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1Multi-touch (MT) Protocol
2-------------------------
3 Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se>
4
5
6Introduction
7------------
8
9In order to utilize the full power of the new multi-touch and multi-user
10devices, a way to report detailed data from multiple contacts, i.e.,
11objects in direct contact with the device surface, is needed. This
12document describes the multi-touch (MT) protocol which allows kernel
13drivers to report details for an arbitrary number of contacts.
14
15The protocol is divided into two types, depending on the capabilities of the
16hardware. For devices handling anonymous contacts (type A), the protocol
17describes how to send the raw data for all contacts to the receiver. For
18devices capable of tracking identifiable contacts (type B), the protocol
19describes how to send updates for individual contacts via event slots.
20
21
22Protocol Usage
23--------------
24
25Contact details are sent sequentially as separate packets of ABS_MT
26events. Only the ABS_MT events are recognized as part of a contact
27packet. Since these events are ignored by current single-touch (ST)
28applications, the MT protocol can be implemented on top of the ST protocol
29in an existing driver.
30
31Drivers for type A devices separate contact packets by calling
32input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
33event, which instructs the receiver to accept the data for the current
34contact and prepare to receive another.
35
36Drivers for type B devices separate contact packets by calling
37input_mt_slot(), with a slot as argument, at the beginning of each packet.
38This generates an ABS_MT_SLOT event, which instructs the receiver to
39prepare for updates of the given slot.
40
41All drivers mark the end of a multi-touch transfer by calling the usual
42input_sync() function. This instructs the receiver to act upon events
43accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
44of events/packets.
45
46The main difference between the stateless type A protocol and the stateful
47type B slot protocol lies in the usage of identifiable contacts to reduce
48the amount of data sent to userspace. The slot protocol requires the use of
49the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
50the raw data [5].
51
52For type A devices, the kernel driver should generate an arbitrary
53enumeration of the full set of anonymous contacts currently on the
54surface. The order in which the packets appear in the event stream is not
55important. Event filtering and finger tracking is left to user space [3].
56
57For type B devices, the kernel driver should associate a slot with each
58identified contact, and use that slot to propagate changes for the contact.
59Creation, replacement and destruction of contacts is achieved by modifying
60the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
61is interpreted as a contact, and the value -1 denotes an unused slot. A
62tracking id not previously present is considered new, and a tracking id no
63longer present is considered removed. Since only changes are propagated,
64the full state of each initiated contact has to reside in the receiving
65end. Upon receiving an MT event, one simply updates the appropriate
66attribute of the current slot.
67
68Some devices identify and/or track more contacts than they can report to the
69driver. A driver for such a device should associate one type B slot with each
70contact that is reported by the hardware. Whenever the identity of the
71contact associated with a slot changes, the driver should invalidate that
72slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
73tracking more contacts than it is currently reporting, the driver should use
74a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
75being tracked by the hardware at that moment. The driver should do this by
76explicitly sending the corresponding BTN_TOOL_*TAP event and setting
77use_count to false when calling input_mt_report_pointer_emulation().
78The driver should only advertise as many slots as the hardware can report.
79Userspace can detect that a driver can report more total contacts than slots
80by noting that the largest supported BTN_TOOL_*TAP event is larger than the
81total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
82
83The minimum value of the ABS_MT_SLOT axis must be 0.
84
85Protocol Example A
86------------------
87
88Here is what a minimal event sequence for a two-contact touch would look
89like for a type A device:
90
91 ABS_MT_POSITION_X x[0]
92 ABS_MT_POSITION_Y y[0]
93 SYN_MT_REPORT
94 ABS_MT_POSITION_X x[1]
95 ABS_MT_POSITION_Y y[1]
96 SYN_MT_REPORT
97 SYN_REPORT
98
99The sequence after moving one of the contacts looks exactly the same; the
100raw data for all present contacts are sent between every synchronization
101with SYN_REPORT.
