linux/Documentation/input/multi-touch-protocol.txt
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   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
  83Protocol Example A
  84------------------
  85
  86Here is what a minimal event sequence for a two-contact touch would look
  87like for a type A device:
  88
  89   ABS_MT_POSITION_X x[0]
  90   ABS_MT_POSITION_Y y[0]
  91   SYN_MT_REPORT
  92   ABS_MT_POSITION_X x[1]
  93   ABS_MT_POSITION_Y y[1]
  94   SYN_MT_REPORT
  95   SYN_REPORT
  96
  97The sequence after moving one of the contacts looks exactly the same; the
  98raw data for all present contacts are sent between every synchronization
  99with SYN_REPORT.
 100
 101Here is the sequence after lifting the first contact:
 102
 103   ABS_MT_POSITION_X x[1]
 104   ABS_MT_POSITION_Y y[1]
 105   SYN_MT_REPORT
 106   SYN_REPORT
 107
 108And here is the sequence after lifting the second contact:
 109
 110   SYN_MT_REPORT
 111   SYN_REPORT
 112
 113If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
 114ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
 115last SYN_REPORT will be dropped by the input core, resulting in no
 116zero-contact event reaching userland.
 117
 118
 119Protocol Example B
 120------------------
 121
 122Here is what a minimal event sequence for a two-contact touch would look
 123like for a type B device:
 124
 125   ABS_MT_SLOT 0
 126   ABS_MT_TRACKING_ID 45
 127   ABS_MT_POSITION_X x[0]
 128   ABS_MT_POSITION_Y y[0]
 129   ABS_MT_SLOT 1
 130   ABS_MT_TRACKING_ID 46
 131   ABS_MT_POSITION_X x[1]
 132   ABS_MT_POSITION_Y y[1]
 133   SYN_REPORT
 134
 135Here is the sequence after moving contact 45 in the x direction:
 136
 137   ABS_MT_SLOT 0
 138   ABS_MT_POSITION_X x[0]
 139   SYN_REPORT
 140
 141Here is the sequence after lifting the contact in slot 0:
 142
 143   ABS_MT_TRACKING_ID -1
 144   SYN_REPORT
 145
 146The slot being modified is already 0, so the ABS_MT_SLOT is omitted.  The
 147message removes the association of slot 0 with contact 45, thereby
 148destroying contact 45 and freeing slot 0 to be reused for another contact.
 149
 150Finally, here is the sequence after lifting the second contact:
 151
 152   ABS_MT_SLOT 1
 153   ABS_MT_TRACKING_ID -1
 154   SYN_REPORT
 155
 156
 157Event Usage
 158-----------
 159
 160A set of ABS_MT events with the desired properties is defined. The events
 161are divided into categories, to allow for partial implementation.  The
 162minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
 163allows for multiple contacts to be tracked.  If the device supports it, the
 164ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
 165of the contact area and approaching tool, respectively.
 166
 167The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
 168looking through a window at someone gently holding a finger against the
 169glass.  You will see two regions, one inner region consisting of the part
 170of the finger actually touching the glass, and one outer region formed by
 171the perimeter of the finger. The center of the touching region (a) is
 172ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
 173ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
 174diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
 175harder against the glass. The touch region will increase, and in general,
 176the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
 177than unity, is related to the contact pressure. For pressure-based devices,
 178ABS_MT_PRESSURE may be used to provide the pressure on the contact area
 179instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
 180indicate the distance between the contact and the surface.
 181
 182
 183          Linux MT                               Win8
 184         __________                     _______________________
 185        /          \                   |                       |
 186       /            \                  |                       |
 187      /     ____     \                 |                       |
 188     /     /    \     \                |                       |
 189     \     \  a  \     \               |       a               |
 190      \     \____/      \              |                       |
 191       \                 \             |                       |
 192        \        b        \            |           b           |
 193         \                 \           |                       |
 194          \                 \          |                       |
 195           \                 \         |                       |
 196            \                /         |                       |
 197             \              /          |                       |
 198              \            /           |                       |
 199               \__________/            |_______________________|
 200
 201
 202In addition to the MAJOR parameters, the oval shape of the touch and finger
 203regions can be described by adding the MINOR parameters, such that MAJOR
 204and MINOR are the major and minor axis of an ellipse. The orientation of
 205the touch ellipse can be described with the ORIENTATION parameter, and the
 206direction of the finger ellipse is given by the vector (a - b).
