1.. include:: <isonum.txt>
2
3=========================
4Multi-touch (MT) Protocol
5=========================
6
7:Copyright: |copy| 2009-2010	Henrik Rydberg <rydberg@euromail.se>
8
9
10Introduction
11------------
12
13In order to utilize the full power of the new multi-touch and multi-user
14devices, a way to report detailed data from multiple contacts, i.e.,
15objects in direct contact with the device surface, is needed.  This
16document describes the multi-touch (MT) protocol which allows kernel
17drivers to report details for an arbitrary number of contacts.
18
19The protocol is divided into two types, depending on the capabilities of the
20hardware. For devices handling anonymous contacts (type A), the protocol
21describes how to send the raw data for all contacts to the receiver. For
22devices capable of tracking identifiable contacts (type B), the protocol
23describes how to send updates for individual contacts via event slots.
24
25.. note::
26   MT protocol type A is obsolete, all kernel drivers have been
27   converted to use type B.
28
29Protocol Usage
30--------------
31
32Contact details are sent sequentially as separate packets of ABS_MT
33events. Only the ABS_MT events are recognized as part of a contact
34packet. Since these events are ignored by current single-touch (ST)
35applications, the MT protocol can be implemented on top of the ST protocol
36in an existing driver.
37
38Drivers for type A devices separate contact packets by calling
39input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
40event, which instructs the receiver to accept the data for the current
41contact and prepare to receive another.
42
43Drivers for type B devices separate contact packets by calling
44input_mt_slot(), with a slot as argument, at the beginning of each packet.
45This generates an ABS_MT_SLOT event, which instructs the receiver to
46prepare for updates of the given slot.
47
48All drivers mark the end of a multi-touch transfer by calling the usual
49input_sync() function. This instructs the receiver to act upon events
50accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
51of events/packets.
52
53The main difference between the stateless type A protocol and the stateful
54type B slot protocol lies in the usage of identifiable contacts to reduce
55the amount of data sent to userspace. The slot protocol requires the use of
56the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
57the raw data [#f5]_.
58
59For type A devices, the kernel driver should generate an arbitrary
60enumeration of the full set of anonymous contacts currently on the
61surface. The order in which the packets appear in the event stream is not
62important.  Event filtering and finger tracking is left to user space [#f3]_.
63
64For type B devices, the kernel driver should associate a slot with each
65identified contact, and use that slot to propagate changes for the contact.
66Creation, replacement and destruction of contacts is achieved by modifying
67the ABS_MT_TRACKING_ID of the associated slot.  A non-negative tracking id
68is interpreted as a contact, and the value -1 denotes an unused slot.  A
69tracking id not previously present is considered new, and a tracking id no
70longer present is considered removed.  Since only changes are propagated,
71the full state of each initiated contact has to reside in the receiving
72end.  Upon receiving an MT event, one simply updates the appropriate
73attribute of the current slot.
74
75Some devices identify and/or track more contacts than they can report to the
76driver.  A driver for such a device should associate one type B slot with each
77contact that is reported by the hardware.  Whenever the identity of the
78contact associated with a slot changes, the driver should invalidate that
79slot by changing its ABS_MT_TRACKING_ID.  If the hardware signals that it is
80tracking more contacts than it is currently reporting, the driver should use
81a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
82being tracked by the hardware at that moment.  The driver should do this by
83explicitly sending the corresponding BTN_TOOL_*TAP event and setting
84use_count to false when calling input_mt_report_pointer_emulation().
85The driver should only advertise as many slots as the hardware can report.
86Userspace can detect that a driver can report more total contacts than slots
87by noting that the largest supported BTN_TOOL_*TAP event is larger than the
88total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
89
90The minimum value of the ABS_MT_SLOT axis must be 0.
