linux/Documentation/input/multi-touch-protocol.txt
<<
>>
Prefs
   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 and
 316MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
 317drivers should instead use input_mt_report_slot_state().
 318
 319ABS_MT_BLOB_ID
 320
 321The BLOB_ID groups several packets together into one arbitrarily shaped
 322contact. The sequence of points forms a polygon which defines the shape of
 323the contact. This is a low-level anonymous grouping for type A devices, and
 324should not be confused with the high-level trackingID [5]. Most type A
 325devices do not have blob capability, so drivers can safely omit this event.
 326
 327ABS_MT_TRACKING_ID
 328
 329The TRACKING_ID identifies an initiated contact throughout its life cycle
 330[5]. The value range of the TRACKING_ID should be large enough to ensure
 331unique identification of a contact maintained over an extended period of
 332time. For type B devices, this event is handled by input core; drivers
 333should instead use input_mt_report_slot_state().
 334
 335
 336Event Computation
 337-----------------
 338
 339The flora of different hardware unavoidably leads to some devices fitting
 340better to the MT protocol than others. To simplify and unify the mapping,
 341this section gives recipes for how to compute certain events.
 342
 343For devices reporting contacts as rectangular shapes, signed orientation
 344cannot be obtained. Assuming X and Y are the lengths of the sides of the
 345touching rectangle, here is a simple formula that retains the most
 346information 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
 352The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
 353the device can distinguish between a finger along the Y axis (0) and a
 354finger along the X axis (1).
 355
 356For 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
 363Unfortunately, there is not enough information to specify both the touching
 364ellipse and the tool ellipse, so one has to resort to approximations.  One
 365simple 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
 373Rationale: We have no information about the orientation of the touching
 374ellipse, so approximate it with an inscribed circle instead. The tool
 375ellipse should align with the the vector (T - C), so the diameter must
 376increase with distance(T, C). Finally, assume that the touch diameter is
 377equal to the tool thickness, and we arrive at the formulas above.
 378
 379Finger Tracking
 380---------------
 381
 382The process of finger tracking, i.e., to assign a unique trackingID to each
 383initiated contact on the surface, is a Euclidian Bipartite Matching
 384problem.  At each event synchronization, the set of actual contacts is
 385matched to the set of contacts from the previous synchronization. A full
 386implementation can be found in [3].
 387
 388
 389Gestures
 390--------
 391
 392In the specific application of creating gesture events, the TOUCH and WIDTH
 393parameters can be used to, e.g., approximate finger pressure or distinguish
 394between index finger and thumb. With the addition of the MINOR parameters,
 395one can also distinguish between a sweeping finger and a pointing finger,
 396and with ORIENTATION, one can detect twisting of fingers.
 397
 398
 399Notes
 400-----
 401
 402In order to stay compatible with existing applications, the data reported
 403in a finger packet must not be recognized as single-touch events.
 404
 405For type A devices, all finger data bypasses input filtering, since
 406subsequent events of the same type refer to different fingers.
 407
 408For example usage of the type A protocol, see the bcm5974 driver. For
 409example usage of the type B protocol, see the hid-egalax driver.
 410
 411[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
 412[2] The list can of course be extended.
 413[3] The mtdev project: http://bitmath.org/code/mtdev/.
 414[4] See the section on event computation.
 415[5] See the section on finger tracking.
 416
lxr.linux.no kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.