1 Multi-touch (MT) Protocol
2 -------------------------
3 Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se>
9 In order to utilize the full power of the new multi-touch and multi-user
10 devices, a way to report detailed data from multiple contacts, i.e.,
11 objects in direct contact with the device surface, is needed. This
12 document describes the multi-touch (MT) protocol which allows kernel
13 drivers to report details for an arbitrary number of contacts.
15 The protocol is divided into two types, depending on the capabilities of the
16 hardware. For devices handling anonymous contacts (type A), the protocol
17 describes how to send the raw data for all contacts to the receiver. For
18 devices capable of tracking identifiable contacts (type B), the protocol
19 describes how to send updates for individual contacts via event slots.
25 Contact details are sent sequentially as separate packets of ABS_MT
26 events. Only the ABS_MT events are recognized as part of a contact
27 packet. Since these events are ignored by current single-touch (ST)
28 applications, the MT protocol can be implemented on top of the ST protocol
29 in an existing driver.
31 Drivers for type A devices separate contact packets by calling
32 input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
33 event, which instructs the receiver to accept the data for the current
34 contact and prepare to receive another.
36 Drivers for type B devices separate contact packets by calling
37 input_mt_slot(), with a slot as argument, at the beginning of each packet.
38 This generates an ABS_MT_SLOT event, which instructs the receiver to
39 prepare for updates of the given slot.
41 All drivers mark the end of a multi-touch transfer by calling the usual
42 input_sync() function. This instructs the receiver to act upon events
43 accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
46 The main difference between the stateless type A protocol and the stateful
47 type B slot protocol lies in the usage of identifiable contacts to reduce
48 the amount of data sent to userspace. The slot protocol requires the use of
49 the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
52 For type A devices, the kernel driver should generate an arbitrary
53 enumeration of the full set of anonymous contacts currently on the
54 surface. The order in which the packets appear in the event stream is not
55 important. Event filtering and finger tracking is left to user space [3].
57 For type B devices, the kernel driver should associate a slot with each
58 identified contact, and use that slot to propagate changes for the contact.
59 Creation, replacement and destruction of contacts is achieved by modifying
60 the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
61 is interpreted as a contact, and the value -1 denotes an unused slot. A
62 tracking id not previously present is considered new, and a tracking id no
63 longer present is considered removed. Since only changes are propagated,
64 the full state of each initiated contact has to reside in the receiving
65 end. Upon receiving an MT event, one simply updates the appropriate
66 attribute of the current slot.
68 Some devices identify and/or track more contacts than they can report to the
69 driver. A driver for such a device should associate one type B slot with each
70 contact that is reported by the hardware. Whenever the identity of the
71 contact associated with a slot changes, the driver should invalidate that
72 slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
73 tracking more contacts than it is currently reporting, the driver should use
74 a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
75 being tracked by the hardware at that moment. The driver should do this by
76 explicitly sending the corresponding BTN_TOOL_*TAP event and setting
77 use_count to false when calling input_mt_report_pointer_emulation().
78 The driver should only advertise as many slots as the hardware can report.
79 Userspace can detect that a driver can report more total contacts than slots
80 by noting that the largest supported BTN_TOOL_*TAP event is larger than the
81 total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
86 Here is what a minimal event sequence for a two-contact touch would look
87 like for a type A device:
89 ABS_MT_POSITION_X x[0]
90 ABS_MT_POSITION_Y y[0]
92 ABS_MT_POSITION_X x[1]
93 ABS_MT_POSITION_Y y[1]
97 The sequence after moving one of the contacts looks exactly the same; the
98 raw data for all present contacts are sent between every synchronization
101 Here is the sequence after lifting the first contact:
103 ABS_MT_POSITION_X x[1]
104 ABS_MT_POSITION_Y y[1]
108 And here is the sequence after lifting the second contact:
113 If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
114 ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
115 last SYN_REPORT will be dropped by the input core, resulting in no
116 zero-contact event reaching userland.
