Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.
1 Multi-touch (MT) Protocol 2 ------------------------- 3 Copyright (C) 2009-2010 Henrik Rydberg <email@example.com> 4 5 6 Introduction 7 ------------ 8 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 . 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 . 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 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. 82 83 The minimum value of the ABS_MT_SLOT axis must be 0. 84 85 Protocol Example A 86 ------------------ 87 88 Here is what a minimal event sequence for a two-contact touch would look 89 like for a type A device: 90 91 ABS_MT_POSITION_X x 92 ABS_MT_POSITION_Y y 93 SYN_MT_REPORT 94 ABS_MT_POSITION_X x 95 ABS_MT_POSITION_Y y 96 SYN_MT_REPORT 97 SYN_REPORT 98 99 The sequence after moving one of the contacts looks exactly the same; the 100 raw data for all present contacts are sent between every synchronization 101 with SYN_REPORT. 102 103 Here is the sequence after lifting the first contact: 104 105 ABS_MT_POSITION_X x 106 ABS_MT_POSITION_Y y 107 SYN_MT_REPORT 108 SYN_REPORT 109 110 And here is the sequence after lifting the second contact: 111 112 SYN_MT_REPORT 113 SYN_REPORT 114 115 If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the 116 ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the 117 last SYN_REPORT will be dropped by the input core, resulting in no 118 zero-contact event reaching userland. 119 120 121 Protocol Example B 122 ------------------ 123 124 Here is what a minimal event sequence for a two-contact touch would look 125 like for a type B device: 126 127 ABS_MT_SLOT 0 128 ABS_MT_TRACKING_ID 45 129 ABS_MT_POSITION_X x 130 ABS_MT_POSITION_Y y 131 ABS_MT_SLOT 1 132 ABS_MT_TRACKING_ID 46 133 ABS_MT_POSITION_X x 134 ABS_MT_POSITION_Y y 135 SYN_REPORT 136 137 Here is the sequence after moving contact 45 in the x direction: 138 139 ABS_MT_SLOT 0 140 ABS_MT_POSITION_X x 141 SYN_REPORT 142 143 Here is the sequence after lifting the contact in slot 0: 144 145 ABS_MT_TRACKING_ID -1 146 SYN_REPORT 147 148 The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The 149 message removes the association of slot 0 with contact 45, thereby 150 destroying contact 45 and freeing slot 0 to be reused for another contact. 151 152 Finally, here is the sequence after lifting the second contact: 153 154 ABS_MT_SLOT 1 155 ABS_MT_TRACKING_ID -1 156 SYN_REPORT 157 158 159 Event Usage 160 ----------- 161 162 A set of ABS_MT events with the desired properties is defined. The events 163 are divided into categories, to allow for partial implementation. The 164 minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which 165 allows for multiple contacts to be tracked. If the device supports it, the 166 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size 167 of the contact area and approaching tool, respectively. 168 169 The TOUCH and WIDTH parameters have a geometrical interpretation; imagine 170 looking through a window at someone gently holding a finger against the 171 glass. You will see two regions, one inner region consisting of the part 172 of the finger actually touching the glass, and one outer region formed by 173 the perimeter of the finger. The center of the touching region (a) is 174 ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is 175 ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger 176 diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger 177 harder against the glass. The touch region will increase, and in general, 178 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller 179 than unity, is related to the contact pressure. For pressure-based devices, 180 ABS_MT_PRESSURE may be used to provide the pressure on the contact area 181 instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to 182 indicate 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 \__________/ |_______________________| 202 203 204 In addition to the MAJOR parameters, the oval shape of the touch and finger 205 regions can be described by adding the MINOR parameters, such that MAJOR 206 and MINOR are the major and minor axis of an ellipse. The orientation of 207 the touch ellipse can be described with the ORIENTATION parameter, and the 208 direction of the finger ellipse is given by the vector (a - b). 209 210 For type A devices, further specification of the touch shape is possible 211 via ABS_MT_BLOB_ID. 212 213 The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a 214 finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event 215 may be used to track identified contacts over time . 216 217 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are 218 implicitly handled by input core; drivers should instead call 219 input_mt_report_slot_state(). 220 221 222 Event Semantics 223 --------------- 224 225 ABS_MT_TOUCH_MAJOR 226 227 The length of the major axis of the contact. The length should be given in 228 surface units. If the surface has an X times Y resolution, the largest 229 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal . 230 231 ABS_MT_TOUCH_MINOR 232 233 The length, in surface units, of the minor axis of the contact. If the 234 contact is circular, this event can be omitted . 235 236 ABS_MT_WIDTH_MAJOR 237 238 The length, in surface units, of the major axis of the approaching 239 tool. This should be understood as the size of the tool itself. The 240 orientation of the contact and the approaching tool are assumed to be the 241 same . 242 243 ABS_MT_WIDTH_MINOR 244 245 The length, in surface units, of the minor axis of the approaching 246 tool. Omit if circular . 247 248 The above four values can be used to derive additional information about 249 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates 250 the notion of pressure. The fingers of the hand and the palm all have 251 different characteristic widths. 252 253 ABS_MT_PRESSURE 254 255 The pressure, in arbitrary units, on the contact area. May be used instead 256 of TOUCH and WIDTH for pressure-based devices or any device with a spatial 257 signal intensity distribution. 258 259 ABS_MT_DISTANCE 260 261 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. 