416 lines
18 KiB
ReStructuredText
416 lines
18 KiB
ReStructuredText
|
.. include:: <isonum.txt>
|
||
|
|
|
||
|
|
=========================
|
||
|
|
Multi-touch (MT) Protocol
|
||
|
|
=========================
|
||
|
|
|
||
|
|
:Copyright: |copy| 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.
|
||
|
|
|
||
|
|
.. note::
|
||
|
|
MT protocol type A is obsolete, all kernel drivers have been
|
||
|
|
converted to use type B.
|
||
|
|
|
||
|
|
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 [#f5]_.
|
||
|
|
|
||
|
|
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 [#f3]_.
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
The minimum value of the ABS_MT_SLOT axis must be 0.
|
||
|
|
|
||
|
|
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 tool, 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 center of the touching region (a) is
|
||
|
|
ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
|
||
|
|
ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
|
||
|
|
diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
|
||
|
|
harder against the glass. The touch 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.
|
||
|
|
|
||
|
|
::
|
||
|
|
|
||
|
|
|
||
|
|
Linux MT Win8
|
||
|
|
__________ _______________________
|
||
|
|
/ \ | |
|
||
|
|
/ \ | |
|
||
|
|
/ ____ \ | |
|
||
|
|
/ / \ \ | |
|
||
|
|
\ \ a \ \ | a |
|
||
|
|
\ \____/ \ | |
|
||
|
|
\ \ | |
|
||
|
|
\ b \ | b |
|
||
|
|
\ \ | |
|
||
|
|
\ \ | |
|
||
|
|
\ \ | |
|
||
|
|
\ / | |
|
||
|
|
\ / | |
|
||
|
|
\ / | |
|
||
|
|
\__________/ |_______________________|
|
||
|
|
|
||
|
|
|
||
|
|
In addition to the MAJOR parameters, the oval shape of the touch and finger
|
||
|
|
regions can be described by adding the MINOR parameters, such that MAJOR
|
||
|
|
and MINOR are the major and minor axis of an ellipse. The orientation of
|
||
|
|
the touch ellipse can be described with the ORIENTATION parameter, and the
|
||
|
|
direction of the finger ellipse is given by the vector (a - b).
|
||
|
|
|
||
|
|
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 [#f5]_.
|
||
|
|
|
||
|
|
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 [#f4]_.
|
||
|
|
|
||
|
|
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 [#f4]_.
|
||
|
|
|
||
|
|
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 [#f4]_.
|
||
|
|
|
||
|
|
ABS_MT_WIDTH_MINOR
|
||
|
|
The length, in surface units, of the minor axis of the approaching
|
||
|
|
tool. Omit if circular [#f4]_.
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
If the resolution is zero, the pressure data is in arbitrary units.
|
||
|
|
If the resolution is non-zero, the pressure data is in units/gram. See
|
||
|
|
:ref:`input-event-codes` for details.
|
||
|
|
|
||
|
|
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 touching 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 an ellipse aligned with
|
||
|
|
the Y axis (north) of the surface, a negative value when the ellipse is
|
||
|
|
turned to the left, and a positive value when the ellipse is turned to the
|
||
|
|
right. When aligned with the X axis in the positive direction, the range
|
||
|
|
max should be returned; when aligned with the X axis in the negative
|
||
|
|
direction, the range -max should be returned.
|
||
|
|
|
||
|
|
Touch ellipses are symmetrical by default. For devices capable of true 360
|
||
|
|
degree orientation, the reported orientation must exceed the range max to
|
||
|
|
indicate more than a quarter of a revolution. For an upside-down finger,
|
||
|
|
range max * 2 should be returned.
|
||
|
|
|
||
|
|
Orientation can be omitted if the touch area 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 axes, but
|
||
|
|
not (uniquely) any values in between. In such cases, the range of
|
||
|
|
ABS_MT_ORIENTATION should be [0, 1] [#f4]_.
|
||
|
|
|
||
|
|
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_X
|
||
|
|
The surface X coordinate of the center of the approaching tool. Omit if
|
||
|
|
the device cannot distinguish between the intended touch point and the
|
||
|
|
tool itself.
|
||
|
|
|
||
|
|
ABS_MT_TOOL_Y
|
||
|
|
The surface Y coordinate of the center of the approaching tool. Omit if the
|
||
|
|
device cannot distinguish between the intended touch point and the tool
|
||
|
|
itself.
|
||
|
|
|
||
|
|
The four position values can be used to separate the position of the touch
|
||
|
|
from the position of the tool. If both positions are present, the major
|
||
|
|
tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
|
||
|
|
aligned with the touch axes.
|
||
|
|
|
||
|
|
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 mainly supports
|
||
|
|
MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_.
|
||
|
|
For type B devices, this event is handled by input core; drivers should
|
||
|
|
instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may
|
||
|
|
change over time while still touching the device, because the firmware may
|
||
|
|
not be able to determine which tool is being used when it first appears.
|
||
|
|
|
||
|
|
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 [#f5]_. 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
|
||
|
|
[#f5]_. 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).
|
||
|
|
|
||
|
|
For Win8 devices with both T and C coordinates, the position mapping is::
|
||
|
|
|
||
|
|
ABS_MT_POSITION_X := T_X
|
||
|
|
ABS_MT_POSITION_Y := T_Y
|
||
|
|
ABS_MT_TOOL_X := C_X
|
||
|
|
ABS_MT_TOOL_Y := C_Y
|
||
|
|
|
||
|
|
Unfortunately, there is not enough information to specify both the touching
|
||
|
|
ellipse and the tool ellipse, so one has to resort to approximations. One
|
||
|
|
simple scheme, which is compatible with earlier usage, is::
|
||
|
|
|
||
|
|
ABS_MT_TOUCH_MAJOR := min(X, Y)
|
||
|
|
ABS_MT_TOUCH_MINOR := <not used>
|
||
|
|
ABS_MT_ORIENTATION := <not used>
|
||
|
|
ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
|
||
|
|
ABS_MT_WIDTH_MINOR := min(X, Y)
|
||
|
|
|
||
|
|
Rationale: We have no information about the orientation of the touching
|
||
|
|
ellipse, so approximate it with an inscribed circle instead. The tool
|
||
|
|
ellipse should align with the vector (T - C), so the diameter must
|
||
|
|
increase with distance(T, C). Finally, assume that the touch diameter is
|
||
|
|
equal to the tool thickness, and we arrive at the formulas above.
|
||
|
|
|
||
|
|
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 [#f3]_.
|
||
|
|
|
||
|
|
|
||
|
|
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.
|
||
|
|
|
||
|
|
.. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
|
||
|
|
.. [#f2] The list can of course be extended.
|
||
|
|
.. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
|
||
|
|
.. [#f4] See the section on event computation.
|
||
|
|
.. [#f5] See the section on finger tracking.
|