1 .. include:: <isonum.txt>
2
3 =========================
4 Multi-touch (MT) Protocol
5 =========================
6
7 :Copyright: |copy| 2009-2010 Henrik Rydberg<
8
9
10 Introduction
11 ------------
12
13 In order to utilize the full power of the new
14 devices, a way to report detailed data from mu
15 objects in direct contact with the device surf
16 document describes the multi-touch (MT) protoc
17 drivers to report details for an arbitrary num
18
19 The protocol is divided into two types, depend
20 hardware. For devices handling anonymous conta
21 describes how to send the raw data for all con
22 devices capable of tracking identifiable conta
23 describes how to send updates for individual c
24
25 .. note::
26 MT protocol type A is obsolete, all kernel
27 converted to use type B.
28
29 Protocol Usage
30 --------------
31
32 Contact details are sent sequentially as separ
33 events. Only the ABS_MT events are recognized
34 packet. Since these events are ignored by curr
35 applications, the MT protocol can be implement
36 in an existing driver.
37
38 Drivers for type A devices separate contact pa
39 input_mt_sync() at the end of each packet. Thi
40 event, which instructs the receiver to accept
41 contact and prepare to receive another.
42
43 Drivers for type B devices separate contact pa
44 input_mt_slot(), with a slot as argument, at t
45 This generates an ABS_MT_SLOT event, which ins
46 prepare for updates of the given slot.
47
48 All drivers mark the end of a multi-touch tran
49 input_sync() function. This instructs the rece
50 accumulated since last EV_SYN/SYN_REPORT and p
51 of events/packets.
52
53 The main difference between the stateless type
54 type B slot protocol lies in the usage of iden
55 the amount of data sent to userspace. The slot
56 the ABS_MT_TRACKING_ID, either provided by the
57 the raw data [#f5]_.
58
59 For type A devices, the kernel driver should g
60 enumeration of the full set of anonymous conta
61 surface. The order in which the packets appear
62 important. Event filtering and finger trackin
63
64 For type B devices, the kernel driver should a
65 identified contact, and use that slot to propa
66 Creation, replacement and destruction of conta
67 the ABS_MT_TRACKING_ID of the associated slot.
68 is interpreted as a contact, and the value -1
69 tracking id not previously present is consider
70 longer present is considered removed. Since o
71 the full state of each initiated contact has t
72 end. Upon receiving an MT event, one simply u
73 attribute of the current slot.
74
75 Some devices identify and/or track more contac
76 driver. A driver for such a device should ass
77 contact that is reported by the hardware. Whe
78 contact associated with a slot changes, the dr
79 slot by changing its ABS_MT_TRACKING_ID. If t
80 tracking more contacts than it is currently re
81 a BTN_TOOL_*TAP event to inform userspace of t
82 being tracked by the hardware at that moment.
83 explicitly sending the corresponding BTN_TOOL_
84 use_count to false when calling input_mt_repor
85 The driver should only advertise as many slots
86 Userspace can detect that a driver can report
87 by noting that the largest supported BTN_TOOL_
88 total number of type B slots reported in the a
89
90 The minimum value of the ABS_MT_SLOT axis must
91
92 Protocol Example A
93 ------------------
94
95 Here is what a minimal event sequence for a tw
96 like 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
106 The sequence after moving one of the contacts
107 raw data for all present contacts are sent bet
108 with SYN_REPORT.
109
110 Here is the sequence after lifting the first c
111
112 ABS_MT_POSITION_X x[1]
113 ABS_MT_POSITION_Y y[1]
114 SYN_MT_REPORT
115 SYN_REPORT
116
117 And here is the sequence after lifting the sec
118
119 SYN_MT_REPORT
120 SYN_REPORT
121
122 If the driver reports one of BTN_TOUCH or ABS_
123 ABS_MT events, the last SYN_MT_REPORT event ma
124 last SYN_REPORT will be dropped by the input c
125 zero-contact event reaching userland.
