1 Multi-touch (MT) Protocol
2 -------------------------
3 Copyright (C) 2009 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.
72 Here is what a minimal event sequence for a two-contact touch would look
73 like for a type A device:
75 ABS_MT_POSITION_X x[0]
76 ABS_MT_POSITION_Y y[0]
78 ABS_MT_POSITION_X x[1]
79 ABS_MT_POSITION_Y y[1]
83 The sequence after moving one of the contacts looks exactly the same; the
84 raw data for all present contacts are sent between every synchronization
87 Here is the sequence after lifting the first contact:
89 ABS_MT_POSITION_X x[1]
90 ABS_MT_POSITION_Y y[1]
94 And here is the sequence after lifting the second contact:
99 If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
100 ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
101 last SYN_REPORT will be dropped by the input core, resulting in no
102 zero-contact event reaching userland.
108 Here is what a minimal event sequence for a two-contact touch would look
109 like for a type B device:
112 ABS_MT_TRACKING_ID 45
113 ABS_MT_POSITION_X x[0]
114 ABS_MT_POSITION_Y y[0]
116 ABS_MT_TRACKING_ID 46
117 ABS_MT_POSITION_X x[1]
118 ABS_MT_POSITION_Y y[1]
121 Here is the sequence after moving contact 45 in the x direction:
124 ABS_MT_POSITION_X x[0]
127 Here is the sequence after lifting the contact in slot 0:
129 ABS_MT_TRACKING_ID -1
132 The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
133 message removes the association of slot 0 with contact 45, thereby
134 destroying contact 45 and freeing slot 0 to be reused for another contact.
136 Finally, here is the sequence after lifting the second contact:
139 ABS_MT_TRACKING_ID -1
146 A set of ABS_MT events with the desired properties is defined. The events
147 are divided into categories, to allow for partial implementation. The
148 minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
149 allows for multiple contacts to be tracked. If the device supports it, the
150 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
151 of the contact area and approaching contact, respectively.
153 The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
154 looking through a window at someone gently holding a finger against the
155 glass. You will see two regions, one inner region consisting of the part
156 of the finger actually touching the glass, and one outer region formed by
157 the perimeter of the finger. The diameter of the inner region is the
158 ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
159 ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
160 against the glass. The inner region will increase, and in general, the
161 ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
162 unity, is related to the contact pressure. For pressure-based devices,
163 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
164 instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
165 indicate the distance between the contact and the surface.
167 In addition to the MAJOR parameters, the oval shape of the contact can be
168 described by adding the MINOR parameters, such that MAJOR and MINOR are the
169 major and minor axis of an ellipse. Finally, the orientation of the oval
170 shape can be describe with the ORIENTATION parameter.
172 The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
173 contact or a pen or something else. Devices with more granular information
174 may specify general shapes as blobs, i.e., as a sequence of rectangular
175 shapes grouped together by an ABS_MT_BLOB_ID. Finally, for the few devices
176 that currently support it, the ABS_MT_TRACKING_ID event may be used to
177 report contact tracking from hardware [5].
185 The length of the major axis of the contact. The length should be given in
186 surface units. If the surface has an X times Y resolution, the largest
187 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
191 The length, in surface units, of the minor axis of the contact. If the
192 contact is circular, this event can be omitted [4].
196 The length, in surface units, of the major axis of the approaching
197 tool. This should be understood as the size of the tool itself. The
198 orientation of the contact and the approaching tool are assumed to be the
203 The length, in surface units, of the minor axis of the approaching
204 tool. Omit if circular [4].
206 The above four values can be used to derive additional information about
207 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
208 the notion of pressure. The fingers of the hand and the palm all have
209 different characteristic widths [1].
213 The pressure, in arbitrary units, on the contact area. May be used instead
214 of TOUCH and WIDTH for pressure-based devices or any device with a spatial
215 signal intensity distribution.
219 The distance, in surface units, between the contact and the surface. Zero
220 distance means the contact is touching the surface. A positive number means
221 the contact is hovering above the surface.
225 The orientation of the ellipse. The value should describe a signed quarter
226 of a revolution clockwise around the touch center. The signed value range
227 is arbitrary, but zero should be returned for a finger aligned along the Y
228 axis of the surface, a negative value when finger is turned to the left, and
229 a positive value when finger turned to the right. When completely aligned with
230 the X axis, the range max should be returned. Orientation can be omitted
231 if the touching object is circular, or if the information is not available
232 in the kernel driver. Partial orientation support is possible if the device
233 can distinguish between the two axis, but not (uniquely) any values in
234 between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1]
239 The surface X coordinate of the center of the touching ellipse.
243 The surface Y coordinate of the center of the touching ellipse.
247 The type of approaching tool. A lot of kernel drivers cannot distinguish
248 between different tool types, such as a finger or a pen. In such cases, the
249 event should be omitted. The protocol currently supports MT_TOOL_FINGER and
254 The BLOB_ID groups several packets together into one arbitrarily shaped
255 contact. This is a low-level anonymous grouping for type A devices, and
256 should not be confused with the high-level trackingID [5]. Most type A
257 devices do not have blob capability, so drivers can safely omit this event.
261 The TRACKING_ID identifies an initiated contact throughout its life cycle
262 [5]. This event is mandatory for type B devices. The value range of the
263 TRACKING_ID should be large enough to ensure unique identification of a
264 contact maintained over an extended period of time.
270 The flora of different hardware unavoidably leads to some devices fitting
271 better to the MT protocol than others. To simplify and unify the mapping,
272 this section gives recipes for how to compute certain events.
274 For devices reporting contacts as rectangular shapes, signed orientation
275 cannot be obtained. Assuming X and Y are the lengths of the sides of the
276 touching rectangle, here is a simple formula that retains the most
277 information possible:
279 ABS_MT_TOUCH_MAJOR := max(X, Y)
280 ABS_MT_TOUCH_MINOR := min(X, Y)
281 ABS_MT_ORIENTATION := bool(X > Y)
283 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
284 the device can distinguish between a finger along the Y axis (0) and a
285 finger along the X axis (1).
291 The process of finger tracking, i.e., to assign a unique trackingID to each
292 initiated contact on the surface, is a Euclidian Bipartite Matching
293 problem. At each event synchronization, the set of actual contacts is
294 matched to the set of contacts from the previous synchronization. A full
295 implementation can be found in [3].
301 In the specific application of creating gesture events, the TOUCH and WIDTH
302 parameters can be used to, e.g., approximate finger pressure or distinguish
303 between index finger and thumb. With the addition of the MINOR parameters,
304 one can also distinguish between a sweeping finger and a pointing finger,
305 and with ORIENTATION, one can detect twisting of fingers.
311 In order to stay compatible with existing applications, the data
312 reported in a finger packet must not be recognized as single-touch
313 events. In addition, all finger data must bypass input filtering,
314 since subsequent events of the same type refer to different fingers.
316 The first kernel driver to utilize the MT protocol is the bcm5974 driver,
317 where examples can be found.
319 [1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
320 difference between the contact position and the approaching tool position
321 could be used to derive tilt.
322 [2] The list can of course be extended.
323 [3] Multitouch X driver project: http://bitmath.org/code/multitouch/.
324 [4] See the section on event computation.
325 [5] See the section on finger tracking.