Le 08/11/2010 04:51, Peter Hutterer a écrit :
fwiw, I'm not sure "arbitrate" is the right word here, filtering seems
easier to understand in this context. I guess "arbitrate" would apply more
if we emit the events across multiple devices like in the bamboo case.
that's mostly bikeshedding though, my points below apply regardless of what
word we choose :)
note that we also have two different approaches - single kernel device or
multiple kernel devices and depending on the approach the device uses the
options below have different advantages and disadvantages.
the tablets I've dealt with so far exposed a single event device, so that's
what I'm focusing on in this email.
On Fri, Nov 05, 2010 at 11:47:28AM -0700, Ping Cheng wrote:
Recent changes and discussion about MT support at LKML, UDS, and
xorg-devel encouraged me to migrate Wacom MT devices to the slot-based
MT protocol (introduced in kernel 2.6.36). Since Wacom supports both
digitizer and touch devices, I need to decide how to report touch data
when the pen is in proximity.
My goal is to understand how X server would like the MT data to be
reported from the kernel. I hope to keep kernel and X server driver MT
support in sync so we can avoid unnecessary confusion or extra work in
the userland.
The existing solution for single touch events is to arbitrate touch
when pen is in prox. This is based on the assumption that we do not
want to have two cursors competing on the screen.
With the introduction of MT, the touch data are most likely translated
into something other than pointer events. So, reporting both pen and
touch data makes sense now. However, I want to assure a smooth
tansition from single touch to MT for end users so they still get the
single touch behavior as they used to be. I gathered the following
approaches:
1. Arbitrate all touch data in the kernel.
This is the simplest solution for device driver developers. But I do
not feel it is end user and userland client friendly.
I'm strongly opposed to this. kernel filtering of these devices is hard to
circumvent and there _will_ be use-cases where we need more than one tool to
work simultaneously. right now we're worrying about pen + touch, but what
stops tablets from becoming large enough to be used by 2+ users with 2+
pens simultaneously?
from a purely event-stream focused viewpoint: why do we even care whether
something is a pen or a touch? both are just tools and how these should be
used is mostly up to the clients anyway. IMO, the whole point of
MT_TOOL_TYPE is that we don't have to assume use-cases for the tools but
just forward the information to someone who knows how to deal with this.
2. Report first finger touch as ABS_X/Y events when pen is not in
prox. Arbitrating single touch data when pen is in prox. Pen data is
reported as ABS_X/Y events. Both ABS_X/Y for pen or the first finger
and ABS_MT_* for MT data are reported.
This approach reduces the overhead in dealing with two cursors in userland.
3. Report first finger touch as ABS_X/Y events when pen is not in prox;
Report pen data as ABS_X/Y events when there is no finger touch;
Report touch data as MT_TOOL_TOUCH and pen data as MT_TOOL_PEN
events when both pen and touch data are received. No ABS_X/Y are
reported when pen and tocuh or multi-touch data are received.
I feel this one makes sense to userland since pen can be considered as
another touch.
4. Report first finger touch as ABS_X/Y events when pen is not in prox;
Report pen data as ABS_X/Y events when there is no finger touch;
Report touch data as MT_TOOL_TOUCH and pen data as MT_TOOL_PEN
events when both pen and touch data are received. ABS_X/Y are also
reported for pen when both pen and tocuh data are received.
I'd vote for this one. It provides all the data necessary for MT clients
(and all the data the device can support) but has a reasonable single-touch
strategy. Given that wacom tablets are still primarily pen-centric tablets,
the emphasis on pen overriding touch makes sense to me.
Hi,
I'd also vote for this.
I don't think that the kernel should make any assumption on the final
application. The data are available, so we have to pass them.
1. I read that people worry about sending "false" events (touch) while
using the pen. But in my mind, this is a _design_ problem of the final
application. I think the final application will have to filter these
events: for instance, what happens if the user is too lazy to remove his
pen (or just want to keep the hover on the application) out of the
proximity range and want to move its digital sheet of paper in his (her)
design application? The final application will have to choose whether
using or not the touch features (depending on the pressure for instance...).
The solution 4. (*technical solution*) addresses the problem of the
"false" events for the applications (*design problem*) that are not
designed to used multitouch. They will just ignore the touch data.
So I think, it's a good start
2. I would also add that multitouch is not only available for trackpads:
there are also direct devices in absolute coordinate mode. With those
device, the touch data can be directed to an other Xclient that is used
by an other user if the surface is large enough. Currently we only see
relatively small surfaces (bamboo, ntrig devices), but in the future, we
can easily imagine a whole table with both pen and touch.
And this solve Michal's problem as he will be able to use buttons in the
application with the finger.
Cheers,
Benjamin
This one makes sense to userland too. It eases the backward
compatibility support for those clients that don't support MT at all.
Which approach do you like? Or do you have other suggestions share?
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