Commit | Line | Data |
---|---|---|
eba31a3a MCC |
1 | .. include:: <isonum.txt> |
2 | ||
3 | ========================= | |
eacaad01 | 4 | Multi-touch (MT) Protocol |
eba31a3a MCC |
5 | ========================= |
6 | ||
7 | :Copyright: |copy| 2009-2010 Henrik Rydberg <rydberg@euromail.se> | |
eacaad01 HR |
8 | |
9 | ||
10 | Introduction | |
11 | ------------ | |
12 | ||
72c8a94a HR |
13 | In order to utilize the full power of the new multi-touch and multi-user |
14 | devices, a way to report detailed data from multiple contacts, i.e., | |
15 | objects in direct contact with the device surface, is needed. This | |
16 | document describes the multi-touch (MT) protocol which allows kernel | |
17 | drivers to report details for an arbitrary number of contacts. | |
18 | ||
19 | The protocol is divided into two types, depending on the capabilities of the | |
20 | hardware. For devices handling anonymous contacts (type A), the protocol | |
21 | describes how to send the raw data for all contacts to the receiver. For | |
22 | devices capable of tracking identifiable contacts (type B), the protocol | |
23 | describes how to send updates for individual contacts via event slots. | |
24 | ||
25 | ||
26 | Protocol Usage | |
27 | -------------- | |
28 | ||
29 | Contact details are sent sequentially as separate packets of ABS_MT | |
30 | events. Only the ABS_MT events are recognized as part of a contact | |
31 | packet. Since these events are ignored by current single-touch (ST) | |
32 | applications, the MT protocol can be implemented on top of the ST protocol | |
33 | in an existing driver. | |
34 | ||
35 | Drivers for type A devices separate contact packets by calling | |
36 | input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT | |
37 | event, which instructs the receiver to accept the data for the current | |
38 | contact and prepare to receive another. | |
39 | ||
40 | Drivers for type B devices separate contact packets by calling | |
41 | input_mt_slot(), with a slot as argument, at the beginning of each packet. | |
42 | This generates an ABS_MT_SLOT event, which instructs the receiver to | |
43 | prepare for updates of the given slot. | |
44 | ||
45 | All drivers mark the end of a multi-touch transfer by calling the usual | |
46 | input_sync() function. This instructs the receiver to act upon events | |
47 | accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set | |
48 | of events/packets. | |
49 | ||
50 | The main difference between the stateless type A protocol and the stateful | |
51 | type B slot protocol lies in the usage of identifiable contacts to reduce | |
52 | the amount of data sent to userspace. The slot protocol requires the use of | |
53 | the ABS_MT_TRACKING_ID, either provided by the hardware or computed from | |
eba31a3a | 54 | the raw data [#f5]_. |
72c8a94a HR |
55 | |
56 | For type A devices, the kernel driver should generate an arbitrary | |
57 | enumeration of the full set of anonymous contacts currently on the | |
58 | surface. The order in which the packets appear in the event stream is not | |
eba31a3a | 59 | important. Event filtering and finger tracking is left to user space [#f3]_. |
72c8a94a HR |
60 | |
61 | For type B devices, the kernel driver should associate a slot with each | |
62 | identified contact, and use that slot to propagate changes for the contact. | |
63 | Creation, replacement and destruction of contacts is achieved by modifying | |
64 | the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id | |
65 | is interpreted as a contact, and the value -1 denotes an unused slot. A | |
66 | tracking id not previously present is considered new, and a tracking id no | |
67 | longer present is considered removed. Since only changes are propagated, | |
68 | the full state of each initiated contact has to reside in the receiving | |
69 | end. Upon receiving an MT event, one simply updates the appropriate | |
70 | attribute of the current slot. | |
71 | ||
a93bd154 DK |
72 | Some devices identify and/or track more contacts than they can report to the |
73 | driver. A driver for such a device should associate one type B slot with each | |
74 | contact that is reported by the hardware. Whenever the identity of the | |