264 265 ABS_MT_ORIENTATION 266 267 The orientation of the touching ellipse. The value should describe a signed 268 quarter of a revolution clockwise around the touch center. The signed value 269 range is arbitrary, but zero should be returned for an ellipse aligned with 270 the Y axis of the surface, a negative value when the ellipse is turned to 271 the left, and a positive value when the ellipse is turned to the 272 right. When completely aligned with the X axis, the range max should be 273 returned. 274 275 Touch ellipsis are symmetrical by default. For devices capable of true 360 276 degree orientation, the reported orientation must exceed the range max to 277 indicate more than a quarter of a revolution. For an upside-down finger, 278 range max * 2 should be returned. 279 280 Orientation can be omitted if the touch area is circular, or if the 281 information is not available in the kernel driver. Partial orientation 282 support is possible if the device can distinguish between the two axis, but 283 not (uniquely) any values in between. In such cases, the range of 284 ABS_MT_ORIENTATION should be [0, 1] . 285 286 ABS_MT_POSITION_X 287 288 The surface X coordinate of the center of the touching ellipse. 289 290 ABS_MT_POSITION_Y 291 292 The surface Y coordinate of the center of the touching ellipse. 293 294 ABS_MT_TOOL_X 295 296 The surface X coordinate of the center of the approaching tool. Omit if 297 the device cannot distinguish between the intended touch point and the 298 tool itself. 299 300 ABS_MT_TOOL_Y 301 302 The surface Y coordinate of the center of the approaching tool. Omit if the 303 device cannot distinguish between the intended touch point and the tool 304 itself. 305 306 The four position values can be used to separate the position of the touch 307 from the position of the tool. If both positions are present, the major 308 tool axis points towards the touch point . Otherwise, the tool axes are 309 aligned with the touch axes. 310 311 ABS_MT_TOOL_TYPE 312 313 The type of approaching tool. A lot of kernel drivers cannot distinguish 314 between different tool types, such as a finger or a pen. In such cases, the 315 event should be omitted. The protocol currently supports MT_TOOL_FINGER and 316 MT_TOOL_PEN . For type B devices, this event is handled by input core; 317 drivers should instead use input_mt_report_slot_state(). 318 319 ABS_MT_BLOB_ID 320 321 The BLOB_ID groups several packets together into one arbitrarily shaped 322 contact. The sequence of points forms a polygon which defines the shape of 323 the contact. This is a low-level anonymous grouping for type A devices, and 324 should not be confused with the high-level trackingID . Most type A 325 devices do not have blob capability, so drivers can safely omit this event. 326 327 ABS_MT_TRACKING_ID 328 329 The TRACKING_ID identifies an initiated contact throughout its life cycle 330 . The value range of the TRACKING_ID should be large enough to ensure 331 unique identification of a contact maintained over an extended period of 332 time. For type B devices, this event is handled by input core; drivers 333 should instead use input_mt_report_slot_state(). 334 335 336 Event Computation 337 ----------------- 338 339 The flora of different hardware unavoidably leads to some devices fitting 340 better to the MT protocol than others. To simplify and unify the mapping, 341 this section gives recipes for how to compute certain events. 342 343 For devices reporting contacts as rectangular shapes, signed orientation 344 cannot be obtained. Assuming X and Y are the lengths of the sides of the 345 touching rectangle, here is a simple formula that retains the most 346 information possible: 347 348 ABS_MT_TOUCH_MAJOR := max(X, Y) 349 ABS_MT_TOUCH_MINOR := min(X, Y) 350 ABS_MT_ORIENTATION := bool(X > Y) 351 352 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that 353 the device can distinguish between a finger along the Y axis (0) and a 354 finger along the X axis (1). 355 356 For win8 devices with both T and C coordinates, the position mapping is 357 358 ABS_MT_POSITION_X := T_X 359 ABS_MT_POSITION_Y := T_Y 360 ABS_MT_TOOL_X := C_X 361 ABS_MT_TOOL_X := C_Y 362 363 Unfortunately, there is not enough information to specify both the touching 364 ellipse and the tool ellipse, so one has to resort to approximations. One 365 simple scheme, which is compatible with earlier usage, is: 366 367 ABS_MT_TOUCH_MAJOR := min(X, Y) 368 ABS_MT_TOUCH_MINOR := <not used> 369 ABS_MT_ORIENTATION := <not used> 370 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C) 371 ABS_MT_WIDTH_MINOR := min(X, Y) 372 373 Rationale: We have no information about the orientation of the touching 374 ellipse, so approximate it with an inscribed circle instead. The tool 375 ellipse should align with the vector (T - C), so the diameter must 376 increase with distance(T, C). Finally, assume that the touch diameter is 377 equal to the tool thickness, and we arrive at the formulas above. 378 379 Finger Tracking 380 --------------- 381 382 The process of finger tracking, i.e., to assign a unique trackingID to each 383 initiated contact on the surface, is a Euclidian Bipartite Matching 384 problem. At each event synchronization, the set of actual contacts is 385 matched to the set of contacts from the previous synchronization. A full 386 implementation can be found in . 387 388 389 Gestures 390 -------- 391 392 In the specific application of creating gesture events, the TOUCH and WIDTH 393 parameters can be used to, e.g., approximate finger pressure or distinguish 394 between index finger and thumb. With the addition of the MINOR parameters, 395 one can also distinguish between a sweeping finger and a pointing finger, 396 and with ORIENTATION, one can detect twisting of fingers. 397 398 399 Notes 400 ----- 401 402 In order to stay compatible with existing applications, the data reported 403 in a finger packet must not be recognized as single-touch events. 404 405 For type A devices, all finger data bypasses input filtering, since 406 subsequent events of the same type refer to different fingers. 407 408 For example usage of the type A protocol, see the bcm5974 driver. For 409 example usage of the type B protocol, see the hid-egalax driver. 410 411  Also, the difference (TOOL_X - POSITION_X) can be used to model tilt. 412  The list can of course be extended. 413  The mtdev project: http://bitmath.org/code/mtdev/. 414  See the section on event computation. 415  See the section on finger tracking.