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Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	Multi-touch (MT) Protocol
2	-------------------------
3		Copyright (C) 2009-2010	Henrik Rydberg <rydberg@euromail.se>
4	
5	
6	Introduction
7	------------
8	
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.
14	
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.
20	
21	
22	Protocol Usage
23	--------------
24	
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.
30	
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.
35	
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.
40	
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
44	of events/packets.
45	
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
50	the raw data [5].
51	
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].
56	
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.
67	
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.
82	
83	The minimum value of the ABS_MT_SLOT axis must be 0.
84	
85	Protocol Example A
86	------------------
87	
88	Here is what a minimal event sequence for a two-contact touch would look
89	like for a type A device:
90	
91	   ABS_MT_POSITION_X x[0]
92	   ABS_MT_POSITION_Y y[0]
93	   SYN_MT_REPORT
94	   ABS_MT_POSITION_X x[1]
95	   ABS_MT_POSITION_Y y[1]
96	   SYN_MT_REPORT
97	   SYN_REPORT
98	
99	The sequence after moving one of the contacts looks exactly the same; the
100	raw data for all present contacts are sent between every synchronization
101	with SYN_REPORT.
102	
103	Here is the sequence after lifting the first contact:
104	
105	   ABS_MT_POSITION_X x[1]
106	   ABS_MT_POSITION_Y y[1]
107	   SYN_MT_REPORT
108	   SYN_REPORT
109	
110	And here is the sequence after lifting the second contact:
111	
112	   SYN_MT_REPORT
113	   SYN_REPORT
114	
115	If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
116	ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
117	last SYN_REPORT will be dropped by the input core, resulting in no
118	zero-contact event reaching userland.
119	
120	
121	Protocol Example B
122	------------------
123	
124	Here is what a minimal event sequence for a two-contact touch would look
125	like for a type B device:
126	
127	   ABS_MT_SLOT 0
128	   ABS_MT_TRACKING_ID 45
129	   ABS_MT_POSITION_X x[0]
130	   ABS_MT_POSITION_Y y[0]
131	   ABS_MT_SLOT 1
132	   ABS_MT_TRACKING_ID 46
133	   ABS_MT_POSITION_X x[1]
134	   ABS_MT_POSITION_Y y[1]
135	   SYN_REPORT
136	
137	Here is the sequence after moving contact 45 in the x direction:
138	
139	   ABS_MT_SLOT 0
140	   ABS_MT_POSITION_X x[0]
141	   SYN_REPORT
142	
143	Here is the sequence after lifting the contact in slot 0:
144	
145	   ABS_MT_TRACKING_ID -1
146	   SYN_REPORT
147	
148	The slot being modified is already 0, so the ABS_MT_SLOT is omitted.  The
149	message removes the association of slot 0 with contact 45, thereby
150	destroying contact 45 and freeing slot 0 to be reused for another contact.
151	
152	Finally, here is the sequence after lifting the second contact:
153	
154	   ABS_MT_SLOT 1
155	   ABS_MT_TRACKING_ID -1
156	   SYN_REPORT
157	
158	
159	Event Usage
160	-----------
161	
162	A set of ABS_MT events with the desired properties is defined. The events
163	are divided into categories, to allow for partial implementation.  The
164	minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
165	allows for multiple contacts to be tracked.  If the device supports it, the
166	ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
167	of the contact area and approaching tool, respectively.
168	
169	The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
170	looking through a window at someone gently holding a finger against the
171	glass.  You will see two regions, one inner region consisting of the part
172	of the finger actually touching the glass, and one outer region formed by
173	the perimeter of the finger. The center of the touching region (a) is
174	ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
175	ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
176	diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
177	harder against the glass. The touch region will increase, and in general,
178	the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
179	than unity, is related to the contact pressure. For pressure-based devices,
180	ABS_MT_PRESSURE may be used to provide the pressure on the contact area
181	instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
182	indicate the distance between the contact and the surface.
183	
184	
185		  Linux MT                               Win8
186	         __________                     _______________________
187	        /          \                   |                       |
188	       /            \                  |                       |
189	      /     ____     \                 |                       |
190	     /     /    \     \                |                       |
191	     \     \  a  \     \               |       a               |
192	      \     \____/      \              |                       |
193	       \                 \             |                       |
194	        \        b        \            |           b           |
195	         \                 \           |                       |
196	          \                 \          |                       |
197	           \                 \         |                       |
198	            \                /         |                       |
199	             \              /          |                       |
200	              \            /           |                       |
201	               \__________/            |_______________________|
202	
203	
204	In addition to the MAJOR parameters, the oval shape of the touch and finger
205	regions can be described by adding the MINOR parameters, such that MAJOR
206	and MINOR are the major and minor axis of an ellipse. The orientation of
207	the touch ellipse can be described with the ORIENTATION parameter, and the
208	direction of the finger ellipse is given by the vector (a - b).
209	
210	For type A devices, further specification of the touch shape is possible
211	via ABS_MT_BLOB_ID.
