multitouch

[Artem Ananiev]

On 3/1/2010 3:41 PM, Daniel Stone wrote:
> On Mon, Mar 01, 2010 at 12:42:40PM +0100, Bradley T. Hughes wrote:
>> On 03/01/2010 12:22 PM, ext Daniel Stone wrote:
>>> and so on, and so forth ... would this be useful enough to let you take
>>> multi-device rather than some unpredictable hybrid?
>>
>> It would for me, absolutely. This avoids the multi-device grab problem
>> described by Peter earlier, but I'm unsure how well it works given that
>> we still lack the user/gesture context (as described by Peter).
>
> Any suggestions? :) Reference to how OS X and/or Windows implement it
> would be welcome too.

In a few words, Windows expects all the subsequent touch events to occur 
on the same window as the first touch. If I press another window, the 
corresponding WM_TOUCH notification is just skipped - not sent to the 
client. I'm not sure about explicit mouse grab (SetCapture() call), though.

Gestures and touch events are mutually exclusive on Windows: one can 
either receive WM_GESTURE or WM_TOUCH messages, but not the both. In the 
latter case, I can feed the gestures engine manually, though, but again, 
if I don't receive touch events for different windows, I can't make the 
gestures engine recognize gestures for different windows.

In other words, Windows doesn't bother about user/gesture context at 
all. If a client needs some complex manipulations (e.g. multiple users 
interacting a large touch table), the native system doesn't provide any 
help for that - just listen to the low-level touch events and write your 
custom gestures recognizer.

Thanks,

Artem

>>> (What happens in the hybrid system when I get an event from finger 1,
>>> decide I like it, take out a grab, and then finger 2 presses on another
>>> window.  Do I respect the event and give the app the finger 2 press it
>>> likely doesn't want, or break the grab and deliver it to another client?
>>> Neither answer is pleasant.)
>>
>> But what happens in the sub-device system? Finger 1's event goes via it's
>> MD to the client under the finger, but what about the second finger if
>> it's over another client's window? Does it go to the client that got
>> finger1 since it's on the same MD (which precludes the multi-user use
>> case)?
>
> You just extend the MD/SD semantics down, so if you have:
> MD ->  Touchscreen ->  Fingers [123...], then:
>     ->  Mouse
> then:
>    * grab on finger 1 results in only finger 1 being grabbed, fingers
>      2345 and the mouse are delivered as normal to any client
>    * grab on the ts results in all fingers being grabbed, but the mouse
>      being delivered as normal
>    * grab on the MD results in everything being grabbed
>    * grab on finger 1 by client A followed by a grab on the MD by client
>      B results in finger 1 being delivered to client A, and all other
>      fingers and the mouse being delivered to client B
>
> So, you just take the existing MD/SD semantics and extend them to n
> levels instead of 2.  Just walk up the list of parents looking for
> grabs/interest, stopping at the first one you find.
>
> Peter, correct me if I'm missing something blindingly obvious, or wholly
> non-obvious. :)
>
> Cheers,
> Daniel
>
>
>
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