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1 | Multi-touch (MT) Protocol |
2 | ------------------------- | |
22f075a8 | 3 | Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se> |
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4 | |
5 | ||
6 | Introduction | |
7 | ------------ | |
8 | ||
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9 | In order to utilize the full power of the new multi-touch and multi-user |
10 | devices, a way to report detailed data from multiple contacts, i.e., | |
11 | objects in direct contact with the device surface, is needed. This | |
12 | document describes the multi-touch (MT) protocol which allows kernel | |
13 | drivers to report details for an arbitrary number of contacts. | |
14 | ||
15 | The protocol is divided into two types, depending on the capabilities of the | |
16 | hardware. For devices handling anonymous contacts (type A), the protocol | |
17 | describes how to send the raw data for all contacts to the receiver. For | |
18 | devices capable of tracking identifiable contacts (type B), the protocol | |
19 | describes how to send updates for individual contacts via event slots. | |
20 | ||
21 | ||
22 | Protocol Usage | |
23 | -------------- | |
24 | ||
25 | Contact details are sent sequentially as separate packets of ABS_MT | |
26 | events. Only the ABS_MT events are recognized as part of a contact | |
27 | packet. Since these events are ignored by current single-touch (ST) | |
28 | applications, the MT protocol can be implemented on top of the ST protocol | |
29 | in an existing driver. | |
30 | ||
31 | Drivers for type A devices separate contact packets by calling | |
32 | input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT | |
33 | event, which instructs the receiver to accept the data for the current | |
34 | contact and prepare to receive another. | |
35 | ||
36 | Drivers for type B devices separate contact packets by calling | |
37 | input_mt_slot(), with a slot as argument, at the beginning of each packet. | |
38 | This generates an ABS_MT_SLOT event, which instructs the receiver to | |
39 | prepare for updates of the given slot. | |
40 | ||
41 | All drivers mark the end of a multi-touch transfer by calling the usual | |
42 | input_sync() function. This instructs the receiver to act upon events | |
43 | accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set | |
44 | of events/packets. | |
45 | ||
46 | The main difference between the stateless type A protocol and the stateful | |
47 | type B slot protocol lies in the usage of identifiable contacts to reduce | |
48 | the amount of data sent to userspace. The slot protocol requires the use of | |
49 | the ABS_MT_TRACKING_ID, either provided by the hardware or computed from | |
50 | the raw data [5]. | |
51 | ||
52 | For type A devices, the kernel driver should generate an arbitrary | |
53 | enumeration of the full set of anonymous contacts currently on the | |
54 | surface. The order in which the packets appear in the event stream is not | |
55 | important. Event filtering and finger tracking is left to user space [3]. | |
56 | ||
57 | For type B devices, the kernel driver should associate a slot with each | |
58 | identified contact, and use that slot to propagate changes for the contact. | |
59 | Creation, replacement and destruction of contacts is achieved by modifying | |
60 | the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id | |
61 | is interpreted as a contact, and the value -1 denotes an unused slot. A | |
62 | tracking id not previously present is considered new, and a tracking id no | |
63 | longer present is considered removed. Since only changes are propagated, | |
64 | the full state of each initiated contact has to reside in the receiving | |
65 | end. Upon receiving an MT event, one simply updates the appropriate | |
66 | attribute of the current slot. | |
67 | ||
a93bd154 DK |
68 | Some devices identify and/or track more contacts than they can report to the |
69 | driver. A driver for such a device should associate one type B slot with each | |
70 | contact that is reported by the hardware. Whenever the identity of the | |
71 | contact associated with a slot changes, the driver should invalidate that | |
72 | slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is | |
