Root/Documentation/input/multi-touch-protocol.txt

1Multi-touch (MT) Protocol
2-------------------------
3    Copyright (C) 2009-2010 Henrik Rydberg <rydberg@euromail.se>
4
5
6Introduction
7------------
8
9In order to utilize the full power of the new multi-touch and multi-user
10devices, a way to report detailed data from multiple contacts, i.e.,
11objects in direct contact with the device surface, is needed. This
12document describes the multi-touch (MT) protocol which allows kernel
13drivers to report details for an arbitrary number of contacts.
14
15The protocol is divided into two types, depending on the capabilities of the
16hardware. For devices handling anonymous contacts (type A), the protocol
17describes how to send the raw data for all contacts to the receiver. For
18devices capable of tracking identifiable contacts (type B), the protocol
19describes how to send updates for individual contacts via event slots.
20
21
22Protocol Usage
23--------------
24
25Contact details are sent sequentially as separate packets of ABS_MT
26events. Only the ABS_MT events are recognized as part of a contact
27packet. Since these events are ignored by current single-touch (ST)
28applications, the MT protocol can be implemented on top of the ST protocol
29in an existing driver.
30
31Drivers for type A devices separate contact packets by calling
32input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
33event, which instructs the receiver to accept the data for the current
34contact and prepare to receive another.
35
36Drivers for type B devices separate contact packets by calling
37input_mt_slot(), with a slot as argument, at the beginning of each packet.
38This generates an ABS_MT_SLOT event, which instructs the receiver to
39prepare for updates of the given slot.
40
41All drivers mark the end of a multi-touch transfer by calling the usual
42input_sync() function. This instructs the receiver to act upon events
43accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
44of events/packets.
45
46The main difference between the stateless type A protocol and the stateful
47type B slot protocol lies in the usage of identifiable contacts to reduce
48the amount of data sent to userspace. The slot protocol requires the use of
49the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
50the raw data [5].
51
52For type A devices, the kernel driver should generate an arbitrary
53enumeration of the full set of anonymous contacts currently on the
54surface. The order in which the packets appear in the event stream is not
55important. Event filtering and finger tracking is left to user space [3].
56
57For type B devices, the kernel driver should associate a slot with each
58identified contact, and use that slot to propagate changes for the contact.
59Creation, replacement and destruction of contacts is achieved by modifying
60the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
61is interpreted as a contact, and the value -1 denotes an unused slot. A
62tracking id not previously present is considered new, and a tracking id no
63longer present is considered removed. Since only changes are propagated,
64the full state of each initiated contact has to reside in the receiving
65end. Upon receiving an MT event, one simply updates the appropriate
66attribute of the current slot.
67
68
69Protocol Example A
70------------------
71
72Here is what a minimal event sequence for a two-contact touch would look
73like for a type A device:
74
75   ABS_MT_POSITION_X x[0]
76   ABS_MT_POSITION_Y y[0]
77   SYN_MT_REPORT
78   ABS_MT_POSITION_X x[1]
79   ABS_MT_POSITION_Y y[1]
80   SYN_MT_REPORT
81   SYN_REPORT
82
83The sequence after moving one of the contacts looks exactly the same; the
84raw data for all present contacts are sent between every synchronization
85with SYN_REPORT.
86
87Here is the sequence after lifting the first contact:
88
89   ABS_MT_POSITION_X x[1]
90   ABS_MT_POSITION_Y y[1]
91   SYN_MT_REPORT
92   SYN_REPORT
93
94And here is the sequence after lifting the second contact:
95
96   SYN_MT_REPORT
97   SYN_REPORT
98
99If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
100ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
101last SYN_REPORT will be dropped by the input core, resulting in no
102zero-contact event reaching userland.
103
104
105Protocol Example B
106------------------
107
108Here is what a minimal event sequence for a two-contact touch would look
109like for a type B device:
110
111   ABS_MT_SLOT 0
112   ABS_MT_TRACKING_ID 45
113   ABS_MT_POSITION_X x[0]
114   ABS_MT_POSITION_Y y[0]
115   ABS_MT_SLOT 1
116   ABS_MT_TRACKING_ID 46
117   ABS_MT_POSITION_X x[1]
118   ABS_MT_POSITION_Y y[1]
119   SYN_REPORT
120
121Here is the sequence after moving contact 45 in the x direction:
122
123   ABS_MT_SLOT 0
124   ABS_MT_POSITION_X x[0]
125   SYN_REPORT
126
127Here is the sequence after lifting the contact in slot 0:
128
129   ABS_MT_TRACKING_ID -1
130   SYN_REPORT
131
132The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
133message removes the association of slot 0 with contact 45, thereby
134destroying contact 45 and freeing slot 0 to be reused for another contact.
135
136Finally, here is the sequence after lifting the second contact:
137
138   ABS_MT_SLOT 1
139   ABS_MT_TRACKING_ID -1
140   SYN_REPORT
141
142
143Event Usage
144-----------
145
146A set of ABS_MT events with the desired properties is defined. The events
147are divided into categories, to allow for partial implementation. The
148minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
149allows for multiple contacts to be tracked. If the device supports it, the
150ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
151of the contact area and approaching contact, respectively.
152
153The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
154looking through a window at someone gently holding a finger against the
155glass. You will see two regions, one inner region consisting of the part
156of the finger actually touching the glass, and one outer region formed by
157the perimeter of the finger. The diameter of the inner region is the
158ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
159ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
160against the glass. The inner region will increase, and in general, the
161ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
162unity, is related to the contact pressure. For pressure-based devices,
163ABS_MT_PRESSURE may be used to provide the pressure on the contact area
164instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
165indicate the distance between the contact and the surface.