75 | contact associated with a slot changes, the driver should invalidate that | |
76 | slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is | |
77 | tracking more contacts than it is currently reporting, the driver should use | |
78 | a BTN_TOOL_*TAP event to inform userspace of the total number of contacts | |
79 | being tracked by the hardware at that moment. The driver should do this by | |
80 | explicitly sending the corresponding BTN_TOOL_*TAP event and setting | |
81 | use_count to false when calling input_mt_report_pointer_emulation(). | |
82 | The driver should only advertise as many slots as the hardware can report. | |
83 | Userspace can detect that a driver can report more total contacts than slots | |
84 | by noting that the largest supported BTN_TOOL_*TAP event is larger than the | |
85 | total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis. | |
72c8a94a | 86 | |
257867dc PH |
87 | The minimum value of the ABS_MT_SLOT axis must be 0. |
88 | ||
72c8a94a HR |
89 | Protocol Example A |
90 | ------------------ | |
91 | ||
92 | Here is what a minimal event sequence for a two-contact touch would look | |
eba31a3a | 93 | like for a type A device:: |
72c8a94a HR |
94 | |
95 | ABS_MT_POSITION_X x[0] | |
96 | ABS_MT_POSITION_Y y[0] | |
97 | SYN_MT_REPORT | |
98 | ABS_MT_POSITION_X x[1] | |
99 | ABS_MT_POSITION_Y y[1] | |
100 | SYN_MT_REPORT | |
101 | SYN_REPORT | |
eacaad01 | 102 | |
72c8a94a HR |
103 | The sequence after moving one of the contacts looks exactly the same; the |
104 | raw data for all present contacts are sent between every synchronization | |
105 | with SYN_REPORT. | |
eacaad01 | 106 | |
eba31a3a | 107 | Here is the sequence after lifting the first contact:: |
eacaad01 | 108 | |
72c8a94a HR |
109 | ABS_MT_POSITION_X x[1] |
110 | ABS_MT_POSITION_Y y[1] | |
111 | SYN_MT_REPORT | |
112 | SYN_REPORT | |
113 | ||
eba31a3a | 114 | And here is the sequence after lifting the second contact:: |
72c8a94a HR |
115 | |
116 | SYN_MT_REPORT | |
117 | SYN_REPORT | |
118 | ||
119 | If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the | |
120 | ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the | |
121 | last SYN_REPORT will be dropped by the input core, resulting in no | |
122 | zero-contact event reaching userland. | |
123 | ||
124 | ||
125 | Protocol Example B | |
126 | ------------------ | |
127 | ||
128 | Here is what a minimal event sequence for a two-contact touch would look | |
eba31a3a | 129 | like for a type B device:: |
72c8a94a HR |
130 | |
131 | ABS_MT_SLOT 0 | |
132 | ABS_MT_TRACKING_ID 45 | |
133 | ABS_MT_POSITION_X x[0] | |
134 | ABS_MT_POSITION_Y y[0] | |
135 | ABS_MT_SLOT 1 | |
136 | ABS_MT_TRACKING_ID 46 | |
137 | ABS_MT_POSITION_X x[1] | |
138 | ABS_MT_POSITION_Y y[1] | |
139 | SYN_REPORT | |
140 | ||
eba31a3a | 141 | Here is the sequence after moving contact 45 in the x direction:: |
72c8a94a HR |
142 | |
143 | ABS_MT_SLOT 0 | |
144 | ABS_MT_POSITION_X x[0] | |
145 | SYN_REPORT | |
146 | ||
eba31a3a | 147 | Here is the sequence after lifting the contact in slot 0:: |
72c8a94a HR |
148 | |
149 | ABS_MT_TRACKING_ID -1 | |
150 | SYN_REPORT | |
151 | ||
152 | The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The | |
153 | message removes the association of slot 0 with contact 45, thereby | |
154 | destroying contact 45 and freeing slot 0 to be reused for another contact. | |
155 | ||
eba31a3a | 156 | Finally, here is the sequence after lifting the second contact:: |
72c8a94a HR |
157 | |
158 | ABS_MT_SLOT 1 | |
159 | ABS_MT_TRACKING_ID -1 | |
160 | SYN_REPORT | |
161 | ||
162 | ||
163 | Event Usage | |
164 | ----------- | |
eacaad01 HR |
165 | |
166 | A set of ABS_MT events with the desired properties is defined. The events | |
167 | are divided into categories, to allow for partial implementation. The | |
f6bdc230 | 168 | minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which |
72c8a94a | 169 | allows for multiple contacts to be tracked. If the device supports it, the |
f6bdc230 | 170 | ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size |
cab7faca | 171 | of the contact area and approaching tool, respectively. |
f6bdc230 HR |
172 | |
173 | The TOUCH and WIDTH parameters have a geometrical interpretation; imagine | |
