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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 | ||
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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 | |
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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 |
72c8a94a | 165 | of the contact area and approaching contact, 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 | |
171 | the perimeter of the finger. The diameter of the inner region is the | |
172 | ABS_MT_TOUCH_MAJOR, the diameter of the outer region is | |
173 | ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder | |
174 | against the glass. The inner region will increase, and in general, the | |
175 | ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than | |
72c8a94a | 176 | unity, is related to the contact pressure. For pressure-based devices, |
f6bdc230 | 177 | ABS_MT_PRESSURE may be used to provide the pressure on the contact area |
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178 | instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to |
179 | indicate the distance between the contact and the surface. | |
f6bdc230 | 180 | |
72c8a94a | 181 | In addition to the MAJOR parameters, the oval shape of the contact can be |
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182 | described by adding the MINOR parameters, such that MAJOR and MINOR are the |
183 | major and minor axis of an ellipse. Finally, the orientation of the oval | |
184 | shape can be describe with the ORIENTATION parameter. | |
185 | ||
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186 | For type A devices, further specification of the touch shape is possible |
187 | via ABS_MT_BLOB_ID. | |
188 | ||
f6bdc230 | 189 | The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a |
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190 | finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event |
191 | may be used to track identified contacts over time [5]. | |
192 | ||
193 | In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are | |
194 | implicitly handled by input core; drivers should instead call | |
195 | input_mt_report_slot_state(). | |
f9fcfc3b | 196 | |
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197 | |
198 | Event Semantics | |
199 | --------------- | |
200 | ||
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201 | ABS_MT_TOUCH_MAJOR |
202 | ||
203 | The length of the major axis of the contact. The length should be given in | |
204 | surface units. If the surface has an X times Y resolution, the largest | |
f9fcfc3b | 205 | possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4]. |
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206 | |
207 | ABS_MT_TOUCH_MINOR | |
208 | ||
209 | The length, in surface units, of the minor axis of the contact. If the | |
f9fcfc3b | 210 | contact is circular, this event can be omitted [4]. |
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211 | |
212 | ABS_MT_WIDTH_MAJOR | |
213 | ||
214 | The length, in surface units, of the major axis of the approaching | |
215 | tool. This should be understood as the size of the tool itself. The | |
216 | orientation of the contact and the approaching tool are assumed to be the | |
f9fcfc3b | 217 | same [4]. |
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218 | |
219 | ABS_MT_WIDTH_MINOR | |
220 | ||
221 | The length, in surface units, of the minor axis of the approaching | |
f9fcfc3b | 222 | tool. Omit if circular [4]. |
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223 | |
224 | The above four values can be used to derive additional information about | |
225 | the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates | |
226 | the notion of pressure. The fingers of the hand and the palm all have | |
227 | different characteristic widths [1]. | |
228 | ||
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229 | ABS_MT_PRESSURE |
230 | ||
231 | The pressure, in arbitrary units, on the contact area. May be used instead | |
232 | of TOUCH and WIDTH for pressure-based devices or any device with a spatial | |
233 | signal intensity distribution. | |
234 | ||
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235 | ABS_MT_DISTANCE |
236 | ||
237 | The distance, in surface units, between the contact and the surface. Zero | |
238 | distance means the contact is touching the surface. A positive number means | |
239 | the contact is hovering above the surface. | |
240 | ||
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241 | ABS_MT_ORIENTATION |
242 | ||
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243 | The orientation of the ellipse. The value should describe a signed quarter |
244 | of a revolution clockwise around the touch center. The signed value range | |
245 | is arbitrary, but zero should be returned for a finger aligned along the Y | |
246 | axis of the surface, a negative value when finger is turned to the left, and | |
247 | a positive value when finger turned to the right. When completely aligned with | |
248 | the X axis, the range max should be returned. Orientation can be omitted | |
249 | if the touching object is circular, or if the information is not available | |
250 | in the kernel driver. Partial orientation support is possible if the device | |
251 | can distinguish between the two axis, but not (uniquely) any values in | |
