1Multi-touch (MT) Protocol 2------------------------- 3 Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se> 4 5 6Introduction 7------------ 8 9In order to utilize the full power of the new multi-touch and multi-user 10devices, a way to report detailed data from multiple contacts, i.e., 11objects in direct contact with the device surface, is needed. This 12document describes the multi-touch (MT) protocol which allows kernel 13drivers to report details for an arbitrary number of contacts. 14 15The protocol is divided into two types, depending on the capabilities of the 16hardware. For devices handling anonymous contacts (type A), the protocol 17describes how to send the raw data for all contacts to the receiver. For 18devices capable of tracking identifiable contacts (type B), the protocol 19describes how to send updates for individual contacts via event slots. 20 21 22Protocol Usage 23-------------- 24 25Contact details are sent sequentially as separate packets of ABS_MT 26events. Only the ABS_MT events are recognized as part of a contact 27packet. Since these events are ignored by current single-touch (ST) 28applications, the MT protocol can be implemented on top of the ST protocol 29in an existing driver. 30 31Drivers for type A devices separate contact packets by calling 32input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT 33event, which instructs the receiver to accept the data for the current 34contact and prepare to receive another. 35 36Drivers for type B devices separate contact packets by calling 37input_mt_slot(), with a slot as argument, at the beginning of each packet. 38This generates an ABS_MT_SLOT event, which instructs the receiver to 39prepare for updates of the given slot. 40 41All drivers mark the end of a multi-touch transfer by calling the usual 42input_sync() function. This instructs the receiver to act upon events 43accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set 44of events/packets. 45 46The main difference between the stateless type A protocol and the stateful 47type B slot protocol lies in the usage of identifiable contacts to reduce 48the amount of data sent to userspace. The slot protocol requires the use of 49the ABS_MT_TRACKING_ID, either provided by the hardware or computed from 50the raw data [5]. 51 52For type A devices, the kernel driver should generate an arbitrary 53enumeration of the full set of anonymous contacts currently on the 54surface. The order in which the packets appear in the event stream is not 55important. Event filtering and finger tracking is left to user space [3]. 56 57For type B devices, the kernel driver should associate a slot with each 58identified contact, and use that slot to propagate changes for the contact. 59Creation, replacement and destruction of contacts is achieved by modifying 60the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id 61is interpreted as a contact, and the value -1 denotes an unused slot. A 62tracking id not previously present is considered new, and a tracking id no 63longer present is considered removed. Since only changes are propagated, 64the full state of each initiated contact has to reside in the receiving 65end. Upon receiving an MT event, one simply updates the appropriate 66attribute of the current slot. 67 68Some devices identify and/or track more contacts than they can report to the 69driver. A driver for such a device should associate one type B slot with each 70contact that is reported by the hardware. Whenever the identity of the 71contact associated with a slot changes, the driver should invalidate that 72slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is 73tracking more contacts than it is currently reporting, the driver should use 74a BTN_TOOL_*TAP event to inform userspace of the total number of contacts 75being tracked by the hardware at that moment. The driver should do this by 76explicitly sending the corresponding BTN_TOOL_*TAP event and setting 77use_count to false when calling input_mt_report_pointer_emulation(). 78The driver should only advertise as many slots as the hardware can report. 79Userspace can detect that a driver can report more total contacts than slots 80by noting that the largest supported BTN_TOOL_*TAP event is larger than the 81total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis. 82 83The minimum value of the ABS_MT_SLOT axis must be 0. 84 85Protocol Example A 86------------------ 87 88Here is what a minimal event sequence for a two-contact touch would look 89like for a type A device: 90 91 ABS_MT_POSITION_X x[0] 92 ABS_MT_POSITION_Y y[0] 93 SYN_MT_REPORT 94 ABS_MT_POSITION_X x[1] 95 ABS_MT_POSITION_Y y[1] 96 SYN_MT_REPORT 97 SYN_REPORT 98 99The sequence after moving one of the contacts looks exactly the same; the 100raw data for all present contacts are sent between every synchronization 101with SYN_REPORT. 102 103Here is the sequence after lifting the first contact: 104 105 ABS_MT_POSITION_X x[1] 106 ABS_MT_POSITION_Y y[1] 107 SYN_MT_REPORT 108 SYN_REPORT 109 110And here is the sequence after lifting the second contact: 111 112 SYN_MT_REPORT 113 SYN_REPORT 114 115If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the 116ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the 117last SYN_REPORT will be dropped by the input core, resulting in no 118zero-contact event reaching userland. 