174 | looking through a window at someone gently holding a finger against the | |
175 | glass. You will see two regions, one inner region consisting of the part | |
176 | of the finger actually touching the glass, and one outer region formed by | |
cab7faca HR |
177 | the perimeter of the finger. The center of the touching region (a) is |
178 | ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is | |
179 | ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger | |
180 | diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger | |
181 | harder against the glass. The touch region will increase, and in general, | |
182 | the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller | |
183 | than unity, is related to the contact pressure. For pressure-based devices, | |
f6bdc230 | 184 | ABS_MT_PRESSURE may be used to provide the pressure on the contact area |
e42a98b5 HR |
185 | instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to |
186 | indicate the distance between the contact and the surface. | |
f6bdc230 | 187 | |
eba31a3a MCC |
188 | :: |
189 | ||
cab7faca HR |
190 | |
191 | Linux MT Win8 | |
192 | __________ _______________________ | |
193 | / \ | | | |
194 | / \ | | | |
195 | / ____ \ | | | |
196 | / / \ \ | | | |
197 | \ \ a \ \ | a | | |
198 | \ \____/ \ | | | |
199 | \ \ | | | |
200 | \ b \ | b | | |
201 | \ \ | | | |
202 | \ \ | | | |
203 | \ \ | | | |
204 | \ / | | | |
205 | \ / | | | |
206 | \ / | | | |
207 | \__________/ |_______________________| | |
208 | ||
209 | ||
210 | In addition to the MAJOR parameters, the oval shape of the touch and finger | |
211 | regions can be described by adding the MINOR parameters, such that MAJOR | |
212 | and MINOR are the major and minor axis of an ellipse. The orientation of | |
213 | the touch ellipse can be described with the ORIENTATION parameter, and the | |
214 | direction of the finger ellipse is given by the vector (a - b). | |
f6bdc230 | 215 | |
22f075a8 HR |
216 | For type A devices, further specification of the touch shape is possible |
217 | via ABS_MT_BLOB_ID. | |
218 | ||
f6bdc230 | 219 | The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a |
22f075a8 | 220 | finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event |
eba31a3a | 221 | may be used to track identified contacts over time [#f5]_. |
22f075a8 HR |
222 | |
223 | In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are | |
224 | implicitly handled by input core; drivers should instead call | |
225 | input_mt_report_slot_state(). | |
f9fcfc3b | 226 | |
eacaad01 HR |
227 | |
228 | Event Semantics | |
229 | --------------- | |
230 | ||
eacaad01 | 231 | ABS_MT_TOUCH_MAJOR |
eba31a3a MCC |
232 | The length of the major axis of the contact. The length should be given in |
233 | surface units. If the surface has an X times Y resolution, the largest | |
234 | possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_. | |
eacaad01 HR |
235 | |
236 | ABS_MT_TOUCH_MINOR | |
eba31a3a MCC |
237 | The length, in surface units, of the minor axis of the contact. If the |
238 | contact is circular, this event can be omitted [#f4]_. | |
eacaad01 HR |
239 | |
240 | ABS_MT_WIDTH_MAJOR | |
eba31a3a MCC |
241 | The length, in surface units, of the major axis of the approaching |
242 | tool. This should be understood as the size of the tool itself. The | |
243 | orientation of the contact and the approaching tool are assumed to be the | |
244 | same [#f4]_. | |
eacaad01 HR |
245 | |
246 | ABS_MT_WIDTH_MINOR | |
eba31a3a MCC |
247 | The length, in surface units, of the minor axis of the approaching |
248 | tool. Omit if circular [#f4]_. | |
eacaad01 | 249 | |
eba31a3a MCC |
250 | The above four values can be used to derive additional information about |
251 | the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates | |
252 | the notion of pressure. The fingers of the hand and the palm all have | |
253 | different characteristic widths. | |
eacaad01 | 254 | |
f6bdc230 | 255 | ABS_MT_PRESSURE |
eba31a3a MCC |
256 | The pressure, in arbitrary units, on the contact area. May be used instead |
257 | of TOUCH and WIDTH for pressure-based devices or any device with a spatial | |
