[RFC] Multitouch support, step one

Henrik Rydberg rydberg at euromail.se
Mon Mar 15 07:41:24 PDT 2010

Hi Peter,

> Alrighty, I've been thinking some more about multitouch and here's my
> current proposal:
> Good news first - we can probably make it work.
> Bad news second - not quite just yet, not without kludges.

I hope this feedback will be taken the right way, as a friendly injection into
the multitouch discussion. :-)

> Preamble:
> Multi-touch as defined in this proposal is limited to single input-point
> multi-touch. This is suitable for indirect touch devices (e.g. touchpads)
> and partially suited for direct touch devices provided a touch is equivalent
> to a single-gesture single-application input.

User-space applications need tools to *use* MT devices, not route raw data from
the devices to the application. The latter is not much more complicated than
opening a file, and everyone can do that already. Thus, unless there is a model
for how MT devices work and interact with other MT devices, I see little point
in having an X protocol at all.

> Details:
> The data we get from the (Linux) kernel includes essentially all the ABS_MT
> events, x, y, w, h, etc. We can pack this data into valuators on the device.
> In the simplest case, a device with two touchpoints would thus send 4
> valuators - the first two being the coordinate pair for the first touch
> point, the latter two the coordinates for the second touch point.
> XI2 provides us with axis labels, so we can label the axes accordingly.
> Clients that don't read axis labels are left guessing what the fancy values
> mean, which is exactly what they're doing already anyway.

The idea of a wide set of dimensions to describe a set of fingers for instance,
was considered and dropped for the kernel MT interface. There is a definite
difference between having "three things" and having "two more of the same kind".
The number of dimensions also increases dramatically, as pointed out by Mr.
Poole. It makes much more sense to define contacts as multiple instances of the
same thing, than to define each new contact as potentially something completely

> XI2 DeviceEvents provide a bitmask for the valuators present in a device.
> Hence, a driver can dynamically add and remove valuators from events, thus
> providing information about the presence of these valuators.
> e.g. DeviceEvent with valuators [1-4] means two touchpoints down, if the
> next event only includes valuators [3-4], the first touchpoint has
> disappeared.

The idea of adding and removing contacts dynamically I believe is a good idea. A
contact has a set of attributes (x, y, etc). Why not provide a clean interface
for the contacts as a concept, rather than mapping the not-so-independent x and
y values into separate dynamic entities? As an example of the smallest
meaningful dynamic entity:

struct Contact {
	int tracking_id;
	float x, y;
	etc etc...

> Core requires us to always send x/y, hence for core emulation we should
> always include _some_ coordinates that are easily translated. While the
> server does caching of absolute values, I think it would be worthwile to
> always have an x/y coordinate _independent of the touchpoints_ in the event.
> The driver can decide which x/y coordinates are chosen if the first
> touchpoint becomes invalid.

Seconded, but the single-touch x/y coordinates are properties of a contact
group, not of a single contact. Example:

struct ContactGroup {
	int group_id;
	float x, y;
	ContactList list;
	etc etc...

> Hence, the example with 4 valuators above becomes a device with 6 valuators
> instead. x/y and the two coordinate pairs as mentioned above. If extra data
> is provided by the kernel driver, these pairs are simple extended into
> tuples of values, appropriately labeled.
> Core clients will ignore the touchpoints and always process the first two
> coordinates.
> XI1 clients will have to guess what the valuators mean or manually set it up
> in the client.
> XI2 clients will automagically work since the axes are labeled. Note that
> any client that receives such an event always has access to _all_
> touchpoints on the device. This works fine for say 4-finger swipes on a
> touchpad but isn't overly useful for the multiple client case, see
> above.

This is at the heart of the problem, I believe. In addition to being able to
work with a set of ContactGroups, like ContactGroupList, one needs the
possibility to dynamically regroup them, based on geometric information and what
not. Partitioning is the word. A toolset consisting of at least these functions:

ContactGroupList partition_contacts_geometrically(ContactList all_contacts);
ContactGroupList partition_contacts_by_user(ContactList all_contacts);
ContactGroupList find_contact_groups_in_window(ContactGroupList all_groups);
etc etc

ought to be the minimum requirement on the interface, such that applications can
do something meaningful with the information at hand.

> Since additional touchpoints are valuators only, grabs work as if the
> touches belong to a single device. If any client grabs this device, the
> others will miss out on the touchpoints.
> XI2 allows devices to change at runtime. Hence a device may add or remove
> valuators on-the-fly as touchpoints appear and disappear. There is a chance
> of a race condition here. If a driver decides to add/remove valuators
> together with the touchpoints, a client that skips events may miss out.
> e.g. if a DeviceChanged event that removes an axis is followed by one that
> adds an axis, a client may only take the second one as current, thus
> thinking the axis was never removed. There is nothing in the XI2 specs that
> prohibits this. Anyways, adding removing axes together with touchpoints
> seems superfluous if we use the presence of an axis as indicator for touch.
> Rather, I think a device should be set up with a fixed number of valuators
> describing the default maximum number of touchpoints. Additional ones can be
> added at runtime if necessary.

Some events are, as always, more important than others. If the stream bandwidth
is a concern, there is always the possibility to tag events as "important" and
"less important", in the same manner as focus events normally are more important
than mouse movement events.

> Work needed:
> - drivers: updated to parse ABS_MT_FOO and forward it on.
> - X server: the input API still uses the principle of first + num_valuators
>   instead of the bitmask that the XI2 protocol uses. These calls need to be
>   added and then used by the drivers.
> - Protocol: no protocol changes are necessary, though care must be taken in
>   regards to XI1 clients. 
>   Although the XI2 protocol does allow device changes, this is not specified
>   in the XI1 protocol, suggesting that once a device changes, potential XI1
>   clients should be either ignored or limited to the set of axes present
>   when they issued the ListInputDevices request. Alternatively, the option
>   is to just encourage XI1 clients to go the way of the dodo.
> Corner cases:
> We currently have a MAX_VALUATORS define of 32. This may or may not be
> arbitrary and interesting things may or may not happen if we increase that.
> A device exposing several axes _and_ multitouch axes will need to be
> appropriately managed by the driver. In this case, the "right" thing to do
> is likely to expose non-MT axes first and tack the MT axes onto the back.
> Some mapping may need to be added.
> The future addition of real multitouch will likely require protocol changes.
> These changes will need to include a way of differentiating a device that
> does true multitouch from one that does single-point multi-touch.
> That's it, pretty much (well, not much actually). Feel free to poke holes
> into this proposal.

Ok, in conclusion, my two cents are: Do not add MT functionality as evaluators
in X, but implement a proper Contact interface from the start.


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