102
103Here is the sequence after lifting the first contact:
104
105 ABS_MT_POSITION_X x[1]
106 ABS_MT_POSITION_Y y[1]
107 SYN_MT_REPORT
108 SYN_REPORT
109
110And here is the sequence after lifting the second contact:
111
112 SYN_MT_REPORT
113 SYN_REPORT
114
115If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
116ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
117last SYN_REPORT will be dropped by the input core, resulting in no
118zero-contact event reaching userland.
119
120
121Protocol Example B
122------------------
123
124Here is what a minimal event sequence for a two-contact touch would look
125like for a type B device:
126
127 ABS_MT_SLOT 0
128 ABS_MT_TRACKING_ID 45
129 ABS_MT_POSITION_X x[0]
130 ABS_MT_POSITION_Y y[0]
131 ABS_MT_SLOT 1
132 ABS_MT_TRACKING_ID 46
133 ABS_MT_POSITION_X x[1]
134 ABS_MT_POSITION_Y y[1]
135 SYN_REPORT
136
137Here is the sequence after moving contact 45 in the x direction:
138
139 ABS_MT_SLOT 0
140 ABS_MT_POSITION_X x[0]
141 SYN_REPORT
142
143Here is the sequence after lifting the contact in slot 0:
144
145 ABS_MT_TRACKING_ID -1
146 SYN_REPORT
147
148The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
149message removes the association of slot 0 with contact 45, thereby
150destroying contact 45 and freeing slot 0 to be reused for another contact.
151
152Finally, here is the sequence after lifting the second contact:
153
154 ABS_MT_SLOT 1
155 ABS_MT_TRACKING_ID -1
156 SYN_REPORT
157
158
159Event Usage
160-----------
161
162A set of ABS_MT events with the desired properties is defined. The events
163are divided into categories, to allow for partial implementation. The
164minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
165allows for multiple contacts to be tracked. If the device supports it, the
166ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
167of the contact area and approaching tool, respectively.
168
169The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
170looking through a window at someone gently holding a finger against the
171glass. You will see two regions, one inner region consisting of the part
172of the finger actually touching the glass, and one outer region formed by
173the perimeter of the finger. The center of the touching region (a) is
174ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
175ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
176diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
177harder against the glass. The touch region will increase, and in general,
178the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
179than unity, is related to the contact pressure. For pressure-based devices,
180ABS_MT_PRESSURE may be used to provide the pressure on the contact area
181instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
182indicate the distance between the contact and the surface.
183
184
185 Linux MT Win8
186 __________ _______________________
187 / \ | |
188 / \ | |
189 / ____ \ | |
190 / / \ \ | |
191 \ \ a \ \ | a |
192 \ \____/ \ | |
193 \ \ | |
194 \ b \ | b |
195 \ \ | |
196 \ \ | |
197 \ \ | |
198 \ / | |
199 \ / | |
200 \ / | |
201 \__________/ |_______________________|
202
203
204In addition to the MAJOR parameters, the oval shape of the touch and finger
205regions can be described by adding the MINOR parameters, such that MAJOR
206and MINOR are the major and minor axis of an ellipse. The orientation of
207the touch ellipse can be described with the ORIENTATION parameter, and the
208direction of the finger ellipse is given by the vector (a - b).
209
210For type A devices, further specification of the touch shape is possible
211via ABS_MT_BLOB_ID.
212
213The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
214finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
215may be used to track identified contacts over time [5].
216
217In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
218implicitly handled by input core; drivers should instead call
219input_mt_report_slot_state().
220
221
222Event Semantics
223---------------
224
225ABS_MT_TOUCH_MAJOR
226
227The length of the major axis of the contact. The length should be given in
228surface units. If the surface has an X times Y resolution, the largest
229possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
230
231ABS_MT_TOUCH_MINOR
232
233The length, in surface units, of the minor axis of the contact. If the
234contact is circular, this event can be omitted [4].
235
236ABS_MT_WIDTH_MAJOR
237
238The length, in surface units, of the major axis of the approaching
239tool. This should be understood as the size of the tool itself. The
240orientation of the contact and the approaching tool are assumed to be the
241same [4].
