How to help on multitouch?

Chase Douglas chase.douglas at
Wed Jun 16 12:27:48 PDT 2010

On Tue, 2010-06-15 at 16:25 +1000, Peter Hutterer wrote:
> On Sun, Jun 13, 2010 at 10:28:04PM -0400, Chase Douglas wrote:
> > Second, I was thinking about the DIDs approach, but I have some concerns
> > about situations where legacy apps don't listen for them. My thought is
> > that if a legacy app has not registered to listen for DIDs, the X server
> > should do some work on its behalf. The server could create one master
> > pointer for each legacy app and warp it to the location of the first
> > touch in the app. 
> The core protocol (i.e. legacy apps) does not permit devices to disappear.
> This isn't much of an issue with the current devices but touchpoints appear
> and disappear all the time. So for each touchpoint that you want to expose
> as a core device, you'd have to keep the device around in case the client
> requests a grab, etc. This is an issue in the MPX model already but much
> less of a problem because one doesn't unplug mice and keyboards all the time.
> Example case: touch comes in, sends button press), touch disappears, client
> requests grab on this device. Now you can grab any other device to be
> protocol-correct but that's not what the user will expect.

I read the following to get a better idea of how things work:

My thinking is that when a legacy client connects (i.e. a client that
does not tell the server it is DID/MPX aware), the server creates a new
master pointer for it. It then core-grabs the new master pointer on
behalf of the client. Now, when the first DID button down event occurs
inside a window of the client, we start sending core events for the
client's master pointer.

(This is starting to sound like reinventing slave devices, only this
time we would be mucking with the heirarchy on the client side of the
server instead of the driver side of the server.)

In legacy apps, when a second master pointer (i.e. representing
traditional mice/trackpads) enters the window, we can intercept their
core events and feed them through to the app as though it came from the
client's own master pointer.

With this approach, all legacy apps see one and only one master pointer.
This avoids some of the grab issues with MPX.

If a client reports that it is DID/MPX aware, then we don't do anything
special with it. It receives core events from master pointers and DID
events through XI2. 

> > Multiple touches beyond the first inside such an app
> > do nothing. This would allow multitouch DIDs to control legacy apps
> > while avoiding some of the master pointer dynamic allocation issues that
> > required multitouchd to have a pool of pre-allocated pointers for.
> How do you define "inside" an application? Clients can register for events
> on windows that they do not own - does this count as "inside"?
> Note that this is already solved with the Grab Ownership approach in MPX,
> but again, touch is trickier than mice/keyboards and grab ownership doesn't
> quite apply here unless you somehow tag new touchpoints with the touch group
> they belong to.

Can't we determine if a DID event is in a client's window by the same
techniques used in DoEnterLeaveEvent()? It already determines which
client and window the event occurs in.

-- Chase

More information about the xorg-devel mailing list