91
92Protocol Example A
93------------------
94
95Here is what a minimal event sequence for a two-contact touch would look
96like for a type A device::
97
98   ABS_MT_POSITION_X x[0]
99   ABS_MT_POSITION_Y y[0]
100   SYN_MT_REPORT
101   ABS_MT_POSITION_X x[1]
102   ABS_MT_POSITION_Y y[1]
103   SYN_MT_REPORT
104   SYN_REPORT
105
106The sequence after moving one of the contacts looks exactly the same; the
107raw data for all present contacts are sent between every synchronization
108with SYN_REPORT.
109
110Here is the sequence after lifting the first contact::
111
112   ABS_MT_POSITION_X x[1]
113   ABS_MT_POSITION_Y y[1]
114   SYN_MT_REPORT
115   SYN_REPORT
116
117And here is the sequence after lifting the second contact::
118
119   SYN_MT_REPORT
120   SYN_REPORT
121
122If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
123ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
124last SYN_REPORT will be dropped by the input core, resulting in no
125zero-contact event reaching userland.
126
127
128Protocol Example B
129------------------
130
131Here is what a minimal event sequence for a two-contact touch would look
132like for a type B device::
133
134   ABS_MT_SLOT 0
135   ABS_MT_TRACKING_ID 45
136   ABS_MT_POSITION_X x[0]
137   ABS_MT_POSITION_Y y[0]
138   ABS_MT_SLOT 1
139   ABS_MT_TRACKING_ID 46
140   ABS_MT_POSITION_X x[1]
141   ABS_MT_POSITION_Y y[1]
142   SYN_REPORT
143
144Here is the sequence after moving contact 45 in the x direction::
145
146   ABS_MT_SLOT 0
147   ABS_MT_POSITION_X x[0]
148   SYN_REPORT
149
150Here is the sequence after lifting the contact in slot 0::
151
152   ABS_MT_TRACKING_ID -1
153   SYN_REPORT
154
155The slot being modified is already 0, so the ABS_MT_SLOT is omitted.  The
156message removes the association of slot 0 with contact 45, thereby
157destroying contact 45 and freeing slot 0 to be reused for another contact.
158
159Finally, here is the sequence after lifting the second contact::
160
161   ABS_MT_SLOT 1
162   ABS_MT_TRACKING_ID -1
163   SYN_REPORT
164
165
166Event Usage
167-----------
168
169A set of ABS_MT events with the desired properties is defined. The events
170are divided into categories, to allow for partial implementation.  The
171minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
172allows for multiple contacts to be tracked.  If the device supports it, the
173ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
174of the contact area and approaching tool, respectively.
175
176The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
177looking through a window at someone gently holding a finger against the
178glass.  You will see two regions, one inner region consisting of the part
179of the finger actually touching the glass, and one outer region formed by
180the perimeter of the finger. The center of the touching region (a) is
181ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
182ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
183diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
184harder against the glass. The touch region will increase, and in general,
185the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
186than unity, is related to the contact pressure. For pressure-based devices,
187ABS_MT_PRESSURE may be used to provide the pressure on the contact area
188instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
189indicate the distance between the contact and the surface.
190
191::
192
193
194	  Linux MT                               Win8
195         __________                     _______________________
196        /          \                   |                       |
197       /            \                  |                       |
198      /     ____     \                 |                       |
199     /     /    \     \                |                       |
200     \     \  a  \     \               |       a               |
201      \     \____/      \              |                       |
202       \                 \             |                       |
203        \        b        \            |           b           |
204         \                 \           |                       |
205          \                 \          |                       |
206           \                 \         |                       |
207            \                /         |                       |
208             \              /          |                       |
209              \            /           |                       |
210               \__________/            |_______________________|
211
212
213In addition to the MAJOR parameters, the oval shape of the touch and finger
214regions can be described by adding the MINOR parameters, such that MAJOR
215and MINOR are the major and minor axis of an ellipse. The orientation of
216the touch ellipse can be described with the ORIENTATION parameter, and the
217direction of the finger ellipse is given by the vector (a - b).
