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1.. include:: <isonum.txt>2 3=========================4Multi-touch (MT) Protocol5=========================6 7:Copyright: |copy| 2009-2010 Henrik Rydberg <rydberg@euromail.se>8 9 10Introduction11------------12 13In order to utilize the full power of the new multi-touch and multi-user14devices, a way to report detailed data from multiple contacts, i.e.,15objects in direct contact with the device surface, is needed. This16document describes the multi-touch (MT) protocol which allows kernel17drivers to report details for an arbitrary number of contacts.18 19The protocol is divided into two types, depending on the capabilities of the20hardware. For devices handling anonymous contacts (type A), the protocol21describes how to send the raw data for all contacts to the receiver. For22devices capable of tracking identifiable contacts (type B), the protocol23describes how to send updates for individual contacts via event slots.24 25.. note::26 MT protocol type A is obsolete, all kernel drivers have been27 converted to use type B.28 29Protocol Usage30--------------31 32Contact details are sent sequentially as separate packets of ABS_MT33events. Only the ABS_MT events are recognized as part of a contact34packet. Since these events are ignored by current single-touch (ST)35applications, the MT protocol can be implemented on top of the ST protocol36in an existing driver.37 38Drivers for type A devices separate contact packets by calling39input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT40event, which instructs the receiver to accept the data for the current41contact and prepare to receive another.42 43Drivers for type B devices separate contact packets by calling44input_mt_slot(), with a slot as argument, at the beginning of each packet.45This generates an ABS_MT_SLOT event, which instructs the receiver to46prepare for updates of the given slot.47 48All drivers mark the end of a multi-touch transfer by calling the usual49input_sync() function. This instructs the receiver to act upon events50accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set51of events/packets.52 53The main difference between the stateless type A protocol and the stateful54type B slot protocol lies in the usage of identifiable contacts to reduce55the amount of data sent to userspace. The slot protocol requires the use of56the ABS_MT_TRACKING_ID, either provided by the hardware or computed from57the raw data [#f5]_.58 59For type A devices, the kernel driver should generate an arbitrary60enumeration of the full set of anonymous contacts currently on the61surface. The order in which the packets appear in the event stream is not62important. Event filtering and finger tracking is left to user space [#f3]_.63 64For type B devices, the kernel driver should associate a slot with each65identified contact, and use that slot to propagate changes for the contact.66Creation, replacement and destruction of contacts is achieved by modifying67the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id68is interpreted as a contact, and the value -1 denotes an unused slot. A69tracking id not previously present is considered new, and a tracking id no70longer present is considered removed. Since only changes are propagated,71the full state of each initiated contact has to reside in the receiving72end. Upon receiving an MT event, one simply updates the appropriate73attribute of the current slot.74 75Some devices identify and/or track more contacts than they can report to the76driver. A driver for such a device should associate one type B slot with each77contact that is reported by the hardware. Whenever the identity of the78contact associated with a slot changes, the driver should invalidate that79slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is80tracking more contacts than it is currently reporting, the driver should use81a BTN_TOOL_*TAP event to inform userspace of the total number of contacts82being tracked by the hardware at that moment. The driver should do this by83explicitly sending the corresponding BTN_TOOL_*TAP event and setting84use_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 slots87by noting that the largest supported BTN_TOOL_*TAP event is larger than the88total 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 A93------------------94 95Here is what a minimal event sequence for a two-contact touch would look96like for a type A device::97 98 ABS_MT_POSITION_X x[0]99 ABS_MT_POSITION_Y y[0]100 SYN_MT_REPORT101 ABS_MT_POSITION_X x[1]102 ABS_MT_POSITION_Y y[1]103 SYN_MT_REPORT104 SYN_REPORT105 106The sequence after moving one of the contacts looks exactly the same; the107raw data for all present contacts are sent between every synchronization108with 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_REPORT115 SYN_REPORT116 117And here is the sequence after lifting the second contact::118 119 SYN_MT_REPORT120 SYN_REPORT121 122If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the123ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the124last SYN_REPORT will be dropped by the input core, resulting in no125zero-contact event reaching userland.126 127 128Protocol Example B129------------------130 131Here is what a minimal event