 207
 208For type A devices, further specification of the touch shape is possible
 209via ABS_MT_BLOB_ID.
 210
 211The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
 212finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
 213may be used to track identified contacts over time [5].
 214
 215In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
 216implicitly handled by input core; drivers should instead call
 217input_mt_report_slot_state().
 218
 219
 220Event Semantics
 221---------------
 222
 223ABS_MT_TOUCH_MAJOR
 224
 225The length of the major axis of the contact. The length should be given in
 226surface units. If the surface has an X times Y resolution, the largest
 227possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
 228
 229ABS_MT_TOUCH_MINOR
 230
 231The length, in surface units, of the minor axis of the contact. If the
 232contact is circular, this event can be omitted [4].
 233
 234ABS_MT_WIDTH_MAJOR
 235
 236The length, in surface units, of the major axis of the approaching
 237tool. This should be understood as the size of the tool itself. The
 238orientation of the contact and the approaching tool are assumed to be the
 239same [4].
 240
 241ABS_MT_WIDTH_MINOR
 242
 243The length, in surface units, of the minor axis of the approaching
 244tool. Omit if circular [4].
 245
 246The above four values can be used to derive additional information about
 247the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
 248the notion of pressure. The fingers of the hand and the palm all have
 249different characteristic widths.
 250
 251ABS_MT_PRESSURE
 252
 253The pressure, in arbitrary units, on the contact area. May be used instead
 254of TOUCH and WIDTH for pressure-based devices or any device with a spatial
 255signal intensity distribution.
 256
 257ABS_MT_DISTANCE
 258
 259The distance, in surface units, between the contact and the surface. Zero
 260distance means the contact is touching the surface. A positive number means
 261the contact is hovering above the surface.
 262
 263ABS_MT_ORIENTATION
 264
 265The orientation of the touching ellipse. The value should describe a signed
 266quarter of a revolution clockwise around the touch center. The signed value
 267range is arbitrary, but zero should be returned for an ellipse aligned with
 268the Y axis of the surface, a negative value when the ellipse is turned to
 269the left, and a positive value when the ellipse is turned to the
 270right. When completely aligned with the X axis, the range max should be
 271returned.
 272
 273Touch ellipsis are symmetrical by default. For devices capable of true 360
 274degree orientation, the reported orientation must exceed the range max to
 275indicate more than a quarter of a revolution. For an upside-down finger,
 276range max * 2 should be returned.
 277
 278Orientation can be omitted if the touch area is circular, or if the
 279information is not available in the kernel driver. Partial orientation
 280support is possible if the device can distinguish between the two axis, but
 281not (uniquely) any values in between. In such cases, the range of
 282ABS_MT_ORIENTATION should be [0, 1] [4].
 283
 284ABS_MT_POSITION_X
 285
 286The surface X coordinate of the center of the touching ellipse.
 287
 288ABS_MT_POSITION_Y
 289
 290The surface Y coordinate of the center of the touching ellipse.
 291
 292ABS_MT_TOOL_X
 293
 294The surface X coordinate of the center of the approaching tool. Omit if
 295the device cannot distinguish between the intended touch point and the
 296tool itself.
 297
 298ABS_MT_TOOL_Y
 299
 300The surface Y coordinate of the center of the approaching tool. Omit if the
 301device cannot distinguish between the intended touch point and the tool
 302itself.
 303
 304The four position values can be used to separate the position of the touch
 305from the position of the tool. If both positions are present, the major
 306tool axis points towards the touch point [1]. Otherwise, the tool axes are
 307aligned with the touch axes.
 308
 309ABS_MT_TOOL_TYPE
 310
 311The type of approaching tool. A lot of kernel drivers cannot distinguish
 312between different tool types, such as a finger or a pen. In such cases, the
 313event should be omitted. The protocol currently supports MT_TOOL_FINGER and
 314MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
 315drivers should instead use input_mt_report_slot_state().