258 | signal intensity distribution. | |
f6bdc230 | 259 | |
e42a98b5 | 260 | ABS_MT_DISTANCE |
eba31a3a MCC |
261 | The distance, in surface units, between the contact and the surface. Zero |
262 | distance means the contact is touching the surface. A positive number means | |
263 | the contact is hovering above the surface. | |
e42a98b5 | 264 | |
eacaad01 | 265 | ABS_MT_ORIENTATION |
eba31a3a MCC |
266 | The orientation of the touching ellipse. The value should describe a signed |
267 | quarter of a revolution clockwise around the touch center. The signed value | |
268 | range is arbitrary, but zero should be returned for an ellipse aligned with | |
269 | the Y axis of the surface, a negative value when the ellipse is turned to | |
270 | the left, and a positive value when the ellipse is turned to the | |
271 | right. When completely aligned with the X axis, the range max should be | |
272 | returned. | |
273 | ||
274 | Touch ellipsis are symmetrical by default. For devices capable of true 360 | |
275 | degree orientation, the reported orientation must exceed the range max to | |
276 | indicate more than a quarter of a revolution. For an upside-down finger, | |
277 | range max * 2 should be returned. | |
278 | ||
279 | Orientation can be omitted if the touch area is circular, or if the | |
280 | information is not available in the kernel driver. Partial orientation | |
281 | support is possible if the device can distinguish between the two axis, but | |
282 | not (uniquely) any values in between. In such cases, the range of | |
283 | ABS_MT_ORIENTATION should be [0, 1] [#f4]_. | |
eacaad01 HR |
284 | |
285 | ABS_MT_POSITION_X | |
eba31a3a | 286 | The surface X coordinate of the center of the touching ellipse. |
eacaad01 HR |
287 | |
288 | ABS_MT_POSITION_Y | |
eba31a3a | 289 | The surface Y coordinate of the center of the touching ellipse. |
eacaad01 | 290 | |
cab7faca | 291 | ABS_MT_TOOL_X |
eba31a3a MCC |
292 | The surface X coordinate of the center of the approaching tool. Omit if |
293 | the device cannot distinguish between the intended touch point and the | |
294 | tool itself. | |
cab7faca HR |
295 | |
296 | ABS_MT_TOOL_Y | |
eba31a3a MCC |
297 | The surface Y coordinate of the center of the approaching tool. Omit if the |
298 | device cannot distinguish between the intended touch point and the tool | |
299 | itself. | |
cab7faca | 300 | |
eba31a3a MCC |
301 | The four position values can be used to separate the position of the touch |
302 | from the position of the tool. If both positions are present, the major | |
303 | tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are | |
304 | aligned with the touch axes. | |
cab7faca | 305 | |
eacaad01 | 306 | ABS_MT_TOOL_TYPE |
eba31a3a MCC |
307 | The type of approaching tool. A lot of kernel drivers cannot distinguish |
308 | between different tool types, such as a finger or a pen. In such cases, the | |
309 | event should be omitted. The protocol currently supports MT_TOOL_FINGER, | |
310 | MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_. For type B devices, this event is | |
311 | handled by input core; drivers should instead use | |
312 | input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may change over | |
313 | time while still touching the device, because the firmware may not be able | |
314 | to determine which tool is being used when it first appears. | |
eacaad01 HR |
315 | |
316 | ABS_MT_BLOB_ID | |
eba31a3a MCC |
317 | The BLOB_ID groups several packets together into one arbitrarily shaped |
318 | contact. The sequence of points forms a polygon which defines the shape of | |
319 | the contact. This is a low-level anonymous grouping for type A devices, and | |
320 | should not be confused with the high-level trackingID [#f5]_. Most type A | |
321 | devices do not have blob capability, so drivers can safely omit this event. | |
f9fcfc3b HR |
322 | |
323 | ABS_MT_TRACKING_ID | |
eba31a3a MCC |
324 | The TRACKING_ID identifies an initiated contact throughout its life cycle |
325 | [#f5]_. The value range of the TRACKING_ID should be large enough to ensure | |
326 | unique identification of a contact maintained over an extended period of | |
