1 .. include:: <isonum.txt>
3 =========================
4 Multi-touch (MT) Protocol
5 =========================
7 :Copyright: |copy| 2009-2010 Henrik Rydberg <rydberg@euromail.se>
13 In order to utilize the full power of the new multi-touch and multi-user
14 devices, a way to report detailed data from multiple contacts, i.e.,
15 objects in direct contact with the device surface, is needed. This
16 document describes the multi-touch (MT) protocol which allows kernel
17 drivers to report details for an arbitrary number of contacts.
19 The protocol is divided into two types, depending on the capabilities of the
20 hardware. For devices handling anonymous contacts (type A), the protocol
21 describes how to send the raw data for all contacts to the receiver. For
22 devices capable of tracking identifiable contacts (type B), the protocol
23 describes how to send updates for individual contacts via event slots.
29 Contact details are sent sequentially as separate packets of ABS_MT
30 events. Only the ABS_MT events are recognized as part of a contact
31 packet. Since these events are ignored by current single-touch (ST)
32 applications, the MT protocol can be implemented on top of the ST protocol
33 in an existing driver.
35 Drivers for type A devices separate contact packets by calling
36 input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
37 event, which instructs the receiver to accept the data for the current
38 contact and prepare to receive another.
40 Drivers for type B devices separate contact packets by calling
41 input_mt_slot(), with a slot as argument, at the beginning of each packet.
42 This generates an ABS_MT_SLOT event, which instructs the receiver to
43 prepare for updates of the given slot.
45 All drivers mark the end of a multi-touch transfer by calling the usual
46 input_sync() function. This instructs the receiver to act upon events
47 accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
50 The main difference between the stateless type A protocol and the stateful
51 type B slot protocol lies in the usage of identifiable contacts to reduce
52 the amount of data sent to userspace. The slot protocol requires the use of
53 the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
56 For type A devices, the kernel driver should generate an arbitrary
57 enumeration of the full set of anonymous contacts currently on the
58 surface. The order in which the packets appear in the event stream is not
59 important. Event filtering and finger tracking is left to user space [#f3]_.
61 For type B devices, the kernel driver should associate a slot with each
62 identified contact, and use that slot to propagate changes for the contact.
63 Creation, replacement and destruction of contacts is achieved by modifying
64 the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
65 is interpreted as a contact, and the value -1 denotes an unused slot. A
66 tracking id not previously present is considered new, and a tracking id no
67 longer present is considered removed. Since only changes are propagated,
68 the full state of each initiated contact has to reside in the receiving
69 end. Upon receiving an MT event, one simply updates the appropriate
70 attribute of the current slot.
72 Some devices identify and/or track more contacts than they can report to the
73 driver. A driver for such a device should associate one type B slot with each
74 contact that is reported by the hardware. Whenever the identity of the
75 contact associated with a slot changes, the driver should invalidate that
76 slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
77 tracking more contacts than it is currently reporting, the driver should use
78 a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
79 being tracked by the hardware at that moment. The driver should do this by
80 explicitly sending the corresponding BTN_TOOL_*TAP event and setting
81 use_count to false when calling input_mt_report_pointer_emulation().
82 The driver should only advertise as many slots as the hardware can report.
83 Userspace can detect that a driver can report more total contacts than slots
84 by noting that the largest supported BTN_TOOL_*TAP event is larger than the
85 total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
87 The minimum value of the ABS_MT_SLOT axis must be 0.
92 Here is what a minimal event sequence for a two-contact touch would look
93 like for a type A device::
95 ABS_MT_POSITION_X x[0]
96 ABS_MT_POSITION_Y y[0]
98 ABS_MT_POSITION_X x[1]
99 ABS_MT_POSITION_Y y[1]
103 The sequence after moving one of the contacts looks exactly the same; the
104 raw data for all present contacts are sent between every synchronization
107 Here is the sequence after lifting the first contact::
109 ABS_MT_POSITION_X x[1]
110 ABS_MT_POSITION_Y y[1]
114 And here is the sequence after lifting the second contact::
119 If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
120 ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
121 last SYN_REPORT will be dropped by the input core, resulting in no
122 zero-contact event reaching userland.