122 Here is what a minimal event sequence for a two-contact touch would look
123 like for a type B device:
126 ABS_MT_TRACKING_ID 45
127 ABS_MT_POSITION_X x[0]
128 ABS_MT_POSITION_Y y[0]
130 ABS_MT_TRACKING_ID 46
131 ABS_MT_POSITION_X x[1]
132 ABS_MT_POSITION_Y y[1]
135 Here is the sequence after moving contact 45 in the x direction:
138 ABS_MT_POSITION_X x[0]
141 Here is the sequence after lifting the contact in slot 0:
143 ABS_MT_TRACKING_ID -1
146 The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
147 message removes the association of slot 0 with contact 45, thereby
148 destroying contact 45 and freeing slot 0 to be reused for another contact.
150 Finally, here is the sequence after lifting the second contact:
153 ABS_MT_TRACKING_ID -1
160 A set of ABS_MT events with the desired properties is defined. The events
161 are divided into categories, to allow for partial implementation. The
162 minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
163 allows for multiple contacts to be tracked. If the device supports it, the
164 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
165 of the contact area and approaching tool, respectively.
167 The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
168 looking through a window at someone gently holding a finger against the
169 glass. You will see two regions, one inner region consisting of the part
170 of the finger actually touching the glass, and one outer region formed by
171 the perimeter of the finger. The center of the touching region (a) is
172 ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
173 ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
174 diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
175 harder against the glass. The touch region will increase, and in general,
176 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
177 than unity, is related to the contact pressure. For pressure-based devices,
178 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
179 instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
180 indicate the distance between the contact and the surface.
184 __________ _______________________
199 \__________/ |_______________________|
202 In addition to the MAJOR parameters, the oval shape of the touch and finger
203 regions can be described by adding the MINOR parameters, such that MAJOR
204 and MINOR are the major and minor axis of an ellipse. The orientation of
205 the touch ellipse can be described with the ORIENTATION parameter, and the
206 direction of the finger ellipse is given by the vector (a - b).
208 For type A devices, further specification of the touch shape is possible
211 The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
212 finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
213 may be used to track identified contacts over time [5].
215 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
216 implicitly handled by input core; drivers should instead call
217 input_mt_report_slot_state().
225 The length of the major axis of the contact. The length should be given in
226 surface units. If the surface has an X times Y resolution, the largest
227 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
231 The length, in surface units, of the minor axis of the contact. If the
232 contact is circular, this event can be omitted [4].
236 The length, in surface units, of the major axis of the approaching
237 tool. This should be understood as the size of the tool itself. The
238 orientation of the contact and the approaching tool are assumed to be the
243 The length, in surface units, of the minor axis of the approaching
244 tool. Omit if circular [4].
246 The above four values can be used to derive additional information about
247 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
248 the notion of pressure. The fingers of the hand and the palm all have
249 different characteristic widths.
253 The pressure, in arbitrary units, on the contact area. May be used instead
254 of TOUCH and WIDTH for pressure-based devices or any device with a spatial
255 signal intensity distribution.
259 The distance, in surface units, between the contact and the surface. Zero
260 distance means the contact is touching the surface. A positive number means
261 the contact is hovering above the surface.
265 The orientation of the touching ellipse. The value should describe a signed
266 quarter of a revolution clockwise around the touch center. The signed value
267 range is arbitrary, but zero should be returned for an ellipse aligned with
268 the Y axis of the surface, a negative value when the ellipse is turned to
269 the left, and a positive value when the ellipse is turned to the
270 right. When completely aligned with the X axis, the range max should be
273 Touch ellipsis are symmetrical by default. For devices capable of true 360
274 degree orientation, the reported orientation must exceed the range max to
275 indicate more than a quarter of a revolution. For an upside-down finger,
276 range max * 2 should be returned.