166
167In addition to the MAJOR parameters, the oval shape of the contact can be
168described by adding the MINOR parameters, such that MAJOR and MINOR are the
169major and minor axis of an ellipse. Finally, the orientation of the oval
170shape can be describe with the ORIENTATION parameter.
171
172For type A devices, further specification of the touch shape is possible
173via ABS_MT_BLOB_ID.
174
175The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
176finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
177may be used to track identified contacts over time [5].
178
179In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
180implicitly handled by input core; drivers should instead call
181input_mt_report_slot_state().
182
183
184Event Semantics
185---------------
186
187ABS_MT_TOUCH_MAJOR
188
189The length of the major axis of the contact. The length should be given in
190surface units. If the surface has an X times Y resolution, the largest
191possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
192
193ABS_MT_TOUCH_MINOR
194
195The length, in surface units, of the minor axis of the contact. If the
196contact is circular, this event can be omitted [4].
197
198ABS_MT_WIDTH_MAJOR
199
200The length, in surface units, of the major axis of the approaching
201tool. This should be understood as the size of the tool itself. The
202orientation of the contact and the approaching tool are assumed to be the
203same [4].
204
205ABS_MT_WIDTH_MINOR
206
207The length, in surface units, of the minor axis of the approaching
208tool. Omit if circular [4].
209
210The above four values can be used to derive additional information about
211the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
212the notion of pressure. The fingers of the hand and the palm all have
213different characteristic widths [1].
214
215ABS_MT_PRESSURE
216
217The pressure, in arbitrary units, on the contact area. May be used instead
218of TOUCH and WIDTH for pressure-based devices or any device with a spatial
219signal intensity distribution.
220
221ABS_MT_DISTANCE
222
223The distance, in surface units, between the contact and the surface. Zero
224distance means the contact is touching the surface. A positive number means
225the contact is hovering above the surface.
226
227ABS_MT_ORIENTATION
228
229The orientation of the ellipse. The value should describe a signed quarter
230of a revolution clockwise around the touch center. The signed value range
231is arbitrary, but zero should be returned for a finger aligned along the Y
232axis of the surface, a negative value when finger is turned to the left, and
233a positive value when finger turned to the right. When completely aligned with
234the X axis, the range max should be returned. Orientation can be omitted
235if the touching object is circular, or if the information is not available
236in the kernel driver. Partial orientation support is possible if the device
237can distinguish between the two axis, but not (uniquely) any values in
238between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1]
239[4].
240
241ABS_MT_POSITION_X
242
243The surface X coordinate of the center of the touching ellipse.
244
245ABS_MT_POSITION_Y
246
247The surface Y coordinate of the center of the touching ellipse.
248
249ABS_MT_TOOL_TYPE
250
251The type of approaching tool. A lot of kernel drivers cannot distinguish
252between different tool types, such as a finger or a pen. In such cases, the
253event should be omitted. The protocol currently supports MT_TOOL_FINGER and
254MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
255drivers should instead use input_mt_report_slot_state().
256
257ABS_MT_BLOB_ID
258
259The BLOB_ID groups several packets together into one arbitrarily shaped
260contact. The sequence of points forms a polygon which defines the shape of
261the contact. This is a low-level anonymous grouping for type A devices, and
262should not be confused with the high-level trackingID [5]. Most type A
263devices do not have blob capability, so drivers can safely omit this event.
264
265ABS_MT_TRACKING_ID
266
267The TRACKING_ID identifies an initiated contact throughout its life cycle
268[5]. The value range of the TRACKING_ID should be large enough to ensure
269unique identification of a contact maintained over an extended period of
270time. For type B devices, this event is handled by input core; drivers
271should instead use input_mt_report_slot_state().
272
273
274Event Computation
275-----------------
276
277The flora of different hardware unavoidably leads to some devices fitting
278better to the MT protocol than others. To simplify and unify the mapping,
279this section gives recipes for how to compute certain events.
280
281For devices reporting contacts as rectangular shapes, signed orientation
282cannot be obtained. Assuming X and Y are the lengths of the sides of the
283touching rectangle, here is a simple formula that retains the most
284information possible:
285
286   ABS_MT_TOUCH_MAJOR := max(X, Y)
287   ABS_MT_TOUCH_MINOR := min(X, Y)
288   ABS_MT_ORIENTATION := bool(X > Y)
289
290The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
291the device can distinguish between a finger along the Y axis (0) and a
292finger along the X axis (1).
293
294
295Finger Tracking
296---------------
297
298The process of finger tracking, i.e., to assign a unique trackingID to each
299initiated contact on the surface, is a Euclidian Bipartite Matching
300problem. At each event synchronization, the set of actual contacts is
301matched to the set of contacts from the previous synchronization. A full
302implementation can be found in [3].
303
304
305Gestures
306--------
307
308In the specific application of creating gesture events, the TOUCH and WIDTH
309parameters can be used to, e.g., approximate finger pressure or distinguish
310between index finger and thumb. With the addition of the MINOR parameters,
311one can also distinguish between a sweeping finger and a pointing finger,
312and with ORIENTATION, one can detect twisting of fingers.
313
314
315Notes
316-----
317
318In order to stay compatible with existing applications, the data reported
319in a finger packet must not be recognized as single-touch events.
320
321For type A devices, all finger data bypasses input filtering, since
322subsequent events of the same type refer to different fingers.
323
324For example usage of the type A protocol, see the bcm5974 driver. For
325example usage of the type B protocol, see the hid-egalax driver.
326
327[1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
328difference between the contact position and the approaching tool position
329could be used to derive tilt.
330[2] The list can of course be extended.
331[3] The mtdev project: http://bitmath.org/code/mtdev/.
332[4] See the section on event computation.
333[5] See the section on finger tracking.
334

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