are set
based on a constant index into the event valuators. This works fine if
the event valuator range always begins at 0. However, if only the 3rd
and 4th valuators changed, the valuator range from the server would be 3
to 4, and the indexes of valid data would be 0 and 1 (shifted down by
2). See /usr/include/X11/extensions/XInput.h, XDevice*Event.first_axis
and XDevice*Event.axes_count for the valuator range of an event.

This would also explain why the issue is seen sporadically with pressure
data but not with tilt. When pressure changes, X and Y is likely to also
change. This causes the valuator range to start at 0, so the axis
indices used in Qt are ok. However, when tilt changes, maybe it usually
is the only valuator that changes value. Thus, the valuator range starts
at whatever the tilt valuator is, and the indices used by Qt are wrong.

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https://bugs.launchpad.net/bugs/799202

Title:
  wacom stylus jumps with latest xserver-xorg-core

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