278 Orientation can be omitted if the touch area is circular, or if the
279 information is not available in the kernel driver. Partial orientation
280 support is possible if the device can distinguish between the two axis, but
281 not (uniquely) any values in between. In such cases, the range of
282 ABS_MT_ORIENTATION should be [0, 1] [4].
286 The surface X coordinate of the center of the touching ellipse.
290 The surface Y coordinate of the center of the touching ellipse.
294 The surface X coordinate of the center of the approaching tool. Omit if
295 the device cannot distinguish between the intended touch point and the
300 The surface Y coordinate of the center of the approaching tool. Omit if the
301 device cannot distinguish between the intended touch point and the tool
304 The four position values can be used to separate the position of the touch
305 from the position of the tool. If both positions are present, the major
306 tool axis points towards the touch point [1]. Otherwise, the tool axes are
307 aligned with the touch axes.
311 The type of approaching tool. A lot of kernel drivers cannot distinguish
312 between different tool types, such as a finger or a pen. In such cases, the
313 event should be omitted. The protocol currently supports MT_TOOL_FINGER and
314 MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
315 drivers should instead use input_mt_report_slot_state().
319 The BLOB_ID groups several packets together into one arbitrarily shaped
320 contact. The sequence of points forms a polygon which defines the shape of
321 the contact. This is a low-level anonymous grouping for type A devices, and
322 should not be confused with the high-level trackingID [5]. Most type A
323 devices do not have blob capability, so drivers can safely omit this event.
327 The 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
329 unique identification of a contact maintained over an extended period of
330 time. For type B devices, this event is handled by input core; drivers
331 should instead use input_mt_report_slot_state().
337 The flora of different hardware unavoidably leads to some devices fitting
338 better to the MT protocol than others. To simplify and unify the mapping,
339 this section gives recipes for how to compute certain events.
341 For devices reporting contacts as rectangular shapes, signed orientation
342 cannot be obtained. Assuming X and Y are the lengths of the sides of the
343 touching rectangle, here is a simple formula that retains the most
344 information possible:
346 ABS_MT_TOUCH_MAJOR := max(X, Y)
347 ABS_MT_TOUCH_MINOR := min(X, Y)
348 ABS_MT_ORIENTATION := bool(X > Y)
350 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
351 the device can distinguish between a finger along the Y axis (0) and a
352 finger along the X axis (1).
354 For win8 devices with both T and C coordinates, the position mapping is
356 ABS_MT_POSITION_X := T_X
357 ABS_MT_POSITION_Y := T_Y
361 Unfortunately, there is not enough information to specify both the touching
362 ellipse and the tool ellipse, so one has to resort to approximations. One
363 simple scheme, which is compatible with earlier usage, is:
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)
371 Rationale: We have no information about the orientation of the touching
372 ellipse, so approximate it with an inscribed circle instead. The tool
373 ellipse should align with the the vector (T - C), so the diameter must
374 increase with distance(T, C). Finally, assume that the touch diameter is
375 equal to the tool thickness, and we arrive at the formulas above.
380 The process of finger tracking, i.e., to assign a unique trackingID to each
381 initiated contact on the surface, is a Euclidian Bipartite Matching
382 problem. At each event synchronization, the set of actual contacts is
383 matched to the set of contacts from the previous synchronization. A full
384 implementation can be found in [3].
390 In the specific application of creating gesture events, the TOUCH and WIDTH
391 parameters can be used to, e.g., approximate finger pressure or distinguish
392 between index finger and thumb. With the addition of the MINOR parameters,
393 one can also distinguish between a sweeping finger and a pointing finger,
394 and with ORIENTATION, one can detect twisting of fingers.
400 In order to stay compatible with existing applications, the data reported
401 in a finger packet must not be recognized as single-touch events.
403 For type A devices, all finger data bypasses input filtering, since
404 subsequent events of the same type refer to different fingers.
406 For example usage of the type A protocol, see the bcm5974 driver. For
407 example usage of the type B protocol, see the hid-egalax driver.
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.
projects / firefly-linux-kernel-4.4.55.git / blob