218
219For type A devices, further specification of the touch shape is possible
220via ABS_MT_BLOB_ID.
221
222The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
223finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
224may be used to track identified contacts over time [#f5]_.
225
226In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
227implicitly handled by input core; drivers should instead call
228input_mt_report_slot_state().
229
230
231Event Semantics
232---------------
233
234ABS_MT_TOUCH_MAJOR
235    The length of the major axis of the contact. The length should be given in
236    surface units. If the surface has an X times Y resolution, the largest
237    possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.
238
239ABS_MT_TOUCH_MINOR
240    The length, in surface units, of the minor axis of the contact. If the
241    contact is circular, this event can be omitted [#f4]_.
242
243ABS_MT_WIDTH_MAJOR
244    The length, in surface units, of the major axis of the approaching
245    tool. This should be understood as the size of the tool itself. The
246    orientation of the contact and the approaching tool are assumed to be the
247    same [#f4]_.
248
249ABS_MT_WIDTH_MINOR
250    The length, in surface units, of the minor axis of the approaching
251    tool. Omit if circular [#f4]_.
252
253    The above four values can be used to derive additional information about
254    the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
255    the notion of pressure. The fingers of the hand and the palm all have
256    different characteristic widths.
257
258ABS_MT_PRESSURE
259    The pressure, in arbitrary units, on the contact area. May be used instead
260    of TOUCH and WIDTH for pressure-based devices or any device with a spatial
261    signal intensity distribution.
262
263    If the resolution is zero, the pressure data is in arbitrary units.
264    If the resolution is non-zero, the pressure data is in units/gram. See
265    :ref:`input-event-codes` for details.
266
267ABS_MT_DISTANCE
268    The distance, in surface units, between the contact and the surface. Zero
269    distance means the contact is touching the surface. A positive number means
270    the contact is hovering above the surface.
271
272ABS_MT_ORIENTATION
273    The orientation of the touching ellipse. The value should describe a signed
274    quarter of a revolution clockwise around the touch center. The signed value
275    range is arbitrary, but zero should be returned for an ellipse aligned with
276    the Y axis (north) of the surface, a negative value when the ellipse is
277    turned to the left, and a positive value when the ellipse is turned to the
278    right. When aligned with the X axis in the positive direction, the range
279    max should be returned; when aligned with the X axis in the negative
280    direction, the range -max should be returned.
281
282    Touch ellipses are symmetrical by default. For devices capable of true 360
283    degree orientation, the reported orientation must exceed the range max to
284    indicate more than a quarter of a revolution. For an upside-down finger,
285    range max * 2 should be returned.
286
287    Orientation can be omitted if the touch area is circular, or if the
288    information is not available in the kernel driver. Partial orientation
289    support is possible if the device can distinguish between the two axes, but
290    not (uniquely) any values in between. In such cases, the range of
291    ABS_MT_ORIENTATION should be [0, 1] [#f4]_.
292
293ABS_MT_POSITION_X
294    The surface X coordinate of the center of the touching ellipse.
295
296ABS_MT_POSITION_Y
297    The surface Y coordinate of the center of the touching ellipse.
298
299ABS_MT_TOOL_X
300    The surface X coordinate of the center of the approaching tool. Omit if
301    the device cannot distinguish between the intended touch point and the
302    tool itself.
303
304ABS_MT_TOOL_Y
305    The surface Y coordinate of the center of the approaching tool. Omit if the
306    device cannot distinguish between the intended touch point and the tool
307    itself.
308
309    The four position values can be used to separate the position of the touch
310    from the position of the tool. If both positions are present, the major
311    tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
312    aligned with the touch axes.