sequence for a two-contact touch would look132like for a type B device::133 134 ABS_MT_SLOT 0135 ABS_MT_TRACKING_ID 45136 ABS_MT_POSITION_X x[0]137 ABS_MT_POSITION_Y y[0]138 ABS_MT_SLOT 1139 ABS_MT_TRACKING_ID 46140 ABS_MT_POSITION_X x[1]141 ABS_MT_POSITION_Y y[1]142 SYN_REPORT143 144Here is the sequence after moving contact 45 in the x direction::145 146 ABS_MT_SLOT 0147 ABS_MT_POSITION_X x[0]148 SYN_REPORT149 150Here is the sequence after lifting the contact in slot 0::151 152 ABS_MT_TRACKING_ID -1153 SYN_REPORT154 155The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The156message removes the association of slot 0 with contact 45, thereby157destroying 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 1162 ABS_MT_TRACKING_ID -1163 SYN_REPORT164 165 166Event Usage167-----------168 169A set of ABS_MT events with the desired properties is defined. The events170are divided into categories, to allow for partial implementation. The171minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which172allows for multiple contacts to be tracked. If the device supports it, the173ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size174of the contact area and approaching tool, respectively.175 176The TOUCH and WIDTH parameters have a geometrical interpretation; imagine177looking through a window at someone gently holding a finger against the178glass. You will see two regions, one inner region consisting of the part179of the finger actually touching the glass, and one outer region formed by180the perimeter of the finger. The center of the touching region (a) is181ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is182ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger183diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger184harder against the glass. The touch region will increase, and in general,185the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller186than unity, is related to the contact pressure. For pressure-based devices,187ABS_MT_PRESSURE may be used to provide the pressure on the contact area188instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to189indicate the distance between the contact and the surface.190 191::192 193 194 Linux MT Win8195 __________ _______________________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 finger214regions can be described by adding the MINOR parameters, such that MAJOR215and MINOR are the major and minor axis of an ellipse. The orientation of216the touch ellipse can be described with the ORIENTATION parameter, and the217direction of the finger ellipse is given by the vector (a - b).218 219For type A devices, further specification of the touch shape is possible220via ABS_MT_BLOB_ID.221 222The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a223finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event224may be used to track identified contacts over time [#f5]_.225 226In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are227implicitly handled by input core; drivers should instead call228input_mt_report_slot_state().229 230 231Event Semantics232---------------233 234ABS_MT_TOUCH_MAJOR235 The length of the major axis of the contact. The length should be given in236 surface units. If the surface has an X times Y resolution, the largest237 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.238 239ABS_MT_TOUCH_MINOR240 The length, in surface units, of the minor axis of the contact. If the241 contact is circular, this event can be omitted [#f4]_.242 243ABS_MT_WIDTH_MAJOR244 The length, in surface units, of the major axis of the approaching245 tool. This should be understood as the size of the tool itself. The246 orientation of the contact and the approaching tool are assumed to be the247 same [#f4]_.248 249ABS_MT_WIDTH_MINOR250 The length, in surface units, of the minor axis of the approaching251 tool. Omit if circular [#f4]_.252 253 The above four values can be used to derive additional information about254 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates255 the notion of pressure. The fingers of the hand and the palm all have256 different characteristic widths.257 258ABS_MT_PRESSURE259 The pressure, in arbitrary units, on the contact area. May be used instead260 of TOUCH and WIDTH for pressure-based devices or any device with a spatial261 signal intensity distribution.262 263 If the resolution is zero, the pressure data is in arbitrary units.264 If the resolution is non-zero, the pressure data is in units/gram. See265 :ref:`input-event-codes` for details.266 267ABS_MT_DISTANCE268 The distance, in surface units, between the contact and the surface. Zero269 distance means the contact is touching the surface. A positive number means270 the contact is hovering above the surface.271 272ABS_MT_ORIENTATION273 The orientation of the touching ellipse. The value should describe a signed274 quarter of a revolution clockwise around the touch center. The signed value275 range is arbitrary, but zero should be returned for an ellipse aligned with276 the Y axis (north) of the surface, a negative value when the ellipse is277 turned to the left, and a positive value when the ellipse is turned to the278 right. When aligned with the X axis in the positive direction, the range279 max should be