 316
 317ABS_MT_BLOB_ID
 318
 319The BLOB_ID groups several packets together into one arbitrarily shaped
 320contact. The sequence of points forms a polygon which defines the shape of
 321the contact. This is a low-level anonymous grouping for type A devices, and
 322should not be confused with the high-level trackingID [5]. Most type A
 323devices do not have blob capability, so drivers can safely omit this event.
 324
 325ABS_MT_TRACKING_ID
 326
 327The TRACKING_ID identifies an initiated contact throughout its life cycle
 328[5]. The value range of the TRACKING_ID should be large enough to ensure
 329unique identification of a contact maintained over an extended period of
 330time. For type B devices, this event is handled by input core; drivers
 331should instead use input_mt_report_slot_state().
 332
 333
 334Event Computation
 335-----------------
 336
 337The flora of different hardware unavoidably leads to some devices fitting
 338better to the MT protocol than others. To simplify and unify the mapping,
 339this section gives recipes for how to compute certain events.
 340
 341For devices reporting contacts as rectangular shapes, signed orientation
 342cannot be obtained. Assuming X and Y are the lengths of the sides of the
 343touching rectangle, here is a simple formula that retains the most
 344information possible:
 345
 346   ABS_MT_TOUCH_MAJOR := max(X, Y)
 347   ABS_MT_TOUCH_MINOR := min(X, Y)
 348   ABS_MT_ORIENTATION := bool(X > Y)
 349
 350The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
 351the device can distinguish between a finger along the Y axis (0) and a
 352finger along the X axis (1).
 353
 354For win8 devices with both T and C coordinates, the position mapping is
 355
 356   ABS_MT_POSITION_X := T_X
 357   ABS_MT_POSITION_Y := T_Y
 358   ABS_MT_TOOL_X := C_X
 359   ABS_MT_TOOL_X := C_Y
 360
 361Unfortunately, there is not enough information to specify both the touching
 362ellipse and the tool ellipse, so one has to resort to approximations.  One
 363simple scheme, which is compatible with earlier usage, is:
 364
 365   ABS_MT_TOUCH_MAJOR := min(X, Y)
 366   ABS_MT_TOUCH_MINOR := <not used>
 367   ABS_MT_ORIENTATION := <not used>
 368   ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
 369   ABS_MT_WIDTH_MINOR := min(X, Y)
 370
 371Rationale: We have no information about the orientation of the touching
 372ellipse, so approximate it with an inscribed circle instead. The tool
 373ellipse should align with the the vector (T - C), so the diameter must
 374increase with distance(T, C). Finally, assume that the touch diameter is
 375equal to the tool thickness, and we arrive at the formulas above.
 376
 377Finger Tracking
 378---------------
 379
 380The process of finger tracking, i.e., to assign a unique trackingID to each
 381initiated contact on the surface, is a Euclidian Bipartite Matching
 382problem.  At each event synchronization, the set of actual contacts is
 383matched to the set of contacts from the previous synchronization. A full
 384implementation can be found in [3].
 385
 386
 387Gestures
 388--------
 389
 390In the specific application of creating gesture events, the TOUCH and WIDTH
 391parameters can be used to, e.g., approximate finger pressure or distinguish
 392between index finger and thumb. With the addition of the MINOR parameters,
 393one can also distinguish between a sweeping finger and a pointing finger,
 394and with ORIENTATION, one can detect twisting of fingers.
 395
 396
 397Notes
 398-----
 399
 400In order to stay compatible with existing applications, the data reported
 401in a finger packet must not be recognized as single-touch events.
 402
 403For type A devices, all finger data bypasses input filtering, since
 404subsequent events of the same type refer to different fingers.
 405
 406For example usage of the type A protocol, see the bcm5974 driver. For
 407example usage of the type B protocol, see the hid-egalax driver.
 408
 409[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
 410[2] The list can of course be extended.
 411[3] The mtdev project: http://bitmath.org/code/mtdev/.
 412[4] See the section on event computation.
 413[5] See the section on finger tracking.
 414
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