Another approach to multitouch handling

Rafi Rubin rafi at
Sun Jun 6 22:20:39 PDT 2010

On 06/07/2010 12:26 AM, Peter Hutterer wrote:
> On Wed, Jun 02, 2010 at 04:40:34PM +0200, Carlos Garnacho wrote:
>> I've been discussing with Peter Hutterer about the convenience of the
>> "touchpoints as multiple valuators" approach, and how it could (IMHO)
>> delay adoption in the short/mid term for anything related to multitouch.
> [...]
>> =The proposal=
>>          The multitouch capable hw device would have a main device
>>          created, which is able to send core events and be attached to a
>>          MD, the evdev driver would also create several floating devices
>>          (one for each touchpoint), unable to send core events nor to be
>>          attached to a MD (I've disabled XI86_POINTER_CAPABLE for these,
>>          but the server doesn't seem to honor that).
>>          The only purpose for the main device would be routing events for
>>          one of the floating touchpoints. Whenever a new touch happens,
>>          and the main device isn't already routing events from another
>>          touch, the events that such touchpoint generates would be sent
>>          through the main device instead.
>>          This means that there would be N+1 devices for N touchpoints, so
>>          at least 1 of these devices wouldn't be sending events, this
>>          makes touchpoints somewhat anonymous for multitouch purposes,
>>          but the routed touchpoint would remain constant as long as it's
>>          operating on the device (press ->  ... ->  release). This also
>>          provides sane backwards compatibility, non-XI2 clients would
>>          just see core events from the main device.
>>          I've been experimenting with this concept, and together with a
>>          ~200LOC patch to GTK+ master (master is already XI2 capable)
>>          I've got things working out of the box, also wrt hotplugging.
>> =The code=
>>          I've started off Benjamin's multitouch-subdevs branch for this
>>          proof of concept.
>> Ideas? comments?
> some more background here for others because some of the talks were on
> private email:
> I've continuously failed to get a multitouch proposal together where touch
> points act like pointers to clients. It always runs into the same walls with
> the biggest one being that the transitive nature of a touchpoint is quite
> imcompatible with many of the core protocol's assumptions.
> For example, the only way the X server can legally break a grab is by
> unmapping the window. That, combined with the race conditions exposed by a
> client delayingly grabbing a pointer that's not even there anymore make it
> rather hard.
> One of the reasons the current approach with stuffing MT data into valuators
> was picked was because it is implementable right now and at least had some
> positive reception.
> Recently, I changed my requirements and figured that we may not need to have
> MT core event support in the protocol but rather leave this up to the
> toolkits. So instead of having core events from MT devices we send MT events
> down the wire and the MT-aware toolkit converts those into the required
> callbacks.
> When I asked Carlos about this, he had already started the work above which
> overlaps to a large degree (though his implementation is different for
> technical reasons).
> The main concept that I think we might need eventually here is twofold:
> - A new device type (let's call it "Direct Input Device", DID) that does not
>    require the abstraction between physical and virtual input device that we
>    have with the MD/SD hierarchy. Unlike a mouse, where you interact on the
>    physical device is where you want the interaction to happen.
> - DID's are non-core devices that are _not_ core devices and thus only send
>    XI2 events. This allows them to be transient with the protocol crafted
>    around their requirements.
> The first DID could act like MD and thus send core events, leaving
> rudimentary single-touch capabilities. Because core falls away, we can
> sidestep the grab handling on the whole lot.
> What's not sorted out yet is sane keyboard handling, it most likely requires
> the introduction of touch groups that share input focus between multiple
> DIDs and of course keyboard would then need to be attached to DIDs instead
> of SD's making it interesting for XI 2.0 clients.
> Carlos' implementation gets us similar effects already by using slave
> devices instead of a new type of devices but especially when we think about
> new event types I think having DIDs might be the long-term solution.
> So yeah, I'm rather optimistic about this approach though there are some
> issues yet to be solved. If you want to chime in, please do so.
> (Also, no code exists yet, this is just hot air from my side so far)
> Cheers,
>    Peter

So are you saying you actually want to be able to subscribe to events from a mt 
finger in window that's next to the window with the rest of the fingers?  Is 
that really a good idea?

Perhaps I should clarify my current understanding and thoughts.

I thought we were talking about having a pointer with a single conventional 
group/cluster position.  In this model, fingers show up as mt positions 
annotated on the pointer, and the cluster position may or may not be the 
position of one of those fingers.  I see that cluster position both as focus 
control for the mt contact set as well as a way to use mt as a conventional pointer.

I see the selection of cluster position as a bit of an arbitrary implementation 
detail (or even a normal option to accommodate differing preferences).  Four 
ideas come to mind:
1.  eldest active contact
2.  first contact (cluster doesn't move if that first finger stays off the sensor)
3.  geometric center
4.  completely independent.

Consider the magic mouse for a moment.  It has a real conventional pointing 
device and an independent mt surface.  I think that clients that want to 
subscribe to finger positions on that touch surface should see them as some how 
related to the position of the conventional pointer.

I think we should eventually talk about supporting mt from a one or more sensors 
in multiple windows/locations.  But I would like to think in terms of spatial 
clustering.  For example, we can cluster based on the size of a hand.  Each hand 
gets a core pointer with one or more fingers annotated.

As for sub-sub-devices vs. valuators, that doesn't matter all that much to me. 
And I think if you establish the meaning, it shouldn't really matter to all that 
many people.  If you have a clean concept down, then it won't change the client 
side code all that much if you switch from one to the other.

I will say if you're talking about a device hierarchy, I think you already have 
1 distinction too many.  I think as far as the client side interface is 
concerned, an input should be an input.  The input should be free to support 
children, if desired (the Virtual core pointer being an example of a children 
fan-in driver).  In that light, the individual fingers are a new special type of 
device, they are just children of a node in the hierarchy.  I suppose that will 
get slightly messier when we start worrying about multiple core pointers for a 
single MT sensor, but I don't think this point of view makes that any trickier 
than the alternatives.


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