313
314ABS_MT_TOOL_TYPE
315    The type of approaching tool. A lot of kernel drivers cannot distinguish
316    between different tool types, such as a finger or a pen. In such cases, the
317    event should be omitted. The protocol currently mainly supports
318    MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_.
319    For type B devices, this event is handled by input core; drivers should
320    instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may
321    change over time while still touching the device, because the firmware may
322    not be able to determine which tool is being used when it first appears.
323
324ABS_MT_BLOB_ID
325    The BLOB_ID groups several packets together into one arbitrarily shaped
326    contact. The sequence of points forms a polygon which defines the shape of
327    the contact. This is a low-level anonymous grouping for type A devices, and
328    should not be confused with the high-level trackingID [#f5]_. Most type A
329    devices do not have blob capability, so drivers can safely omit this event.
330
331ABS_MT_TRACKING_ID
332    The TRACKING_ID identifies an initiated contact throughout its life cycle
333    [#f5]_. The value range of the TRACKING_ID should be large enough to ensure
334    unique identification of a contact maintained over an extended period of
335    time. For type B devices, this event is handled by input core; drivers
336    should instead use input_mt_report_slot_state().
337
338
339Event Computation
340-----------------
341
342The flora of different hardware unavoidably leads to some devices fitting
343better to the MT protocol than others. To simplify and unify the mapping,
344this section gives recipes for how to compute certain events.
345
346For devices reporting contacts as rectangular shapes, signed orientation
347cannot be obtained. Assuming X and Y are the lengths of the sides of the
348touching rectangle, here is a simple formula that retains the most
349information possible::
350
351   ABS_MT_TOUCH_MAJOR := max(X, Y)
352   ABS_MT_TOUCH_MINOR := min(X, Y)
353   ABS_MT_ORIENTATION := bool(X > Y)
354
355The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
356the device can distinguish between a finger along the Y axis (0) and a
357finger along the X axis (1).
358
359For Win8 devices with both T and C coordinates, the position mapping is::
360
361   ABS_MT_POSITION_X := T_X
362   ABS_MT_POSITION_Y := T_Y
363   ABS_MT_TOOL_X := C_X
364   ABS_MT_TOOL_Y := C_Y
365
366Unfortunately, there is not enough information to specify both the touching
367ellipse and the tool ellipse, so one has to resort to approximations.  One
368simple scheme, which is compatible with earlier usage, is::
369
370   ABS_MT_TOUCH_MAJOR := min(X, Y)
371   ABS_MT_TOUCH_MINOR := <not used>
372   ABS_MT_ORIENTATION := <not used>
373   ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
374   ABS_MT_WIDTH_MINOR := min(X, Y)
375
376Rationale: We have no information about the orientation of the touching
377ellipse, so approximate it with an inscribed circle instead. The tool
378ellipse should align with the vector (T - C), so the diameter must
379increase with distance(T, C). Finally, assume that the touch diameter is
380equal to the tool thickness, and we arrive at the formulas above.
381
382Finger Tracking
383---------------
384
385The process of finger tracking, i.e., to assign a unique trackingID to each
386initiated contact on the surface, is a Euclidian Bipartite Matching
387problem.  At each event synchronization, the set of actual contacts is
388matched to the set of contacts from the previous synchronization. A full
389implementation can be found in [#f3]_.
390
391
392Gestures
393--------
394
395In the specific application of creating gesture events, the TOUCH and WIDTH
396parameters can be used to, e.g., approximate finger pressure or distinguish
397between index finger and thumb. With the addition of the MINOR parameters,
398one can also distinguish between a sweeping finger and a pointing finger,
399and with ORIENTATION, one can detect twisting of fingers.
400
401
402Notes
403-----
404
405In order to stay compatible with existing applications, the data reported
406in a finger packet must not be recognized as single-touch events.
407
408For type A devices, all finger data bypasses input filtering, since
409subsequent events of the same type refer to different fingers.
410
411.. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
412.. [#f2] The list can of course be extended.
413.. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
414.. [#f4] See the section on event computation.
415.. [#f5] See the section on finger tracking.
416