327 | time. For type B devices, this event is handled by input core; drivers | |
328 | should instead use input_mt_report_slot_state(). | |
f9fcfc3b HR |
329 | |
330 | ||
331 | Event Computation | |
332 | ----------------- | |
333 | ||
334 | The flora of different hardware unavoidably leads to some devices fitting | |
335 | better to the MT protocol than others. To simplify and unify the mapping, | |
336 | this section gives recipes for how to compute certain events. | |
337 | ||
338 | For devices reporting contacts as rectangular shapes, signed orientation | |
339 | cannot be obtained. Assuming X and Y are the lengths of the sides of the | |
340 | touching rectangle, here is a simple formula that retains the most | |
eba31a3a | 341 | information possible:: |
f9fcfc3b HR |
342 | |
343 | ABS_MT_TOUCH_MAJOR := max(X, Y) | |
344 | ABS_MT_TOUCH_MINOR := min(X, Y) | |
345 | ABS_MT_ORIENTATION := bool(X > Y) | |
346 | ||
347 | The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that | |
348 | the device can distinguish between a finger along the Y axis (0) and a | |
349 | finger along the X axis (1). | |
eacaad01 | 350 | |
eba31a3a | 351 | For win8 devices with both T and C coordinates, the position mapping is:: |
cab7faca HR |
352 | |
353 | ABS_MT_POSITION_X := T_X | |
354 | ABS_MT_POSITION_Y := T_Y | |
355 | ABS_MT_TOOL_X := C_X | |
b1452723 | 356 | ABS_MT_TOOL_Y := C_Y |
cab7faca HR |
357 | |
358 | Unfortunately, there is not enough information to specify both the touching | |
359 | ellipse and the tool ellipse, so one has to resort to approximations. One | |
eba31a3a | 360 | simple scheme, which is compatible with earlier usage, is:: |
cab7faca HR |
361 | |
362 | ABS_MT_TOUCH_MAJOR := min(X, Y) | |
363 | ABS_MT_TOUCH_MINOR := <not used> | |
364 | ABS_MT_ORIENTATION := <not used> | |
365 | ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C) | |
366 | ABS_MT_WIDTH_MINOR := min(X, Y) | |
367 | ||
368 | Rationale: We have no information about the orientation of the touching | |
369 | ellipse, so approximate it with an inscribed circle instead. The tool | |
df5cbb27 | 370 | ellipse should align with the vector (T - C), so the diameter must |
cab7faca HR |
371 | increase with distance(T, C). Finally, assume that the touch diameter is |
372 | equal to the tool thickness, and we arrive at the formulas above. | |
eacaad01 HR |
373 | |
374 | Finger Tracking | |
375 | --------------- | |
376 | ||
f9fcfc3b | 377 | The process of finger tracking, i.e., to assign a unique trackingID to each |
72c8a94a HR |
378 | initiated contact on the surface, is a Euclidian Bipartite Matching |
379 | problem. At each event synchronization, the set of actual contacts is | |
380 | matched to the set of contacts from the previous synchronization. A full | |
eba31a3a | 381 | implementation can be found in [#f3]_. |
f9fcfc3b HR |
382 | |
383 | ||
f6bdc230 HR |
384 | Gestures |
385 | -------- | |
386 | ||
387 | In the specific application of creating gesture events, the TOUCH and WIDTH | |
388 | parameters can be used to, e.g., approximate finger pressure or distinguish | |
389 | between index finger and thumb. With the addition of the MINOR parameters, | |
390 | one can also distinguish between a sweeping finger and a pointing finger, | |
391 | and with ORIENTATION, one can detect twisting of fingers. | |
392 | ||
393 | ||
eacaad01 HR |
394 | Notes |
395 | ----- | |
396 | ||
22f075a8 HR |
397 | In order to stay compatible with existing applications, the data reported |
398 | in a finger packet must not be recognized as single-touch events. | |
399 | ||
400 | For type A devices, all finger data bypasses input filtering, since | |
401 | subsequent events of the same type refer to different fingers. | |
eacaad01 | 402 | |
22f075a8 HR |
403 | For example usage of the type A protocol, see the bcm5974 driver. For |
404 | example usage of the type B protocol, see the hid-egalax driver. | |
eacaad01 | 405 | |
eba31a3a MCC |
406 | .. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt. |
407 | .. [#f2] The list can of course be extended. | |
408 | .. [#f3] The mtdev project: http://bitmath.org/code/mtdev/. | |
409 | .. [#f4] See the section on event computation. | |
410 | .. [#f5] See the section on finger tracking. |