128 Here is what a minimal event sequence for a two-contact touch would look
129 like for a type B device::
132 ABS_MT_TRACKING_ID 45
133 ABS_MT_POSITION_X x[0]
134 ABS_MT_POSITION_Y y[0]
136 ABS_MT_TRACKING_ID 46
137 ABS_MT_POSITION_X x[1]
138 ABS_MT_POSITION_Y y[1]
141 Here is the sequence after moving contact 45 in the x direction::
144 ABS_MT_POSITION_X x[0]
147 Here is the sequence after lifting the contact in slot 0::
149 ABS_MT_TRACKING_ID -1
152 The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
153 message removes the association of slot 0 with contact 45, thereby
154 destroying contact 45 and freeing slot 0 to be reused for another contact.
156 Finally, here is the sequence after lifting the second contact::
159 ABS_MT_TRACKING_ID -1
166 A set of ABS_MT events with the desired properties is defined. The events
167 are divided into categories, to allow for partial implementation. The
168 minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
169 allows for multiple contacts to be tracked. If the device supports it, the
170 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
171 of the contact area and approaching tool, respectively.
173 The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
174 looking through a window at someone gently holding a finger against the
175 glass. You will see two regions, one inner region consisting of the part
176 of the finger actually touching the glass, and one outer region formed by
177 the perimeter of the finger. The center of the touching region (a) is
178 ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
179 ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
180 diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
181 harder against the glass. The touch region will increase, and in general,
182 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
183 than unity, is related to the contact pressure. For pressure-based devices,
184 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
185 instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
186 indicate the distance between the contact and the surface.
192 __________ _______________________
207 \__________/ |_______________________|
210 In addition to the MAJOR parameters, the oval shape of the touch and finger
211 regions can be described by adding the MINOR parameters, such that MAJOR
212 and MINOR are the major and minor axis of an ellipse. The orientation of
213 the touch ellipse can be described with the ORIENTATION parameter, and the
214 direction of the finger ellipse is given by the vector (a - b).
216 For type A devices, further specification of the touch shape is possible
219 The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
220 finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
221 may be used to track identified contacts over time [#f5]_.
223 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
224 implicitly handled by input core; drivers should instead call
225 input_mt_report_slot_state().
232 The length of the major axis of the contact. The length should be given in
233 surface units. If the surface has an X times Y resolution, the largest
234 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.
237 The length, in surface units, of the minor axis of the contact. If the
238 contact is circular, this event can be omitted [#f4]_.
241 The length, in surface units, of the major axis of the approaching
242 tool. This should be understood as the size of the tool itself. The
243 orientation of the contact and the approaching tool are assumed to be the
247 The length, in surface units, of the minor axis of the approaching
248 tool. Omit if circular [#f4]_.
250 The above four values can be used to derive additional information about
251 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
252 the notion of pressure. The fingers of the hand and the palm all have
253 different characteristic widths.
256 The pressure, in arbitrary units, on the contact area. May be used instead
257 of TOUCH and WIDTH for pressure-based devices or any device with a spatial
258 signal intensity distribution.
261 The distance, in surface units, between the contact and the surface. Zero
262 distance means the contact is touching the surface. A positive number means
263 the contact is hovering above the surface.
266 The orientation of the touching ellipse. The value should describe a signed
267 quarter of a revolution clockwise around the touch center. The signed value
268 range is arbitrary, but zero should be returned for an ellipse aligned with
269 the Y axis of the surface, a negative value when the ellipse is turned to
270 the left, and a positive value when the ellipse is turned to the
271 right. When completely aligned with the X axis, the range max should be
274 Touch ellipsis are symmetrical by default. For devices capable of true 360
275 degree orientation, the reported orientation must exceed the range max to
276 indicate more than a quarter of a revolution. For an upside-down finger,
277 range max * 2 should be returned.
279 Orientation can be omitted if the touch area is circular, or if the
280 information is not available in the kernel driver. Partial orientation
281 support is possible if the device can distinguish between the two axis, but
282 not (uniquely) any values in between. In such cases, the range of
283 ABS_MT_ORIENTATION should be [0, 1] [#f4]_.
286 The surface X coordinate of the center of the touching ellipse.
289 The surface Y coordinate of the center of the touching ellipse.
292 The surface X coordinate of the center of the approaching tool. Omit if
293 the device cannot distinguish between the intended touch point and the
297 The surface Y coordinate of the center of the approaching tool. Omit if the
298 device cannot distinguish between the intended touch point and the tool
301 The four position values can be used to separate the position of the touch
302 from the position of the tool. If both positions are present, the major
303 tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
304 aligned with the touch axes.
