Multi-touch (MT) Protocol
-------------------------
Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se>
Introduction
------------
In order to utilize the full power of the new multi-touch and multi-user
devices, a way to report detailed data from multiple contacts, i.e.,
objects in direct contact with the device surface, is needed. This
document describes the multi-touch (MT) protocol which allows kernel
drivers to report details for an arbitrary number of contacts.
The protocol is divided into two types, depending on the capabilities of the
hardware. For devices handling anonymous contacts (type A), the protocol
describes how to send the raw data for all contacts to the receiver. For
devices capable of tracking identifiable contacts (type B), the protocol
describes how to send updates for individual contacts via event slots.
Protocol Usage
--------------
Contact details are sent sequentially as separate packets of ABS_MT
events. Only the ABS_MT events are recognized as part of a contact
packet. Since these events are ignored by current single-touch (ST)
applications, the MT protocol can be implemented on top of the ST protocol
in an existing driver.
Drivers for type A devices separate contact packets by calling
input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
event, which instructs the receiver to accept the data for the current
contact and prepare to receive another.
Drivers for type B devices separate contact packets by calling
input_mt_slot(), with a slot as argument, at the beginning of each packet.
This generates an ABS_MT_SLOT event, which instructs the receiver to
prepare for updates of the given slot.
All drivers mark the end of a multi-touch transfer by calling the usual
input_sync() function. This instructs the receiver to act upon events
accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
of events/packets.
The main difference between the stateless type A protocol and the stateful
type B slot protocol lies in the usage of identifiable contacts to reduce
the amount of data sent to userspace. The slot protocol requires the use of
the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
the raw data [5].
For type A devices, the kernel driver should generate an arbitrary
enumeration of the full set of anonymous contacts currently on the
surface. The order in which the packets appear in the event stream is not
important. Event filtering and finger tracking is left to user space [3].
For type B devices, the kernel driver should associate a slot with each
identified contact, and use that slot to propagate changes for the contact.
Creation, replacement and destruction of contacts is achieved by modifying
the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
is interpreted as a contact, and the value -1 denotes an unused slot. A
tracking id not previously present is considered new, and a tracking id no
longer present is considered removed. Since only changes are propagated,
the full state of each initiated contact has to reside in the receiving
end. Upon receiving an MT event, one simply updates the appropriate
attribute of the current slot.
Some devices identify and/or track more contacts than they can report to the
driver. A driver for such a device should associate one type B slot with each
contact that is reported by the hardware. Whenever the identity of the
contact associated with a slot changes, the driver should invalidate that
slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
tracking more contacts than it is currently reporting, the driver should use
a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
being tracked by the hardware at that moment. The driver should do this by
explicitly sending the corresponding BTN_TOOL_*TAP event and setting
use_count to false when calling input_mt_report_pointer_emulation().
The driver should only advertise as many slots as the hardware can report.
Userspace can detect that a driver can report more total contacts than slots
by noting that the largest supported BTN_TOOL_*TAP event is larger than the
total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
Protocol Example A
------------------
Here is what a minimal event sequence for a two-contact touch would look
like for a type A device:
ABS_MT_POSITION_X x[0]
ABS_MT_POSITION_Y y[0]
SYN_MT_REPORT
ABS_MT_POSITION_X x[1]
ABS_MT_POSITION_Y y[1]
SYN_MT_REPORT
SYN_REPORT
The sequence after moving one of the contacts looks exactly the same; the
raw data for all present contacts are sent between every synchronization
with SYN_REPORT.
Here is the sequence after lifting the first contact:
ABS_MT_POSITION_X x[1]
ABS_MT_POSITION_Y y[1]
SYN_MT_REPORT
SYN_REPORT
And here is the sequence after lifting the second contact:
SYN_MT_REPORT
SYN_REPORT
If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
last SYN_REPORT will be dropped by the input core, resulting in no
zero-contact event reaching userland.
Protocol Example B
------------------
Here is what a minimal event sequence for a two-contact touch would look
like for a type B device:
ABS_MT_SLOT 0
ABS_MT_TRACKING_ID 45
ABS_MT_POSITION_X x[0]
ABS_MT_POSITION_Y y[0]
ABS_MT_SLOT 1
ABS_MT_TRACKING_ID 46
ABS_MT_POSITION_X x[1]
ABS_MT_POSITION_Y y[1]
SYN_REPORT
Here is the sequence after moving contact 45 in the x direction:
ABS_MT_SLOT 0
ABS_MT_POSITION_X x[0]
SYN_REPORT
Here is the sequence after lifting the contact in slot 0:
ABS_MT_TRACKING_ID -1
SYN_REPORT
The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
message removes the association of slot 0 with contact 45, thereby
destroying contact 45 and freeing slot 0 to be reused for another contact.
Finally, here is the sequence after lifting the second contact:
ABS_MT_SLOT 1
ABS_MT_TRACKING_ID -1
SYN_REPORT
Event Usage
-----------
A set of ABS_MT events with the desired properties is defined. The events
are divided into categories, to allow for partial implementation. The
minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
allows for multiple contacts to be tracked. If the device supports it, the
ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
of the contact area and approaching contact, respectively.
The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
looking through a window at someone gently holding a finger against the
glass. You will see two regions, one inner region consisting of the part
of the finger actually touching the glass, and one outer region formed by
the perimeter of the finger. The diameter of the inner region is the
ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
against the glass. The inner region will increase, and in general, the
ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
unity, is related to the contact pressure. For pressure-based devices,
ABS_MT_PRESSURE may be used to provide the pressure on the contact area
instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
indicate the distance between the contact and the surface.
