Multi-touch Protocol

MT protocol type과 contact slot

새 multi-touch와 multi-user 장치를 온전히 사용하려면 장치 표면에 직접 닿은 여러 contact의 상세 data를 보고할 방법이 필요합니다. MT protocol은 kernel driver가 임의 개수 contact의 세부 정보를 전달하게 합니다.

Hardware가 contact를 식별하지 못하는 type A는 모든 anonymous contact의 raw data를 수신자에게 전송합니다. 식별 가능한 contact를 추적하는 type B는 event slot별로 개별 contact 변경만 전송합니다. Type A는 obsolete이며 모든 kernel driver가 type B로 전환됐습니다.

Contact 세부 정보는 `ABS_MT` event packet으로 순서대로 전송하며 `ABS_MT` event만 contact packet 일부로 인식합니다. 기존 single-touch application은 이를 무시하므로 기존 driver의 ST protocol 위에 MT protocol을 추가할 수 있습니다.

Type A driver는 contact packet 끝에 `input_mt_sync()`를 호출해 `SYN_MT_REPORT`를 생성합니다. Type B driver는 각 packet 시작에 slot 인자를 넣어 `input_mt_slot()`을 호출해 `ABS_MT_SLOT`을 생성합니다. 모든 driver는 전체 multi-touch transfer 끝에 `input_sync()`를 호출해 `SYN_REPORT` 이전에 누적된 packet을 적용하게 합니다.

Type B는 identifiable contact를 사용해 변경된 값만 사용자 공간에 보내 data 양을 줄이며 hardware가 제공하거나 raw data에서 계산한 `ABS_MT_TRACKING_ID`가 필수입니다.

Type A는 현재 표면의 anonymous contact 전체를 임의 순서로 열거하며 event filtering과 finger tracking은 사용자 공간이 담당합니다. Type B는 각 contact에 slot을 연결하고 그 slot으로 변경을 보냅니다.

Type B slot의 `ABS_MT_TRACKING_ID`를 바꾸어 contact 생성·교체·삭제를 나타냅니다. Non-negative ID는 활성 contact, -1은 사용하지 않는 slot입니다. 이전에 없던 ID는 새 contact, 더 이상 없는 ID는 제거된 contact입니다. 변경만 전송하므로 수신자는 시작된 각 contact의 전체 상태를 보관하고 MT event가 오면 현재 slot의 해당 attribute만 갱신합니다.

일부 hardware는 driver에 보고할 수 있는 수보다 더 많은 contact를 식별·추적합니다. Driver는 실제로 보고되는 각 contact에 type B slot 하나를 연결하고 slot의 contact identity가 바뀔 때 tracking ID를 바꿔 무효화해야 합니다.

Hardware가 보고 중인 수보다 더 많은 contact를 추적한다고 알리면 `BTN_TOOL_*TAP`을 명시적으로 보내 총 contact 수를 사용자 공간에 알려야 하며 `input_mt_report_pointer_emulation()` 호출 때 `use_count=false`로 둡니다. Driver는 hardware가 실제 보고할 수 있는 수만큼만 slot을 광고합니다.

사용자 공간은 가장 큰 `BTN_TOOL_*TAP` 지원 수가 `ABS_MT_SLOT` absinfo의 slot 총수보다 큰지 비교해 총 추적 contact 수가 slot 수보다 많음을 알 수 있습니다. `ABS_MT_SLOT` axis의 최소값은 반드시 0입니다.

.. 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.

Type A와 type B event sequence

Type A의 두 contact touch는 첫 contact X·Y와 `SYN_MT_REPORT`, 둘째 contact X·Y와 `SYN_MT_REPORT`, 마지막 `SYN_REPORT` 순서입니다. Contact 하나만 움직여도 매 `SYN_REPORT` 사이에 현재 존재하는 모든 contact의 raw data를 다시 보냅니다.

Type A에서 첫 contact를 떼면 둘째 contact X·Y와 `SYN_MT_REPORT`, `SYN_REPORT`만 보냅니다. 둘째 contact까지 떼면 `SYN_MT_REPORT`, `SYN_REPORT`을 보냅니다.

Driver가 MT event 외에 `BTN_TOUCH` 또는 `ABS_PRESSURE`도 보고하면 마지막 `SYN_MT_REPORT`을 생략할 수 있습니다. 그렇지 않으면 input core가 마지막 `SYN_REPORT`을 drop해 zero-contact event가 사용자 공간에 도달하지 않습니다.