73 | tracking more contacts than it is currently reporting, the driver should use | |
74 | a BTN_TOOL_*TAP event to inform userspace of the total number of contacts | |
75 | being tracked by the hardware at that moment. The driver should do this by | |
76 | explicitly sending the corresponding BTN_TOOL_*TAP event and setting | |
77 | use_count to false when calling input_mt_report_pointer_emulation(). | |
78 | The driver should only advertise as many slots as the hardware can report. | |
79 | Userspace can detect that a driver can report more total contacts than slots | |
80 | by noting that the largest supported BTN_TOOL_*TAP event is larger than the | |
81 | total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis. | |
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82 | |
83 | Protocol Example A | |
84 | ------------------ | |
85 | ||
86 | Here is what a minimal event sequence for a two-contact touch would look | |
87 | like for a type A device: | |
88 | ||
89 | ABS_MT_POSITION_X x[0] | |
90 | ABS_MT_POSITION_Y y[0] | |
91 | SYN_MT_REPORT | |
92 | ABS_MT_POSITION_X x[1] | |
93 | ABS_MT_POSITION_Y y[1] | |
94 | SYN_MT_REPORT | |
95 | SYN_REPORT | |
eacaad01 | 96 | |
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97 | The sequence after moving one of the contacts looks exactly the same; the |
98 | raw data for all present contacts are sent between every synchronization | |
99 | with SYN_REPORT. | |
eacaad01 | 100 | |
72c8a94a | 101 | Here is the sequence after lifting the first contact: |
eacaad01 | 102 | |
72c8a94a HR |
103 | ABS_MT_POSITION_X x[1] |
104 | ABS_MT_POSITION_Y y[1] | |
105 | SYN_MT_REPORT | |
106 | SYN_REPORT | |
107 | ||
108 | And here is the sequence after lifting the second contact: | |
109 | ||
110 | SYN_MT_REPORT | |
111 | SYN_REPORT | |
112 | ||
113 | If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the | |
114 | ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the | |
115 | last SYN_REPORT will be dropped by the input core, resulting in no | |
116 | zero-contact event reaching userland. | |
117 | ||
118 | ||
119 | Protocol Example B | |
120 | ------------------ | |
121 | ||
122 | Here is what a minimal event sequence for a two-contact touch would look | |
123 | like for a type B device: | |
124 | ||
125 | ABS_MT_SLOT 0 | |
126 | ABS_MT_TRACKING_ID 45 | |
127 | ABS_MT_POSITION_X x[0] | |
128 | ABS_MT_POSITION_Y y[0] | |
129 | ABS_MT_SLOT 1 | |
130 | ABS_MT_TRACKING_ID 46 | |
131 | ABS_MT_POSITION_X x[1] | |
132 | ABS_MT_POSITION_Y y[1] | |
133 | SYN_REPORT | |
134 | ||
135 | Here is the sequence after moving contact 45 in the x direction: | |
136 | ||
137 | ABS_MT_SLOT 0 | |
138 | ABS_MT_POSITION_X x[0] | |
139 | SYN_REPORT | |
140 | ||
141 | Here is the sequence after lifting the contact in slot 0: | |
142 | ||
143 | ABS_MT_TRACKING_ID -1 | |
144 | SYN_REPORT | |
145 | ||
146 | The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The | |
147 | message removes the association of slot 0 with contact 45, thereby | |
148 | destroying contact 45 and freeing slot 0 to be reused for another contact. | |
149 | ||
150 | Finally, here is the sequence after lifting the second contact: | |
151 | ||
152 | ABS_MT_SLOT 1 | |
153 | ABS_MT_TRACKING_ID -1 | |
154 | SYN_REPORT | |
155 | ||
156 | ||
157 | Event Usage | |
158 | ----------- | |
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159 | |
160 | A set of ABS_MT events with the desired properties is defined. The events | |
161 | are divided into categories, to allow for partial implementation. The | |
f6bdc230 | 162 | minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which |
72c8a94a | 163 | allows for multiple contacts to be tracked. If the device supports it, the |
f6bdc230 | 164 | ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size |
cab7faca | 165 | of the contact area and approaching tool, respectively. |
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166 | |
167 | The TOUCH and WIDTH parameters have a geometrical interpretation; imagine | |
168 | looking through a window at someone gently holding a finger against the | |
169 | glass. You will see two regions, one inner region consisting of the part | |
170 | of the finger actually touching the glass, and one outer region formed by | |
