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 contact, 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 diameter of the inner region is the
172 ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
173 ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
174 against the glass. The inner region will increase, and in general, the
175 ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
176 unity, is related to the contact pressure. For pressure-based devices,
177 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
178 instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
179 indicate the distance between the contact and the surface.
181 In addition to the MAJOR parameters, the oval shape of the contact can be
182 described by adding the MINOR parameters, such that MAJOR and MINOR are the
183 major and minor axis of an ellipse. Finally, the orientation of the oval
184 shape can be describe with the ORIENTATION parameter.
186 For type A devices, further specification of the touch shape is possible
189 The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
190 finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
191 may be used to track identified contacts over time [5].
193 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
194 implicitly handled by input core; drivers should instead call
195 input_mt_report_slot_state().
203 The length of the major axis of the contact. The length should be given in
204 surface units. If the surface has an X times Y resolution, the largest
205 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
209 The length, in surface units, of the minor axis of the contact. If the
210 contact is circular, this event can be omitted [4].
214 The length, in surface units, of the major axis of the approaching
215 tool. This should be understood as the size of the tool itself. The
216 orientation of the contact and the approaching tool are assumed to be the
221 The length, in surface units, of the minor axis of the approaching
222 tool. Omit if circular [4].
224 The above four values can be used to derive additional information about
225 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
226 the notion of pressure. The fingers of the hand and the palm all have
227 different characteristic widths [1].
231 The pressure, in arbitrary units, on the contact area. May be used instead
232 of TOUCH and WIDTH for pressure-based devices or any device with a spatial
233 signal intensity distribution.
237 The distance, in surface units, between the contact and the surface. Zero
238 distance means the contact is touching the surface. A positive number means
239 the contact is hovering above the surface.
243 The orientation of the ellipse. The value should describe a signed quarter
244 of a revolution clockwise around the touch center. The signed value range
245 is arbitrary, but zero should be returned for a finger aligned along the Y
246 axis of the surface, a negative value when finger is turned to the left, and
247 a positive value when finger turned to the right. When completely aligned with
248 the X axis, the range max should be returned. Orientation can be omitted
249 if the touching object is circular, or if the information is not available
250 in the kernel driver. Partial orientation support is possible if the device
251 can distinguish between the two axis, but not (uniquely) any values in
252 between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1]
257 The surface X coordinate of the center of the touching ellipse.
261 The surface Y coordinate of the center of the touching ellipse.
265 The type of approaching tool. A lot of kernel drivers cannot distinguish
266 between different tool types, such as a finger or a pen. In such cases, the
267 event should be omitted. The protocol currently supports MT_TOOL_FINGER and
268 MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
269 drivers should instead use input_mt_report_slot_state().
273 The BLOB_ID groups several packets together into one arbitrarily shaped
274 contact. The sequence of points forms a polygon which defines the shape of
275 the contact. This is a low-level anonymous grouping for type A devices, and
276 should not be confused with the high-level trackingID [5]. Most type A
277 devices do not have blob capability, so drivers can safely omit this event.
281 The TRACKING_ID identifies an initiated contact throughout its life cycle
282 [5]. The value range of the TRACKING_ID should be large enough to ensure
283 unique identification of a contact maintained over an extended period of
284 time. For type B devices, this event is handled by input core; drivers
285 should instead use input_mt_report_slot_state().
291 The flora of different hardware unavoidably leads to some devices fitting
292 better to the MT protocol than others. To simplify and unify the mapping,
293 this section gives recipes for how to compute certain events.
295 For devices reporting contacts as rectangular shapes, signed orientation
296 cannot be obtained. Assuming X and Y are the lengths of the sides of the
297 touching rectangle, here is a simple formula that retains the most
298 information possible:
300 ABS_MT_TOUCH_MAJOR := max(X, Y)
301 ABS_MT_TOUCH_MINOR := min(X, Y)
302 ABS_MT_ORIENTATION := bool(X > Y)
304 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
305 the device can distinguish between a finger along the Y axis (0) and a
306 finger along the X axis (1).
312 The process of finger tracking, i.e., to assign a unique trackingID to each
313 initiated contact on the surface, is a Euclidian Bipartite Matching
314 problem. At each event synchronization, the set of actual contacts is
315 matched to the set of contacts from the previous synchronization. A full
316 implementation can be found in [3].
322 In the specific application of creating gesture events, the TOUCH and WIDTH
323 parameters can be used to, e.g., approximate finger pressure or distinguish
324 between index finger and thumb. With the addition of the MINOR parameters,
325 one can also distinguish between a sweeping finger and a pointing finger,
326 and with ORIENTATION, one can detect twisting of fingers.
332 In order to stay compatible with existing applications, the data reported
333 in a finger packet must not be recognized as single-touch events.
335 For type A devices, all finger data bypasses input filtering, since
336 subsequent events of the same type refer to different fingers.
338 For example usage of the type A protocol, see the bcm5974 driver. For
339 example usage of the type B protocol, see the hid-egalax driver.
341 [1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
342 difference between the contact position and the approaching tool position
343 could be used to derive tilt.
344 [2] The list can of course be extended.
345 [3] The mtdev project: http://bitmath.org/code/mtdev/.
346 [4] See the section on event computation.
347 [5] See the section on finger tracking.
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