returned; when aligned with the X axis in the negative280 direction, the range -max should be returned.281 282 Touch ellipses are symmetrical by default. For devices capable of true 360283 degree orientation, the reported orientation must exceed the range max to284 indicate more than a quarter of a revolution. For an upside-down finger,285 range max * 2 should be returned.286 287 Orientation can be omitted if the touch area is circular, or if the288 information is not available in the kernel driver. Partial orientation289 support is possible if the device can distinguish between the two axes, but290 not (uniquely) any values in between. In such cases, the range of291 ABS_MT_ORIENTATION should be [0, 1] [#f4]_.292 293ABS_MT_POSITION_X294 The surface X coordinate of the center of the touching ellipse.295 296ABS_MT_POSITION_Y297 The surface Y coordinate of the center of the touching ellipse.298 299ABS_MT_TOOL_X300 The surface X coordinate of the center of the approaching tool. Omit if301 the device cannot distinguish between the intended touch point and the302 tool itself.303 304ABS_MT_TOOL_Y305 The surface Y coordinate of the center of the approaching tool. Omit if the306 device cannot distinguish between the intended touch point and the tool307 itself.308 309 The four position values can be used to separate the position of the touch310 from the position of the tool. If both positions are present, the major311 tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are312 aligned with the touch axes.313 314ABS_MT_TOOL_TYPE315 The type of approaching tool. A lot of kernel drivers cannot distinguish316 between different tool types, such as a finger or a pen. In such cases, the317 event should be omitted. The protocol currently mainly supports318 MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_.319 For type B devices, this event is handled by input core; drivers should320 instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may321 change over time while still touching the device, because the firmware may322 not be able to determine which tool is being used when it first appears.323 324ABS_MT_BLOB_ID325 The BLOB_ID groups several packets together into one arbitrarily shaped326 contact. The sequence of points forms a polygon which defines the shape of327 the contact. This is a low-level anonymous grouping for type A devices, and328 should not be confused with the high-level trackingID [#f5]_. Most type A329 devices do not have blob capability, so drivers can safely omit this event.330 331ABS_MT_TRACKING_ID332 The TRACKING_ID identifies an initiated contact throughout its life cycle333 [#f5]_. The value range of the TRACKING_ID should be large enough to ensure334 unique identification of a contact maintained over an extended period of335 time. For type B devices, this event is handled by input core; drivers336 should instead use input_mt_report_slot_state().337 338 339Event Computation340-----------------341 342The flora of different hardware unavoidably leads to some devices fitting343better 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 orientation347cannot be obtained. Assuming X and Y are the lengths of the sides of the348touching rectangle, here is a simple formula that retains the most349information 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 that356the device can distinguish between a finger along the Y axis (0) and a357finger 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_X362 ABS_MT_POSITION_Y := T_Y363 ABS_MT_TOOL_X := C_X364 ABS_MT_TOOL_Y := C_Y365 366Unfortunately, there is not enough information to specify both the touching367ellipse and the tool ellipse, so one has to resort to approximations. One368simple 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 touching377ellipse, so approximate it with an inscribed circle instead. The tool378ellipse should align with the vector (T - C), so the diameter must379increase with distance(T, C). Finally, assume that the touch diameter is380equal to the tool thickness, and we arrive at the formulas above.381 382Finger Tracking383---------------384 385The process of finger tracking, i.e., to assign a unique trackingID to each386initiated contact on the surface, is a Euclidean Bipartite Matching387problem. At each event synchronization, the set of actual contacts is388matched to the set of contacts from the previous synchronization. A full389implementation can be found in [#f3]_.390 391 392Gestures393--------394 395In the specific application of creating gesture events, the TOUCH and WIDTH396parameters can be used to, e.g., approximate finger pressure or distinguish397between 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 402Notes403-----404 405In order to stay compatible with existing applications, the data reported406in a finger packet must not be recognized as single-touch events.407 408For type A devices, all finger data bypasses input filtering, since409subsequent events of the same type refer to different fingers.410 411.. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.412.. [#f2] The list can of course be extended.413.. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.414.. [#f4] See the section on event computation.415.. [#f5] See the section on finger tracking.416