126
127
128 Protocol Example B
129 ------------------
130
131 Here is what a minimal event sequence for a tw
132 like 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
144 Here is the sequence after moving contact 45 i
145
146 ABS_MT_SLOT 0
147 ABS_MT_POSITION_X x[0]
148 SYN_REPORT
149
150 Here is the sequence after lifting the contact
151
152 ABS_MT_TRACKING_ID -1
153 SYN_REPORT
154
155 The slot being modified is already 0, so the A
156 message removes the association of slot 0 with
157 destroying contact 45 and freeing slot 0 to be
158
159 Finally, here is the sequence after lifting th
160
161 ABS_MT_SLOT 1
162 ABS_MT_TRACKING_ID -1
163 SYN_REPORT
164
165
166 Event Usage
167 -----------
168
169 A set of ABS_MT events with the desired proper
170 are divided into categories, to allow for part
171 minimum set consists of ABS_MT_POSITION_X and
172 allows for multiple contacts to be tracked. I
173 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may
174 of the contact area and approaching tool, resp
175
176 The TOUCH and WIDTH parameters have a geometri
177 looking through a window at someone gently hol
178 glass. You will see two regions, one inner re
179 of the finger actually touching the glass, and
180 the perimeter of the finger. The center of the
181 ABS_MT_POSITION_X/Y and the center of the appr
182 ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_
183 diameter is ABS_MT_WIDTH_MAJOR. Now imagine th
184 harder against the glass. The touch region wil
185 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MA
186 than unity, is related to the contact pressure
187 ABS_MT_PRESSURE may be used to provide the pre
188 instead. Devices capable of contact hovering c
189 indicate the distance between the contact and
190
191 ::
192
193
194 Linux MT
195 __________ ______
196 / \ |
197 / \ |
198 / ____ \ |
199 / / \ \ |
200 \ \ a \ \ |
201 \ \____/ \ |
202 \ \ |
203 \ b \ |
204 \ \ |
205 \ \ |
206 \ \ |
207 \ / |
208 \ / |
209 \ / |
210 \__________/ |______
211
212
213 In addition to the MAJOR parameters, the oval
214 regions can be described by adding the MINOR p
215 and MINOR are the major and minor axis of an e
216 the touch ellipse can be described with the OR
217 direction of the finger ellipse is given by th
218
219 For type A devices, further specification of t
220 via ABS_MT_BLOB_ID.
221
222 The ABS_MT_TOOL_TYPE may be used to specify wh
223 finger or a pen or something else. Finally, th
224 may be used to track identified contacts over
225
226 In the type B protocol, ABS_MT_TOOL_TYPE and A
227 implicitly handled by input core; drivers shou
228 input_mt_report_slot_state().
229
230
231 Event Semantics
232 ---------------
233
234 ABS_MT_TOUCH_MAJOR
235 The length of the major axis of the contac
236 surface units. If the surface has an X tim
237 possible value of ABS_MT_TOUCH_MAJOR is sq
238
239 ABS_MT_TOUCH_MINOR
240 The length, in surface units, of the minor
241 contact is circular, this event can be omi
242
243 ABS_MT_WIDTH_MAJOR
244 The length, in surface units, of the major
245 tool. This should be understood as the siz
246 orientation of the contact and the approac
247 same [#f4]_.
248
249 ABS_MT_WIDTH_MINOR
250 The length, in surface units, of the minor
251 tool. Omit if circular [#f4]_.
252
253 The above four values can be used to deriv
254 the contact. The ratio ABS_MT_TOUCH_MAJOR
255 the notion of pressure. The fingers of the
256 different characteristic widths.
257
258 ABS_MT_PRESSURE
259 The pressure, in arbitrary units, on the c
260 of TOUCH and WIDTH for pressure-based devi
261 signal intensity distribution.
262
263 If the resolution is zero, the pressure da
264 If the resolution is non-zero, the pressur
265 :ref:`input-event-codes` for details.
266
267 ABS_MT_DISTANCE
268 The distance, in surface units, between th
269 distance means the contact is touching the
270 the contact is hovering above the surface.
