1.. include:: <isonum.txt> 2 3========================= 4Multi-touch (MT) Protocol 5========================= 6 7:Copyright: |copy| 2009-2010 Henrik Rydberg <rydberg@euromail.se> 8 9 10Introduction 11------------ 12 13In order to utilize the full power of the new multi-touch and multi-user 14devices, a way to report detailed data from multiple contacts, i.e., 15objects in direct contact with the device surface, is needed. This 16document describes the multi-touch (MT) protocol which allows kernel 17drivers to report details for an arbitrary number of contacts. 18 19The protocol is divided into two types, depending on the capabilities of the 20hardware. For devices handling anonymous contacts (type A), the protocol 21describes how to send the raw data for all contacts to the receiver. For 22devices capable of tracking identifiable contacts (type B), the protocol 23describes how to send updates for individual contacts via event slots. 24 25.. note:: 26 MT potocol type A is obsolete, all kernel drivers have been 27 converted to use type B. 28 29Protocol Usage 30-------------- 31 32Contact details are sent sequentially as separate packets of ABS_MT 33events. Only the ABS_MT events are recognized as part of a contact 34packet. Since these events are ignored by current single-touch (ST) 35applications, the MT protocol can be implemented on top of the ST protocol 36in an existing driver. 37 38Drivers for type A devices separate contact packets by calling 39input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT 40event, which instructs the receiver to accept the data for the current 41contact and prepare to receive another. 42 43Drivers for type B devices separate contact packets by calling 44input_mt_slot(), with a slot as argument, at the beginning of each packet. 45This generates an ABS_MT_SLOT event, which instructs the receiver to 46prepare for updates of the given slot. 47 48All drivers mark the end of a multi-touch transfer by calling the usual 49input_sync() function. This instructs the receiver to act upon events 50accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set 51of events/packets. 52 53The main difference between the stateless type A protocol and the stateful 54type B slot protocol lies in the usage of identifiable contacts to reduce 55the amount of data sent to userspace. The slot protocol requires the use of 56the ABS_MT_TRACKING_ID, either provided by the hardware or computed from 57the raw data [#f5]_. 58 59For type A devices, the kernel driver should generate an arbitrary 60enumeration of the full set of anonymous contacts currently on the 61surface. The order in which the packets appear in the event stream is not 62important. Event filtering and finger tracking is left to user space [#f3]_. 63 64For type B devices, the kernel driver should associate a slot with each 65identified contact, and use that slot to propagate changes for the contact. 66Creation, replacement and destruction of contacts is achieved by modifying 67the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id 68is interpreted as a contact, and the value -1 denotes an unused slot. A 69tracking id not previously present is considered new, and a tracking id no 70longer present is considered removed. Since only changes are propagated, 71the full state of each initiated contact has to reside in the receiving 72end. Upon receiving an MT event, one simply updates the appropriate 73attribute of the current slot. 74 75Some devices identify and/or track more contacts than they can report to the 76driver. A driver for such a device should associate one type B slot with each 77contact that is reported by the hardware. Whenever the identity of the 78contact associated with a slot changes, the driver should invalidate that 79slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is 80tracking more contacts than it is currently reporting, the driver should use 81a BTN_TOOL_*TAP event to inform userspace of the total number of contacts 82being tracked by the hardware at that moment. The driver should do this by 83explicitly sending the corresponding BTN_TOOL_*TAP event and setting 84use_count to false when calling input_mt_report_pointer_emulation(). 85The driver should only advertise as many slots as the hardware can report. 86Userspace can detect that a driver can report more total contacts than slots 87by noting that the largest supported BTN_TOOL_*TAP event is larger than the 88total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis. 89 90The minimum value of the ABS_MT_SLOT axis must be 0. 91 92Protocol Example A 93------------------ 94 95Here is what a minimal event sequence for a two-contact touch would look 96like 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 106The sequence after moving one of the contacts looks exactly the same; the 107raw data for all present contacts are sent between every synchronization 108with SYN_REPORT. 109 110Here is the sequence after lifting the first contact:: 111 112 ABS_MT_POSITION_X x[1] 113 ABS_MT_POSITION_Y y[1] 114 SYN_MT_REPORT 115 SYN_REPORT 116 117And here is the sequence after lifting the second contact:: 118 119 SYN_MT_REPORT 120 SYN_REPORT 121 122If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the 123ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the 124last SYN_REPORT will be dropped by the input core, resulting in no 125zero-contact event reaching userland. 