119 120 121Protocol Example B 122------------------ 123 124Here is what a minimal event sequence for a two-contact touch would look 125like for a type B device: 126 127 ABS_MT_SLOT 0 128 ABS_MT_TRACKING_ID 45 129 ABS_MT_POSITION_X x[0] 130 ABS_MT_POSITION_Y y[0] 131 ABS_MT_SLOT 1 132 ABS_MT_TRACKING_ID 46 133 ABS_MT_POSITION_X x[1] 134 ABS_MT_POSITION_Y y[1] 135 SYN_REPORT 136 137Here is the sequence after moving contact 45 in the x direction: 138 139 ABS_MT_SLOT 0 140 ABS_MT_POSITION_X x[0] 141 SYN_REPORT 142 143Here is the sequence after lifting the contact in slot 0: 144 145 ABS_MT_TRACKING_ID -1 146 SYN_REPORT 147 148The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The 149message removes the association of slot 0 with contact 45, thereby 150destroying contact 45 and freeing slot 0 to be reused for another contact. 151 152Finally, 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 159Event Usage 160----------- 161 162A set of ABS_MT events with the desired properties is defined. The events 163are divided into categories, to allow for partial implementation. The 164minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which 165allows for multiple contacts to be tracked. If the device supports it, the 166ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size 167of the contact area and approaching tool, respectively. 168 169The TOUCH and WIDTH parameters have a geometrical interpretation; imagine 170looking through a window at someone gently holding a finger against the 171glass. You will see two regions, one inner region consisting of the part 172of the finger actually touching the glass, and one outer region formed by 173the perimeter of the finger. The center of the touching region (a) is 174ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is 175ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger 176diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger 177harder against the glass. The touch region will increase, and in general, 178the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller 179than unity, is related to the contact pressure. For pressure-based devices, 180ABS_MT_PRESSURE may be used to provide the pressure on the contact area 181instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to 182indicate 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 204In addition to the MAJOR parameters, the oval shape of the touch and finger 205regions can be described by adding the MINOR parameters, such that MAJOR 206and MINOR are the major and minor axis of an ellipse. The orientation of 207the touch ellipse can be described with the ORIENTATION parameter, and the 208direction of the finger ellipse is given by the vector (a - b). 209 210For type A devices, further specification of the touch shape is possible 211via ABS_MT_BLOB_ID. 212 213The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a 214finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event 215may be used to track identified contacts over time [5]. 216 217In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are 218implicitly handled by input core; drivers should instead call 219input_mt_report_slot_state(). 220 221 222Event Semantics 223--------------- 224 225ABS_MT_TOUCH_MAJOR 226 227The length of the major axis of the contact. The length should be given in 228surface units. If the surface has an X times Y resolution, the largest 229possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4]. 230 231ABS_MT_TOUCH_MINOR 232 233The length, in surface units, of the minor axis of the contact. If the 234contact is circular, this event can be omitted [4]. 235 236ABS_MT_WIDTH_MAJOR 237 238The length, in surface units, of the major axis of the approaching 239tool. This should be understood as the size of the tool itself. The 240orientation of the contact and the approaching tool are assumed to be the 241same [4]. 242 243ABS_MT_WIDTH_MINOR 244 245The length, in surface units, of the minor axis of the approaching 246tool. Omit if circular [4]. 247 248The above four values can be used to derive additional information about 249the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates 250the notion of pressure. The fingers of the hand and the palm all have 251different characteristic widths. 252 253ABS_MT_PRESSURE 254 255The pressure, in arbitrary units, on the contact area. May be used instead 256of TOUCH and WIDTH for pressure-based devices or any device with a spatial 257signal intensity distribution. 258 259ABS_MT_DISTANCE 260 261The distance, in surface units, between the contact and the surface. Zero 262distance means the contact is touching the surface. A positive number means 263the contact is hovering above the surface. 264 265ABS_MT_ORIENTATION 266 267The orientation of the touching ellipse. The value should describe a signed 268quarter of a revolution clockwise around the touch center. The signed value 269range is arbitrary, but zero should be returned for an ellipse aligned with 270the Y axis of the surface, a negative value when the ellipse is turned to 271the left, and a positive value when the ellipse is turned to the 272right. When completely aligned with the X axis, the range max should be 273returned. 274 275Touch ellipsis are symmetrical by default. For devices capable of true 360 276degree orientation, the reported orientation must exceed the range max to 277indicate more than a quarter of a revolution. For an upside-down finger, 278range max * 2 should be returned. 279 280Orientation can be omitted if the touch area is circular, or if the 281information is not available in the kernel driver. Partial orientation 282support is possible if the device can distinguish between the two axis, but 283not (uniquely) any values in between. In such cases, the range of 284ABS_MT_ORIENTATION should be [0, 1] [4]. 