307 The type of approaching tool. A lot of kernel drivers cannot distinguish
308 between different tool types, such as a finger or a pen. In such cases, the
309 event should be omitted. The protocol currently supports MT_TOOL_FINGER,
310 MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_. For type B devices, this event is
311 handled by input core; drivers should instead use
312 input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may change over
313 time while still touching the device, because the firmware may not be able
314 to determine which tool is being used when it first appears.
317 The BLOB_ID groups several packets together into one arbitrarily shaped
318 contact. The sequence of points forms a polygon which defines the shape of
319 the contact. This is a low-level anonymous grouping for type A devices, and
320 should not be confused with the high-level trackingID [#f5]_. Most type A
321 devices do not have blob capability, so drivers can safely omit this event.
324 The TRACKING_ID identifies an initiated contact throughout its life cycle
325 [#f5]_. The value range of the TRACKING_ID should be large enough to ensure
326 unique identification of a contact maintained over an extended period of
327 time. For type B devices, this event is handled by input core; drivers
328 should instead use input_mt_report_slot_state().
334 The flora of different hardware unavoidably leads to some devices fitting
335 better to the MT protocol than others. To simplify and unify the mapping,
336 this section gives recipes for how to compute certain events.
338 For devices reporting contacts as rectangular shapes, signed orientation
339 cannot be obtained. Assuming X and Y are the lengths of the sides of the
340 touching rectangle, here is a simple formula that retains the most
341 information possible::
343 ABS_MT_TOUCH_MAJOR := max(X, Y)
344 ABS_MT_TOUCH_MINOR := min(X, Y)
345 ABS_MT_ORIENTATION := bool(X > Y)
347 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
348 the device can distinguish between a finger along the Y axis (0) and a
349 finger along the X axis (1).
351 For win8 devices with both T and C coordinates, the position mapping is::
353 ABS_MT_POSITION_X := T_X
354 ABS_MT_POSITION_Y := T_Y
358 Unfortunately, there is not enough information to specify both the touching
359 ellipse and the tool ellipse, so one has to resort to approximations. One
360 simple scheme, which is compatible with earlier usage, is::
362 ABS_MT_TOUCH_MAJOR := min(X, Y)
363 ABS_MT_TOUCH_MINOR := <not used>
364 ABS_MT_ORIENTATION := <not used>
365 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
366 ABS_MT_WIDTH_MINOR := min(X, Y)
368 Rationale: We have no information about the orientation of the touching
369 ellipse, so approximate it with an inscribed circle instead. The tool
370 ellipse should align with the vector (T - C), so the diameter must
371 increase with distance(T, C). Finally, assume that the touch diameter is
372 equal to the tool thickness, and we arrive at the formulas above.
377 The process of finger tracking, i.e., to assign a unique trackingID to each
378 initiated contact on the surface, is a Euclidian Bipartite Matching
379 problem. At each event synchronization, the set of actual contacts is
380 matched to the set of contacts from the previous synchronization. A full
381 implementation can be found in [#f3]_.
387 In the specific application of creating gesture events, the TOUCH and WIDTH
388 parameters can be used to, e.g., approximate finger pressure or distinguish
389 between index finger and thumb. With the addition of the MINOR parameters,
390 one can also distinguish between a sweeping finger and a pointing finger,
391 and with ORIENTATION, one can detect twisting of fingers.
397 In order to stay compatible with existing applications, the data reported
398 in a finger packet must not be recognized as single-touch events.
400 For type A devices, all finger data bypasses input filtering, since
401 subsequent events of the same type refer to different fingers.
403 For example usage of the type A protocol, see the bcm5974 driver. For
404 example usage of the type B protocol, see the hid-egalax driver.
406 .. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
407 .. [#f2] The list can of course be extended.
408 .. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
409 .. [#f4] See the section on event computation.
410 .. [#f5] See the section on finger tracking.