126 127 128Protocol Example B 129------------------ 130 131Here is what a minimal event sequence for a two-contact touch would look 132like 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 144Here is the sequence after moving contact 45 in the x direction:: 145 146 ABS_MT_SLOT 0 147 ABS_MT_POSITION_X x[0] 148 SYN_REPORT 149 150Here is the sequence after lifting the contact in slot 0:: 151 152 ABS_MT_TRACKING_ID -1 153 SYN_REPORT 154 155The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The 156message removes the association of slot 0 with contact 45, thereby 157destroying contact 45 and freeing slot 0 to be reused for another contact. 158 159Finally, here is the sequence after lifting the second contact:: 160 161 ABS_MT_SLOT 1 162 ABS_MT_TRACKING_ID -1 163 SYN_REPORT 164 165 166Event Usage 167----------- 168 169A set of ABS_MT events with the desired properties is defined. The events 170are divided into categories, to allow for partial implementation. The 171minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which 172allows for multiple contacts to be tracked. If the device supports it, the 173ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size 174of the contact area and approaching tool, respectively. 175 176The TOUCH and WIDTH parameters have a geometrical interpretation; imagine 177looking through a window at someone gently holding a finger against the 178glass. You will see two regions, one inner region consisting of the part 179of the finger actually touching the glass, and one outer region formed by 180the perimeter of the finger. The center of the touching region (a) is 181ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is 182ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger 183diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger 184harder against the glass. The touch region will increase, and in general, 185the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller 186than unity, is related to the contact pressure. For pressure-based devices, 187ABS_MT_PRESSURE may be used to provide the pressure on the contact area 188instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to 189indicate the distance between the contact and the surface. 190 191:: 192 193 194 Linux MT Win8 195 __________ _______________________ 196 / \ | | 197 / \ | | 198 / ____ \ | | 199 / / \ \ | | 200 \ \ a \ \ | a | 201 \ \____/ \ | | 202 \ \ | | 203 \ b \ | b | 204 \ \ | | 205 \ \ | | 206 \ \ | | 207 \ / | | 208 \ / | | 209 \ / | | 210 \__________/ |_______________________| 211 212 213In addition to the MAJOR parameters, the oval shape of the touch and finger 214regions can be described by adding the MINOR parameters, such that MAJOR 215and MINOR are the major and minor axis of an ellipse. The orientation of 216the touch ellipse can be described with the ORIENTATION parameter, and the 217direction of the finger ellipse is given by the vector (a - b). 218 219For type A devices, further specification of the touch shape is possible 220via ABS_MT_BLOB_ID. 221 222The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a 223finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event 224may be used to track identified contacts over time [#f5]_. 225 226In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are 227implicitly handled by input core; drivers should instead call 228input_mt_report_slot_state(). 229 230 231Event Semantics 232--------------- 233 234ABS_MT_TOUCH_MAJOR 235 The length of the major axis of the contact. The length should be given in 236 surface units. If the surface has an X times Y resolution, the largest 237 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_. 238 239ABS_MT_TOUCH_MINOR 240 The length, in surface units, of the minor axis of the contact. If the 241 contact is circular, this event can be omitted [#f4]_. 242 243ABS_MT_WIDTH_MAJOR 244 The length, in surface units, of the major axis of the approaching 245 tool. This should be understood as the size of the tool itself. The 246 orientation of the contact and the approaching tool are assumed to be the 247 same [#f4]_. 248 249ABS_MT_WIDTH_MINOR 250 The length, in surface units, of the minor axis of the approaching 251 tool. Omit if circular [#f4]_. 252 253 The above four values can be used to derive additional information about 254 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates 255 the notion of pressure. The fingers of the hand and the palm all have 256 different characteristic widths. 257 258ABS_MT_PRESSURE 259 The pressure, in arbitrary units, on the contact area. May be used instead 260 of TOUCH and WIDTH for pressure-based devices or any device with a spatial 261 signal intensity distribution. 262 263ABS_MT_DISTANCE 264 The distance, in surface units, between the contact and the surface. Zero 265 distance means the contact is touching the surface. A positive number means 266 the contact is hovering above the surface. 267 268ABS_MT_ORIENTATION 269 The orientation of the touching ellipse. The value should describe a signed 270 quarter of a revolution clockwise around the touch center. The signed value 271 range is arbitrary, but zero should be returned for an ellipse aligned with 272 the Y axis (north) of the surface, a negative value when the ellipse is 273 turned to the left, and a positive value when the ellipse is turned to the 274 right. When aligned with the X axis in the positive direction, the range 275 max should be returned; when aligned with the X axis in the negative 276 direction, the range -max should be returned. 