Type B의 두 contact 시작은 slot 0, tracking ID 45, X·Y, slot 1, tracking ID 46, X·Y, `SYN_REPORT` 순서입니다.

Contact 45의 X만 움직이면 slot 0을 선택하고 X만 보고한 뒤 `SYN_REPORT`을 보냅니다. Slot 0 contact를 떼면 현재 slot이 이미 0이므로 `ABS_MT_SLOT`을 생략하고 `ABS_MT_TRACKING_ID -1`, `SYN_REPORT`만 보냅니다. 이로써 slot 0과 contact 45의 association을 제거하고 slot을 재사용할 수 있게 합니다.

둘째 contact를 떼면 slot 1을 선택하고 tracking ID -1을 보낸 뒤 `SYN_REPORT`을 보냅니다.

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

Contact·tool 기하와 event 조합

MT property는 `ABS_MT` event 집합으로 정의되며 부분 구현이 가능하도록 category로 나뉩니다. 최소 집합은 여러 contact 위치를 추적하는 `ABS_MT_POSITION_X`, `ABS_MT_POSITION_Y`입니다. Hardware가 지원하면 contact area 크기는 `ABS_MT_TOUCH_MAJOR`, 접근 tool 크기는 `ABS_MT_WIDTH_MAJOR`로 제공합니다.

유리창에 손가락을 대는 모습을 생각하면 실제 유리에 닿는 안쪽 영역과 손가락 둘레가 이루는 바깥 영역이 있습니다. 접촉 영역 중심 a는 `ABS_MT_POSITION_X/Y`, 접근한 손가락 중심 b는 `ABS_MT_TOOL_X/Y`입니다. 접촉 지름은 `ABS_MT_TOUCH_MAJOR`, 손가락 지름은 `ABS_MT_WIDTH_MAJOR`입니다.

더 세게 누르면 touch 영역이 커지며 항상 1보다 작은 `TOUCH_MAJOR / WIDTH_MAJOR` 비율은 접촉 pressure와 관련됩니다. Pressure 기반 장치는 대신 `ABS_MT_PRESSURE`를 사용할 수 있고 hover 장치는 표면과 contact 사이 거리를 `ABS_MT_DISTANCE`로 나타냅니다.

MAJOR에 MINOR를 더하면 touch와 finger 영역의 타원형을 major·minor axis로 표현할 수 있습니다. `ABS_MT_ORIENTATION`은 touch ellipse 방향, vector `(a - b)`는 finger ellipse 방향입니다.

Type A는 `ABS_MT_BLOB_ID`로 touch shape를 더 자세히 지정할 수 있습니다. `ABS_MT_TOOL_TYPE`은 finger, pen 또는 다른 tool 종류를 나타내고 `ABS_MT_TRACKING_ID`는 식별된 contact를 시간에 따라 추적합니다.

Type B에서는 input core가 `ABS_MT_TOOL_TYPE`과 `ABS_MT_TRACKING_ID`를 암묵적으로 처리하므로 driver는 직접 event를 쓰는 대신 `input_mt_report_slot_state()`를 호출해야 합니다.

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().

`ABS_MT_*` event 의미

Touch·tool axis 길이는 surface unit으로 보고합니다. Surface 해상도가 X×Y라면 `ABS_MT_TOUCH_MAJOR` 최대 가능값은 대각선 `sqrt(X^2 + Y^2)`입니다. Circular contact는 `TOUCH_MINOR`, circular tool은 `WIDTH_MINOR`를 생략할 수 있습니다.

`TOUCH_MAJOR / WIDTH_MAJOR` 비율은 pressure를 근사하며 손가락과 손바닥은 특징적인 width가 다릅니다. 공간 signal intensity를 가진 pressure 기반 장치는 TOUCH·WIDTH 대신 `ABS_MT_PRESSURE`를 쓸 수 있습니다. Pressure resolution이 0이면 arbitrary unit, 0이 아니면 units/gram입니다.

`ABS_MT_ORIENTATION`은 touch center 주위로 시계 방향 signed quarter revolution을 나타냅니다. Y axis north와 정렬하면 0, 왼쪽으로 돌면 음수, 오른쪽이면 양수입니다. Positive X 정렬은 range max, negative X 정렬은 -max입니다.