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171 | the perimeter of the finger. The center of the touching region (a) is |
172 | ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is | |
173 | ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger | |
174 | diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger | |
175 | harder against the glass. The touch region will increase, and in general, | |
176 | the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller | |
177 | than unity, is related to the contact pressure. For pressure-based devices, | |
f6bdc230 | 178 | ABS_MT_PRESSURE may be used to provide the pressure on the contact area |
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179 | instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to |
180 | indicate the distance between the contact and the surface. | |
f6bdc230 | 181 | |
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182 | |
183 | Linux MT Win8 | |
184 | __________ _______________________ | |
185 | / \ | | | |
186 | / \ | | | |
187 | / ____ \ | | | |
188 | / / \ \ | | | |
189 | \ \ a \ \ | a | | |
190 | \ \____/ \ | | | |
191 | \ \ | | | |
192 | \ b \ | b | | |
193 | \ \ | | | |
194 | \ \ | | | |
195 | \ \ | | | |
196 | \ / | | | |
197 | \ / | | | |
198 | \ / | | | |
199 | \__________/ |_______________________| | |
200 | ||
201 | ||
202 | In addition to the MAJOR parameters, the oval shape of the touch and finger | |
203 | regions can be described by adding the MINOR parameters, such that MAJOR | |
204 | and MINOR are the major and minor axis of an ellipse. The orientation of | |
205 | the touch ellipse can be described with the ORIENTATION parameter, and the | |
206 | direction of the finger ellipse is given by the vector (a - b). | |
f6bdc230 | 207 | |
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208 | For type A devices, further specification of the touch shape is possible |
209 | via ABS_MT_BLOB_ID. | |
210 | ||
f6bdc230 | 211 | The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a |
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212 | finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event |
213 | may be used to track identified contacts over time [5]. | |
214 | ||
215 | In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are | |
216 | implicitly handled by input core; drivers should instead call | |
217 | input_mt_report_slot_state(). | |
f9fcfc3b | 218 | |
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219 | |
220 | Event Semantics | |
221 | --------------- | |
222 | ||
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223 | ABS_MT_TOUCH_MAJOR |
224 | ||
225 | The length of the major axis of the contact. The length should be given in | |
226 | surface units. If the surface has an X times Y resolution, the largest | |
f9fcfc3b | 227 | possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4]. |
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228 | |
229 | ABS_MT_TOUCH_MINOR | |
230 | ||
231 | The length, in surface units, of the minor axis of the contact. If the | |
f9fcfc3b | 232 | contact is circular, this event can be omitted [4]. |
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233 | |
234 | ABS_MT_WIDTH_MAJOR | |
235 | ||
236 | The length, in surface units, of the major axis of the approaching | |
237 | tool. This should be understood as the size of the tool itself. The | |
238 | orientation of the contact and the approaching tool are assumed to be the | |
f9fcfc3b | 239 | same [4]. |
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240 | |
241 | ABS_MT_WIDTH_MINOR | |
242 | ||
243 | The length, in surface units, of the minor axis of the approaching | |
f9fcfc3b | 244 | tool. Omit if circular [4]. |
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245 | |
246 | The above four values can be used to derive additional information about | |
247 | the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates | |
248 | the notion of pressure. The fingers of the hand and the palm all have | |
cab7faca | 249 | different characteristic widths. |
eacaad01 | 250 | |
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251 | ABS_MT_PRESSURE |
252 | ||
253 | The pressure, in arbitrary units, on the contact area. May be used instead | |