271
272 ABS_MT_ORIENTATION
273 The orientation of the touching ellipse. T
274 quarter of a revolution clockwise around t
275 range is arbitrary, but zero should be ret
276 the Y axis (north) of the surface, a negat
277 turned to the left, and a positive value w
278 right. When aligned with the X axis in the
279 max should be returned; when aligned with
280 direction, the range -max should be return
281
282 Touch ellipses are symmetrical by default.
283 degree orientation, the reported orientati
284 indicate more than a quarter of a revoluti
285 range max * 2 should be returned.
286
287 Orientation can be omitted if the touch ar
288 information is not available in the kernel
289 support is possible if the device can dist
290 not (uniquely) any values in between. In s
291 ABS_MT_ORIENTATION should be [0, 1] [#f4]_
292
293 ABS_MT_POSITION_X
294 The surface X coordinate of the center of
295
296 ABS_MT_POSITION_Y
297 The surface Y coordinate of the center of
298
299 ABS_MT_TOOL_X
300 The surface X coordinate of the center of
301 the device cannot distinguish between the
302 tool itself.
303
304 ABS_MT_TOOL_Y
305 The surface Y coordinate of the center of
306 device cannot distinguish between the inte
307 itself.
308
309 The four position values can be used to se
310 from the position of the tool. If both pos
311 tool axis points towards the touch point [
312 aligned with the touch axes.
313
314 ABS_MT_TOOL_TYPE
315 The type of approaching tool. A lot of ker
316 between different tool types, such as a fi
317 event should be omitted. The protocol curr
318 MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_P
319 For type B devices, this event is handled
320 instead use input_mt_report_slot_state().
321 change over time while still touching the
322 not be able to determine which tool is bei
323
324 ABS_MT_BLOB_ID
325 The BLOB_ID groups several packets togethe
326 contact. The sequence of points forms a po
327 the contact. This is a low-level anonymous
328 should not be confused with the high-level
329 devices do not have blob capability, so dr
330
331 ABS_MT_TRACKING_ID
332 The TRACKING_ID identifies an initiated co
333 [#f5]_. The value range of the TRACKING_ID
334 unique identification of a contact maintai
335 time. For type B devices, this event is ha
336 should instead use input_mt_report_slot_st
337
338
339 Event Computation
340 -----------------
341
342 The flora of different hardware unavoidably le
343 better to the MT protocol than others. To simp
344 this section gives recipes for how to compute
345
346 For devices reporting contacts as rectangular
347 cannot be obtained. Assuming X and Y are the l
348 touching rectangle, here is a simple formula t
349 information 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
355 The range of ABS_MT_ORIENTATION should be set
356 the device can distinguish between a finger al
357 finger along the X axis (1).
358
359 For Win8 devices with both T and C coordinates
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
366 Unfortunately, there is not enough information
367 ellipse and the tool ellipse, so one has to re
368 simple scheme, which is compatible with earlie
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(
374 ABS_MT_WIDTH_MINOR := min(X, Y)
375
376 Rationale: We have no information about the or
377 ellipse, so approximate it with an inscribed c
378 ellipse should align with the vector (T - C),
379 increase with distance(T, C). Finally, assume
380 equal to the tool thickness, and we arrive at
381
382 Finger Tracking
383 ---------------
384
385 The process of finger tracking, i.e., to assig
386 initiated contact on the surface, is a Euclide
387 problem. At each event synchronization, the s
388 matched to the set of contacts from the previo
389 implementation can be found in [#f3]_.
390
391
392 Gestures
393 --------
394
395 In the specific application of creating gestur
396 parameters can be used to, e.g., approximate f
397 between index finger and thumb. With the addit
398 one can also distinguish between a sweeping fi
399 and with ORIENTATION, one can detect twisting
400
401
402 Notes
403 -----
404
405 In order to stay compatible with existing appl
406 in a finger packet must not be recognized as s
407
408 For type A devices, all finger data bypasses i
409 subsequent events of the same type refer to di
410
411 .. [#f1] Also, the difference (TOOL_X - POSITI
412 .. [#f2] The list can of course be extended.
413 .. [#f3] The mtdev project: http://bitmath.org
414 .. [#f4] See the section on event computation.
415 .. [#f5] See the section on finger tracking.
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