기본 touch ellipse는 대칭입니다. 실제 360도 방향을 보고할 수 있으면 quarter revolution을 넘는 값에 range max보다 큰 값을 쓰고 뒤집힌 손가락은 `range max * 2`입니다. Circular touch 또는 kernel driver가 정보를 얻지 못하면 orientation을 생략할 수 있습니다.

장치가 두 axis 방향만 구분하고 중간 각도를 유일하게 판별하지 못하면 `ABS_MT_ORIENTATION` range를 [0, 1]로 둬 partial orientation을 보고할 수 있습니다.

Touch 중심은 `POSITION_X/Y`, tool 중심은 `TOOL_X/Y`입니다. 두 위치가 모두 있으면 tool major axis가 touch point를 향하며, tool 위치를 구분할 수 없으면 tool axis는 touch axis와 정렬되고 `TOOL_X/Y`를 생략합니다.

`ABS_MT_TOOL_TYPE`은 주로 `MT_TOOL_FINGER`, `MT_TOOL_PEN`, `MT_TOOL_PALM`을 지원합니다. Driver가 구분하지 못하면 생략합니다. Type B에서는 core가 처리하므로 `input_mt_report_slot_state()`를 사용합니다. Firmware가 접촉 시작 때 tool을 판별하지 못할 수 있어 같은 contact 중에도 type이 바뀔 수 있습니다.

`ABS_MT_BLOB_ID`는 여러 packet의 point sequence를 polygon으로 묶어 임의 shape contact를 만드는 type A 저수준 anonymous group이며 high-level tracking ID와 다릅니다. Type A 장치 대부분은 blob capability가 없어 생략할 수 있습니다.

`ABS_MT_TRACKING_ID`는 시작된 contact의 전체 lifecycle을 식별합니다. 장기간 유지되는 contact를 unique하게 구분할 만큼 range가 커야 합니다. Type B driver는 직접 다루지 않고 `input_mt_report_slot_state()`를 사용합니다.

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 계산, finger tracking과 gesture

Hardware 차이를 MT protocol에 일관되게 매핑하기 위해 event 계산 규칙을 사용합니다. Rectangle contact의 signed orientation은 알 수 없으므로 변 길이 X, Y에서 major는 `max(X,Y)`, minor는 `min(X,Y)`, orientation은 `bool(X > Y)`로 둡니다.

Orientation [0,1]은 finger가 Y axis 방향인지(0), X axis 방향인지(1)만 구분할 수 있음을 뜻합니다.

T와 C 좌표가 모두 있는 Win8 장치는 touch point T를 `POSITION_X/Y`, tool center C를 `TOOL_X/Y`로 매핑합니다.

Touch ellipse와 tool ellipse를 모두 정할 정보가 부족하므로 근사가 필요합니다. 호환 가능한 단순 방식은 touch major를 `min(X,Y)`, touch minor와 orientation은 미사용, width major를 `min(X,Y) + distance(T,C)`, width minor를 `min(X,Y)`로 둡니다.

Touch orientation 정보가 없으므로 inscribed circle로 근사합니다. Tool ellipse는 vector `(T - C)`와 정렬돼야 하므로 지름은 `distance(T,C)`에 따라 커져야 합니다. Touch 지름이 tool thickness와 같다고 가정하면 위 공식이 나옵니다.

Finger tracking은 표면에서 시작된 각 contact에 unique tracking ID를 할당하는 Euclidean Bipartite Matching 문제입니다. 매 event synchronization마다 현재 contact 집합과 이전 synchronization의 contact 집합을 matching합니다. 전체 구현은 mtdev project를 참고합니다.

Gesture 생성에서는 TOUCH·WIDTH로 finger pressure를 근사하거나 검지와 엄지를 구분할 수 있습니다. MINOR를 추가하면 sweeping finger와 pointing finger를 구분하고 ORIENTATION으로 finger twist를 탐지할 수 있습니다.

기존 application과 호환하려면 finger packet data가 single-touch event로 인식돼서는 안 됩니다. Type A에서는 같은 type의 연속 event가 서로 다른 finger를 뜻하므로 모든 finger data가 input filtering을 우회합니다.

추가 note로 `(TOOL_X - POSITION_X)` 차이는 tilt modeling에 사용할 수 있고 tool type 목록은 확장 가능합니다. Event 계산과 tracking의 상세 근거는 각각 해당 절과 mtdev project를 참고합니다.

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 Euclidean 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.