285 286ABS_MT_POSITION_X 287 288The surface X coordinate of the center of the touching ellipse. 289 290ABS_MT_POSITION_Y 291 292The surface Y coordinate of the center of the touching ellipse. 293 294ABS_MT_TOOL_X 295 296The surface X coordinate of the center of the approaching tool. Omit if 297the device cannot distinguish between the intended touch point and the 298tool itself. 299 300ABS_MT_TOOL_Y 301 302The surface Y coordinate of the center of the approaching tool. Omit if the 303device cannot distinguish between the intended touch point and the tool 304itself. 305 306The four position values can be used to separate the position of the touch 307from the position of the tool. If both positions are present, the major 308tool axis points towards the touch point [1]. Otherwise, the tool axes are 309aligned with the touch axes. 310 311ABS_MT_TOOL_TYPE 312 313The type of approaching tool. A lot of kernel drivers cannot distinguish 314between different tool types, such as a finger or a pen. In such cases, the 315event should be omitted. The protocol currently supports MT_TOOL_FINGER, 316MT_TOOL_PEN, and MT_TOOL_PALM [2]. For type B devices, this event is handled 317by input core; drivers should instead use input_mt_report_slot_state(). 318A contact's ABS_MT_TOOL_TYPE may change over time while still touching the 319device, because the firmware may not be able to determine which tool is being 320used when it first appears. 321 322ABS_MT_BLOB_ID 323 324The BLOB_ID groups several packets together into one arbitrarily shaped 325contact. The sequence of points forms a polygon which defines the shape of 326the contact. This is a low-level anonymous grouping for type A devices, and 327should not be confused with the high-level trackingID [5]. Most type A 328devices do not have blob capability, so drivers can safely omit this event. 329 330ABS_MT_TRACKING_ID 331 332The TRACKING_ID identifies an initiated contact throughout its life cycle 333[5]. The value range of the TRACKING_ID should be large enough to ensure 334unique identification of a contact maintained over an extended period of 335time. For type B devices, this event is handled by input core; drivers 336should instead use input_mt_report_slot_state(). 337 338 339Event Computation 340----------------- 341 342The flora of different hardware unavoidably leads to some devices fitting 343better to the MT protocol than others. To simplify and unify the mapping, 344this section gives recipes for how to compute certain events. 345 346For devices reporting contacts as rectangular shapes, signed orientation 347cannot be obtained. Assuming X and Y are the lengths of the sides of the 348touching rectangle, here is a simple formula that retains the most 349information possible: 350 351 ABS_MT_TOUCH_MAJOR := max(X, Y) 352 ABS_MT_TOUCH_MINOR := min(X, Y) 353 ABS_MT_ORIENTATION := bool(X > Y) 354 355The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that 356the device can distinguish between a finger along the Y axis (0) and a 357finger along the X axis (1). 358 359For win8 devices with both T and C coordinates, the position mapping is 360 361 ABS_MT_POSITION_X := T_X 362 ABS_MT_POSITION_Y := T_Y 363 ABS_MT_TOOL_X := C_X 364 ABS_MT_TOOL_X := C_Y 365 366Unfortunately, there is not enough information to specify both the touching 367ellipse and the tool ellipse, so one has to resort to approximations. One 368simple scheme, which is compatible with earlier usage, is: 369 370 ABS_MT_TOUCH_MAJOR := min(X, Y) 371 ABS_MT_TOUCH_MINOR := <not used> 372 ABS_MT_ORIENTATION := <not used> 373 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C) 374 ABS_MT_WIDTH_MINOR := min(X, Y) 375 376Rationale: We have no information about the orientation of the touching 377ellipse, so approximate it with an inscribed circle instead. The tool 378ellipse should align with the vector (T - C), so the diameter must 379increase with distance(T, C). Finally, assume that the touch diameter is 380equal to the tool thickness, and we arrive at the formulas above. 381 382Finger Tracking 383--------------- 384 385The process of finger tracking, i.e., to assign a unique trackingID to each 386initiated contact on the surface, is a Euclidian Bipartite Matching 387problem. At each event synchronization, the set of actual contacts is 388matched to the set of contacts from the previous synchronization. A full 389implementation can be found in [3]. 390 391 392Gestures 393-------- 394 395In the specific application of creating gesture events, the TOUCH and WIDTH 396parameters can be used to, e.g., approximate finger pressure or distinguish 397between index finger and thumb. With the addition of the MINOR parameters, 398one can also distinguish between a sweeping finger and a pointing finger, 399and with ORIENTATION, one can detect twisting of fingers. 400 401 402Notes 403----- 404 405In order to stay compatible with existing applications, the data reported 406in a finger packet must not be recognized as single-touch events. 407 408For type A devices, all finger data bypasses input filtering, since 409subsequent events of the same type refer to different fingers. 410 411For example usage of the type A protocol, see the bcm5974 driver. For 412example usage of the type B protocol, see the hid-egalax driver. 413 414[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt. 415[2] The list can of course be extended. 416[3] The mtdev project: http://bitmath.org/code/mtdev/. 417[4] See the section on event computation. 418[5] See the section on finger tracking. 419