254 | of TOUCH and WIDTH for pressure-based devices or any device with a spatial | |
255 | signal intensity distribution. | |
256 | ||
e42a98b5 HR |
257 | ABS_MT_DISTANCE |
258 | ||
259 | The distance, in surface units, between the contact and the surface. Zero | |
260 | distance means the contact is touching the surface. A positive number means | |
261 | the contact is hovering above the surface. | |
262 | ||
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263 | ABS_MT_ORIENTATION |
264 | ||
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265 | The orientation of the touching ellipse. The value should describe a signed |
266 | quarter of a revolution clockwise around the touch center. The signed value | |
267 | range is arbitrary, but zero should be returned for an ellipse aligned with | |
268 | the Y axis of the surface, a negative value when the ellipse is turned to | |
269 | the left, and a positive value when the ellipse is turned to the | |
270 | right. When completely aligned with the X axis, the range max should be | |
271 | returned. | |
272 | ||
273 | Touch ellipsis are symmetrical by default. For devices capable of true 360 | |
274 | degree orientation, the reported orientation must exceed the range max to | |
275 | indicate more than a quarter of a revolution. For an upside-down finger, | |
276 | range max * 2 should be returned. | |
277 | ||
278 | Orientation can be omitted if the touch area is circular, or if the | |
279 | information is not available in the kernel driver. Partial orientation | |
280 | support is possible if the device can distinguish between the two axis, but | |
281 | not (uniquely) any values in between. In such cases, the range of | |
282 | ABS_MT_ORIENTATION should be [0, 1] [4]. | |
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283 | |
284 | ABS_MT_POSITION_X | |
285 | ||
286 | The surface X coordinate of the center of the touching ellipse. | |
287 | ||
288 | ABS_MT_POSITION_Y | |
289 | ||
290 | The surface Y coordinate of the center of the touching ellipse. | |
291 | ||
cab7faca HR |
292 | ABS_MT_TOOL_X |
293 | ||
294 | The surface X coordinate of the center of the approaching tool. Omit if | |
295 | the device cannot distinguish between the intended touch point and the | |
296 | tool itself. | |
297 | ||
298 | ABS_MT_TOOL_Y | |
299 | ||
300 | The surface Y coordinate of the center of the approaching tool. Omit if the | |
301 | device cannot distinguish between the intended touch point and the tool | |
302 | itself. | |
303 | ||
304 | The four position values can be used to separate the position of the touch | |
305 | from the position of the tool. If both positions are present, the major | |
306 | tool axis points towards the touch point [1]. Otherwise, the tool axes are | |
307 | aligned with the touch axes. | |
308 | ||
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309 | ABS_MT_TOOL_TYPE |
310 | ||
311 | The type of approaching tool. A lot of kernel drivers cannot distinguish | |
312 | between different tool types, such as a finger or a pen. In such cases, the | |
313 | event should be omitted. The protocol currently supports MT_TOOL_FINGER and | |
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314 | MT_TOOL_PEN [2]. For type B devices, this event is handled by input core; |
315 | drivers should instead use input_mt_report_slot_state(). | |
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316 | |
317 | ABS_MT_BLOB_ID | |
318 | ||
319 | The BLOB_ID groups several packets together into one arbitrarily shaped | |
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320 | contact. The sequence of points forms a polygon which defines the shape of |
321 | the contact. This is a low-level anonymous grouping for type A devices, and | |
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322 | should not be confused with the high-level trackingID [5]. Most type A |
323 | devices do not have blob capability, so drivers can safely omit this event. | |
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324 | |
325 | ABS_MT_TRACKING_ID | |
326 | ||
327 | The TRACKING_ID identifies an initiated contact throughout its life cycle | |
22f075a8 HR |
328 | [5]. The value range of the TRACKING_ID should be large enough to ensure |
329 | unique identification of a contact maintained over an extended period of | |