277 278 Touch ellipsis are symmetrical by default. For devices capable of true 360 279 degree orientation, the reported orientation must exceed the range max to 280 indicate more than a quarter of a revolution. For an upside-down finger, 281 range max * 2 should be returned. 282 283 Orientation can be omitted if the touch area is circular, or if the 284 information is not available in the kernel driver. Partial orientation 285 support is possible if the device can distinguish between the two axis, but 286 not (uniquely) any values in between. In such cases, the range of 287 ABS_MT_ORIENTATION should be [0, 1] [#f4]_. 288 289ABS_MT_POSITION_X 290 The surface X coordinate of the center of the touching ellipse. 291 292ABS_MT_POSITION_Y 293 The surface Y coordinate of the center of the touching ellipse. 294 295ABS_MT_TOOL_X 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 300ABS_MT_TOOL_Y 301 The surface Y coordinate of the center of the approaching tool. Omit if the 302 device cannot distinguish between the intended touch point and the tool 303 itself. 304 305 The four position values can be used to separate the position of the touch 306 from the position of the tool. If both positions are present, the major 307 tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are 308 aligned with the touch axes. 309 310ABS_MT_TOOL_TYPE 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 mainly supports 314 MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_. 315 For type B devices, this event is handled by input core; drivers should 316 instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may 317 change over time while still touching the device, because the firmware may 318 not be able to determine which tool is being used when it first appears. 319 320ABS_MT_BLOB_ID 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 [#f5]_. Most type A 325 devices do not have blob capability, so drivers can safely omit this event. 326 327ABS_MT_TRACKING_ID 328 The TRACKING_ID identifies an initiated contact throughout its life cycle 329 [#f5]_. The value range of the TRACKING_ID should be large enough to ensure 330 unique identification of a contact maintained over an extended period of 331 time. For type B devices, this event is handled by input core; drivers 332 should instead use input_mt_report_slot_state(). 333 334 335Event Computation 336----------------- 337 338The flora of different hardware unavoidably leads to some devices fitting 339better to the MT protocol than others. To simplify and unify the mapping, 340this section gives recipes for how to compute certain events. 341 342For devices reporting contacts as rectangular shapes, signed orientation 343cannot be obtained. Assuming X and Y are the lengths of the sides of the 344touching rectangle, here is a simple formula that retains the most 345information possible:: 346 347 ABS_MT_TOUCH_MAJOR := max(X, Y) 348 ABS_MT_TOUCH_MINOR := min(X, Y) 349 ABS_MT_ORIENTATION := bool(X > Y) 350 351The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that 352the device can distinguish between a finger along the Y axis (0) and a 353finger along the X axis (1). 354 355For win8 devices with both T and C coordinates, the position mapping is:: 356 357 ABS_MT_POSITION_X := T_X 358 ABS_MT_POSITION_Y := T_Y 359 ABS_MT_TOOL_X := C_X 360 ABS_MT_TOOL_Y := C_Y 361 362Unfortunately, there is not enough information to specify both the touching 363ellipse and the tool ellipse, so one has to resort to approximations. One 364simple scheme, which is compatible with earlier usage, is:: 365 366 ABS_MT_TOUCH_MAJOR := min(X, Y) 367 ABS_MT_TOUCH_MINOR := <not used> 368 ABS_MT_ORIENTATION := <not used> 369 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C) 370 ABS_MT_WIDTH_MINOR := min(X, Y) 371 372Rationale: We have no information about the orientation of the touching 373ellipse, so approximate it with an inscribed circle instead. The tool 374ellipse should align with the vector (T - C), so the diameter must 375increase with distance(T, C). Finally, assume that the touch diameter is 376equal to the tool thickness, and we arrive at the formulas above. 377 378Finger Tracking 379--------------- 380 381The process of finger tracking, i.e., to assign a unique trackingID to each 382initiated contact on the surface, is a Euclidian Bipartite Matching 383problem. At each event synchronization, the set of actual contacts is 384matched to the set of contacts from the previous synchronization. A full 385implementation can be found in [#f3]_. 386 387 388Gestures 389-------- 390 391In the specific application of creating gesture events, the TOUCH and WIDTH 392parameters can be used to, e.g., approximate finger pressure or distinguish 393between index finger and thumb. With the addition of the MINOR parameters, 394one can also distinguish between a sweeping finger and a pointing finger, 395and with ORIENTATION, one can detect twisting of fingers. 396 397 398Notes 399----- 400 401In order to stay compatible with existing applications, the data reported 402in a finger packet must not be recognized as single-touch events. 403 404For type A devices, all finger data bypasses input filtering, since 405subsequent events of the same type refer to different fingers. 406 407.. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt. 408.. [#f2] The list can of course be extended. 409.. [#f3] The mtdev project: http://bitmath.org/code/mtdev/. 410.. [#f4] See the section on event computation. 411.. [#f5] See the section on finger tracking. 412