In addition to the MAJOR parameters, the oval shape of the contact can be
described by adding the MINOR parameters, such that MAJOR and MINOR are the
major and minor axis of an ellipse. Finally, the orientation of the oval
shape can be describe with the ORIENTATION parameter.
For type A devices, further specification of the touch shape is possible
via ABS_MT_BLOB_ID.
The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
may be used to track identified contacts over time [5].
In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
implicitly handled by input core; drivers should instead call
input_mt_report_slot_state().
Event Semantics
---------------
ABS_MT_TOUCH_MAJOR
The length of the major axis of the contact. The length should be given in
surface units. If the surface has an X times Y resolution, the largest
possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
ABS_MT_TOUCH_MINOR
The length, in surface units, of the minor axis of the contact. If the
contact is circular, this event can be omitted [4].
ABS_MT_WIDTH_MAJOR
The length, in surface units, of the major axis of the approaching
tool. This should be understood as the size of the tool itself. The
orientation of the contact and the approaching tool are assumed to be the
same [4].
ABS_MT_WIDTH_MINOR
The length, in surface units, of the minor axis of the approaching
tool. Omit if circular [4].
The above four values can be used to derive additional information about
the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
the notion of pressure. The fingers of the hand and the palm all have
different characteristic widths [1].
ABS_MT_PRESSURE
The pressure, in arbitrary units, on the contact area. May be used instead
of TOUCH and WIDTH for pressure-based devices or any device with a spatial
signal intensity distribution.
ABS_MT_DISTANCE
The distance, in surface units, between the contact and the surface. Zero
distance means the contact is touching the surface. A positive number means
the contact is hovering above the surface.
ABS_MT_ORIENTATION
The orientation of the ellipse. The value should describe a signed quarter
of a revolution clockwise around the touch center. The signed value range
is arbitrary, but zero should be returned for a finger aligned along the Y
axis of the surface, a negative value when finger is turned to the left, and
a positive value when finger turned to the right. When completely aligned with
the X axis, the range max should be returned. Orientation can be omitted
if the touching object is circular, or if the information is not available
in the kernel driver. Partial orientation support is possible if the device
can distinguish between the two axis, but not (uniquely) any values in
between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1]
[4].
ABS_MT_POSITION_X
The surface X coordinate of the center of the touching ellipse.
ABS_MT_POSITION_Y
The surface Y coordinate of the center of the touching ellipse.
ABS_MT_TOOL_TYPE
The type of approaching tool. A lot of kernel drivers cannot distinguish
between different tool types, such as a finger or a pen. In such cases, the
event should be omitted. The protocol currently supports MT_TOOL_FINGER and
MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
drivers should instead use input_mt_report_slot_state().
ABS_MT_BLOB_ID
The BLOB_ID groups several packets together into one arbitrarily shaped
contact. The sequence of points forms a polygon which defines the shape of
the contact. This is a low-level anonymous grouping for type A devices, and
should not be confused with the high-level trackingID [5]. Most type A
devices do not have blob capability, so drivers can safely omit this event.
ABS_MT_TRACKING_ID
The TRACKING_ID identifies an initiated contact throughout its life cycle
[5]. The value range of the TRACKING_ID should be large enough to ensure
unique identification of a contact maintained over an extended period of
time. For type B devices, this event is handled by input core; drivers
should instead use input_mt_report_slot_state().
Event Computation
-----------------
The flora of different hardware unavoidably leads to some devices fitting
better to the MT protocol than others. To simplify and unify the mapping,
this section gives recipes for how to compute certain events.
For devices reporting contacts as rectangular shapes, signed orientation
cannot be obtained. Assuming X and Y are the lengths of the sides of the
touching rectangle, here is a simple formula that retains the most
information possible:
ABS_MT_TOUCH_MAJOR := max(X, Y)
ABS_MT_TOUCH_MINOR := min(X, Y)
ABS_MT_ORIENTATION := bool(X > Y)
The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
the device can distinguish between a finger along the Y axis (0) and a
finger along the X axis (1).
Finger Tracking
---------------
The process of finger tracking, i.e., to assign a unique trackingID to each
initiated contact on the surface, is a Euclidian Bipartite Matching
problem. At each event synchronization, the set of actual contacts is
matched to the set of contacts from the previous synchronization. A full
implementation can be found in [3].
Gestures
--------
In the specific application of creating gesture events, the TOUCH and WIDTH
parameters can be used to, e.g., approximate finger pressure or distinguish
between index finger and thumb. With the addition of the MINOR parameters,
one can also distinguish between a sweeping finger and a pointing finger,
and with ORIENTATION, one can detect twisting of fingers.
Notes
-----
In order to stay compatible with existing applications, the data reported
in a finger packet must not be recognized as single-touch events.
For type A devices, all finger data bypasses input filtering, since
subsequent events of the same type refer to different fingers.
For example usage of the type A protocol, see the bcm5974 driver. For
example usage of the type B protocol, see the hid-egalax driver.
[1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
difference between the contact position and the approaching tool position
could be used to derive tilt.
[2] The list can of course be extended.
[3] The mtdev project: http://bitmath.org/code/mtdev/.
[4] See the section on event computation.
[5] See the section on finger tracking.