330 | time. For type B devices, this event is handled by input core; drivers | |
331 | should instead use input_mt_report_slot_state(). | |
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332 | |
333 | ||
334 | Event Computation | |
335 | ----------------- | |
336 | ||
337 | The flora of different hardware unavoidably leads to some devices fitting | |
338 | better to the MT protocol than others. To simplify and unify the mapping, | |
339 | this section gives recipes for how to compute certain events. | |
340 | ||
341 | For devices reporting contacts as rectangular shapes, signed orientation | |
342 | cannot be obtained. Assuming X and Y are the lengths of the sides of the | |
343 | touching rectangle, here is a simple formula that retains the most | |
344 | information possible: | |
345 | ||
346 | ABS_MT_TOUCH_MAJOR := max(X, Y) | |
347 | ABS_MT_TOUCH_MINOR := min(X, Y) | |
348 | ABS_MT_ORIENTATION := bool(X > Y) | |
349 | ||
350 | The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that | |
351 | the device can distinguish between a finger along the Y axis (0) and a | |
352 | finger along the X axis (1). | |
eacaad01 | 353 | |
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354 | For win8 devices with both T and C coordinates, the position mapping is |
355 | ||
356 | ABS_MT_POSITION_X := T_X | |
357 | ABS_MT_POSITION_Y := T_Y | |
358 | ABS_MT_TOOL_X := C_X | |
359 | ABS_MT_TOOL_X := C_Y | |
360 | ||
361 | Unfortunately, there is not enough information to specify both the touching | |
362 | ellipse and the tool ellipse, so one has to resort to approximations. One | |
363 | simple scheme, which is compatible with earlier usage, is: | |
364 | ||
365 | ABS_MT_TOUCH_MAJOR := min(X, Y) | |
366 | ABS_MT_TOUCH_MINOR := <not used> | |
367 | ABS_MT_ORIENTATION := <not used> | |
368 | ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C) | |
369 | ABS_MT_WIDTH_MINOR := min(X, Y) | |
370 | ||
371 | Rationale: We have no information about the orientation of the touching | |
372 | ellipse, so approximate it with an inscribed circle instead. The tool | |
373 | ellipse should align with the the vector (T - C), so the diameter must | |
374 | increase with distance(T, C). Finally, assume that the touch diameter is | |
375 | equal to the tool thickness, and we arrive at the formulas above. | |
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376 | |
377 | Finger Tracking | |
378 | --------------- | |
379 | ||
f9fcfc3b | 380 | The process of finger tracking, i.e., to assign a unique trackingID to each |
72c8a94a HR |
381 | initiated contact on the surface, is a Euclidian Bipartite Matching |
382 | problem. At each event synchronization, the set of actual contacts is | |
383 | matched to the set of contacts from the previous synchronization. A full | |
384 | implementation can be found in [3]. | |
f9fcfc3b HR |
385 | |
386 | ||
f6bdc230 HR |
387 | Gestures |
388 | -------- | |
389 | ||
390 | In the specific application of creating gesture events, the TOUCH and WIDTH | |
391 | parameters can be used to, e.g., approximate finger pressure or distinguish | |
392 | between index finger and thumb. With the addition of the MINOR parameters, | |
393 | one can also distinguish between a sweeping finger and a pointing finger, | |
394 | and with ORIENTATION, one can detect twisting of fingers. | |
395 | ||
396 | ||
eacaad01 HR |
397 | Notes |
398 | ----- | |
399 | ||
22f075a8 HR |
400 | In order to stay compatible with existing applications, the data reported |
401 | in a finger packet must not be recognized as single-touch events. | |
402 | ||
403 | For type A devices, all finger data bypasses input filtering, since | |
404 | subsequent events of the same type refer to different fingers. | |
eacaad01 | 405 | |
22f075a8 HR |
406 | For example usage of the type A protocol, see the bcm5974 driver. For |
407 | example usage of the type B protocol, see the hid-egalax driver. | |
eacaad01 | 408 | |
cab7faca | 409 | [1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt. |
eacaad01 | 410 | [2] The list can of course be extended. |
22f075a8 | 411 | [3] The mtdev project: http://bitmath.org/code/mtdev/. |
f9fcfc3b HR |
412 | [4] See the section on event computation. |
413 | [5] See the section on finger tracking. |