242
243ABS_MT_WIDTH_MINOR
244
245The length, in surface units, of the minor axis of the approaching
246tool. Omit if circular [4].
247
248The above four values can be used to derive additional information about
249the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
250the notion of pressure. The fingers of the hand and the palm all have
251different characteristic widths.
252
253ABS_MT_PRESSURE
254
255The pressure, in arbitrary units, on the contact area. May be used instead
256of TOUCH and WIDTH for pressure-based devices or any device with a spatial
257signal intensity distribution.
258
259ABS_MT_DISTANCE
260
261The distance, in surface units, between the contact and the surface. Zero
262distance means the contact is touching the surface. A positive number means
263the contact is hovering above the surface.
264
265ABS_MT_ORIENTATION
266
267The orientation of the touching ellipse. The value should describe a signed
268quarter of a revolution clockwise around the touch center. The signed value
269range is arbitrary, but zero should be returned for an ellipse aligned with
270the Y axis of the surface, a negative value when the ellipse is turned to
271the left, and a positive value when the ellipse is turned to the
272right. When completely aligned with the X axis, the range max should be
273returned.
274
275Touch ellipsis are symmetrical by default. For devices capable of true 360
276degree orientation, the reported orientation must exceed the range max to
277indicate more than a quarter of a revolution. For an upside-down finger,
278range max * 2 should be returned.
279
280Orientation can be omitted if the touch area is circular, or if the
281information is not available in the kernel driver. Partial orientation
282support is possible if the device can distinguish between the two axis, but
283not (uniquely) any values in between. In such cases, the range of
284ABS_MT_ORIENTATION should be [0, 1] [4].
285
286ABS_MT_POSITION_X
287
288The surface X coordinate of the center of the touching ellipse.
289
290ABS_MT_POSITION_Y
291
292The surface Y coordinate of the center of the touching ellipse.
293
294ABS_MT_TOOL_X
295
296The surface X coordinate of the center of the approaching tool. Omit if
297the device cannot distinguish between the intended touch point and the
298tool itself.
299
300ABS_MT_TOOL_Y
301
302The surface Y coordinate of the center of the approaching tool. Omit if the
303device cannot distinguish between the intended touch point and the tool
304itself.
305
306The four position values can be used to separate the position of the touch
307from the position of the tool. If both positions are present, the major
308tool axis points towards the touch point [1]. Otherwise, the tool axes are
309aligned with the touch axes.
310
311ABS_MT_TOOL_TYPE
312
313The type of approaching tool. A lot of kernel drivers cannot distinguish
314between different tool types, such as a finger or a pen. In such cases, the
315event should be omitted. The protocol currently supports MT_TOOL_FINGER,
316MT_TOOL_PEN, and MT_TOOL_PALM [2]. For type B devices, this event is handled
317by input core; drivers should instead use input_mt_report_slot_state().
318A contact's ABS_MT_TOOL_TYPE may change over time while still touching the
319device, because the firmware may not be able to determine which tool is being
320used when it first appears.
321
322ABS_MT_BLOB_ID
323
324The BLOB_ID groups several packets together into one arbitrarily shaped
325contact. The sequence of points forms a polygon which defines the shape of
326the contact. This is a low-level anonymous grouping for type A devices, and
327should not be confused with the high-level trackingID [5]. Most type A
328devices do not have blob capability, so drivers can safely omit this event.
329
330ABS_MT_TRACKING_ID
331
332The TRACKING_ID identifies an initiated contact throughout its life cycle
333[5]. The value range of the TRACKING_ID should be large enough to ensure
334unique identification of a contact maintained over an extended period of
335time. For type B devices, this event is handled by input core; drivers
336should 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 [3].
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
411For example usage of the type A protocol, see the bcm5974 driver. For
412example usage of the type B protocol, see the hid-egalax driver.
413
414[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
415[2] The list can of course be extended.
416[3] The mtdev project: http://bitmath.org/code/mtdev/.
417[4] See the section on event computation.
418[5] See the section on finger tracking.