252 | between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1] | |
253 | [4]. | |
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254 | |
255 | ABS_MT_POSITION_X | |
256 | ||
257 | The surface X coordinate of the center of the touching ellipse. | |
258 | ||
259 | ABS_MT_POSITION_Y | |
260 | ||
261 | The surface Y coordinate of the center of the touching ellipse. | |
262 | ||
263 | ABS_MT_TOOL_TYPE | |
264 | ||
265 | The type of approaching tool. A lot of kernel drivers cannot distinguish | |
266 | between different tool types, such as a finger or a pen. In such cases, the | |
267 | event should be omitted. The protocol currently supports MT_TOOL_FINGER and | |
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268 | MT_TOOL_PEN [2]. For type B devices, this event is handled by input core; |
269 | drivers should instead use input_mt_report_slot_state(). | |
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270 | |
271 | ABS_MT_BLOB_ID | |
272 | ||
273 | The BLOB_ID groups several packets together into one arbitrarily shaped | |
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274 | contact. The sequence of points forms a polygon which defines the shape of |
275 | the contact. This is a low-level anonymous grouping for type A devices, and | |
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276 | should not be confused with the high-level trackingID [5]. Most type A |
277 | devices do not have blob capability, so drivers can safely omit this event. | |
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278 | |
279 | ABS_MT_TRACKING_ID | |
280 | ||
281 | The TRACKING_ID identifies an initiated contact throughout its life cycle | |
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282 | [5]. The value range of the TRACKING_ID should be large enough to ensure |
283 | unique identification of a contact maintained over an extended period of | |
284 | time. For type B devices, this event is handled by input core; drivers | |
285 | should instead use input_mt_report_slot_state(). | |
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286 | |
287 | ||
288 | Event Computation | |
289 | ----------------- | |
290 | ||
291 | The flora of different hardware unavoidably leads to some devices fitting | |
292 | better to the MT protocol than others. To simplify and unify the mapping, | |
293 | this section gives recipes for how to compute certain events. | |
294 | ||
295 | For devices reporting contacts as rectangular shapes, signed orientation | |
296 | cannot be obtained. Assuming X and Y are the lengths of the sides of the | |
297 | touching rectangle, here is a simple formula that retains the most | |
298 | information possible: | |
299 | ||
300 | ABS_MT_TOUCH_MAJOR := max(X, Y) | |
301 | ABS_MT_TOUCH_MINOR := min(X, Y) | |
302 | ABS_MT_ORIENTATION := bool(X > Y) | |
303 | ||
304 | The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that | |
305 | the device can distinguish between a finger along the Y axis (0) and a | |
306 | finger along the X axis (1). | |
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307 | |
308 | ||
309 | Finger Tracking | |
310 | --------------- | |
311 | ||
f9fcfc3b | 312 | The process of finger tracking, i.e., to assign a unique trackingID to each |
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313 | initiated contact on the surface, is a Euclidian Bipartite Matching |
314 | problem. At each event synchronization, the set of actual contacts is | |
315 | matched to the set of contacts from the previous synchronization. A full | |
316 | implementation can be found in [3]. | |
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317 | |
318 | ||
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319 | Gestures |
320 | -------- | |
321 | ||
322 | In the specific application of creating gesture events, the TOUCH and WIDTH | |
323 | parameters can be used to, e.g., approximate finger pressure or distinguish | |
324 | between index finger and thumb. With the addition of the MINOR parameters, | |
325 | one can also distinguish between a sweeping finger and a pointing finger, | |
326 | and with ORIENTATION, one can detect twisting of fingers. | |
327 | ||
328 | ||
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329 | Notes |
330 | ----- | |
331 | ||
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332 | In order to stay compatible with existing applications, the data reported |
333 | in a finger packet must not be recognized as single-touch events. | |
334 | ||
335 | For type A devices, all finger data bypasses input filtering, since | |
336 | subsequent events of the same type refer to different fingers. | |
eacaad01 | 337 | |
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338 | For example usage of the type A protocol, see the bcm5974 driver. For |
339 | example usage of the type B protocol, see the hid-egalax driver. | |
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340 | |
341 | [1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the | |
342 | difference between the contact position and the approaching tool position | |
343 | could be used to derive tilt. | |
344 | [2] The list can of course be extended. | |
22f075a8 | 345 | [3] The mtdev project: http://bitmath.org/code/mtdev/. |
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346 | [4] See the section on event computation. |
347 | [5] See the section on finger tracking. |