212	
213	The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
214	finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
215	may be used to track identified contacts over time [5].
216	
217	In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
218	implicitly handled by input core; drivers should instead call
219	input_mt_report_slot_state().
220	
221	
222	Event Semantics
223	---------------
224	
225	ABS_MT_TOUCH_MAJOR
226	
227	The length of the major axis of the contact. The length should be given in
228	surface units. If the surface has an X times Y resolution, the largest
229	possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
230	
231	ABS_MT_TOUCH_MINOR
232	
233	The length, in surface units, of the minor axis of the contact. If the
234	contact is circular, this event can be omitted [4].
235	
236	ABS_MT_WIDTH_MAJOR
237	
238	The length, in surface units, of the major axis of the approaching
239	tool. This should be understood as the size of the tool itself. The
240	orientation of the contact and the approaching tool are assumed to be the
241	same [4].
242	
243	ABS_MT_WIDTH_MINOR
244	
245	The length, in surface units, of the minor axis of the approaching
246	tool. Omit if circular [4].
247	
248	The above four values can be used to derive additional information about
249	the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
250	the notion of pressure. The fingers of the hand and the palm all have
251	different characteristic widths.
252	
253	ABS_MT_PRESSURE
254	
255	The pressure, in arbitrary units, on the contact area. May be used instead
256	of TOUCH and WIDTH for pressure-based devices or any device with a spatial
257	signal intensity distribution.
258	
259	ABS_MT_DISTANCE
260	
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.
264	
265	ABS_MT_ORIENTATION
266	
267	The orientation of the touching ellipse. The value should describe a signed
268	quarter of a revolution clockwise around the touch center. The signed value
269	range is arbitrary, but zero should be returned for an ellipse aligned with
270	the Y axis of the surface, a negative value when the ellipse is turned to
271	the left, and a positive value when the ellipse is turned to the
272	right. When completely aligned with the X axis, the range max should be
273	returned.
274	
275	Touch ellipsis are symmetrical by default. For devices capable of true 360
276	degree orientation, the reported orientation must exceed the range max to
277	indicate more than a quarter of a revolution. For an upside-down finger,
278	range max * 2 should be returned.
279	
280	Orientation can be omitted if the touch area is circular, or if the
281	information is not available in the kernel driver. Partial orientation
282	support is possible if the device can distinguish between the two axis, but
283	not (uniquely) any values in between. In such cases, the range of
284	ABS_MT_ORIENTATION should be [0, 1] [4].
285	
286	ABS_MT_POSITION_X
287	
288	The surface X coordinate of the center of the touching ellipse.
289	
290	ABS_MT_POSITION_Y
291	
292	The surface Y coordinate of the center of the touching ellipse.
293	
294	ABS_MT_TOOL_X
295	
296	The surface X coordinate of the center of the approaching tool. Omit if
297	the device cannot distinguish between the intended touch point and the
298	tool itself.
299	
300	ABS_MT_TOOL_Y
301	
302	The surface Y coordinate of the center of the approaching tool. Omit if the
303	device cannot distinguish between the intended touch point and the tool
304	itself.
305	
306	The four position values can be used to separate the position of the touch
307	from the position of the tool. If both positions are present, the major
308	tool axis points towards the touch point [1]. Otherwise, the tool axes are
309	aligned with the touch axes.
310	
311	ABS_MT_TOOL_TYPE
312	
313	The type of approaching tool. A lot of kernel drivers cannot distinguish
314	between different tool types, such as a finger or a pen. In such cases, the
315	event should be omitted. The protocol currently supports MT_TOOL_FINGER and
316	MT_TOOL_PEN [2]. For type B devices, this event is handled by input core;
317	drivers should instead use input_mt_report_slot_state().
318	
319	ABS_MT_BLOB_ID
320	
321	The BLOB_ID groups several packets together into one arbitrarily shaped
322	contact. The sequence of points forms a polygon which defines the shape of
323	the contact. This is a low-level anonymous grouping for type A devices, and
324	should not be confused with the high-level trackingID [5]. Most type A
325	devices do not have blob capability, so drivers can safely omit this event.
326	
327	ABS_MT_TRACKING_ID
328	
329	The TRACKING_ID identifies an initiated contact throughout its life cycle
330	[5]. The value range of the TRACKING_ID should be large enough to ensure
331	unique identification of a contact maintained over an extended period of
332	time. For type B devices, this event is handled by input core; drivers
333	should instead use input_mt_report_slot_state().
334	
335	
336	Event Computation
337	-----------------
338	
339	The flora of different hardware unavoidably leads to some devices fitting
340	better to the MT protocol than others. To simplify and unify the mapping,
341	this section gives recipes for how to compute certain events.
342	
343	For devices reporting contacts as rectangular shapes, signed orientation
344	cannot be obtained. Assuming X and Y are the lengths of the sides of the
345	touching rectangle, here is a simple formula that retains the most
346	information possible:
347	
348	   ABS_MT_TOUCH_MAJOR := max(X, Y)
349	   ABS_MT_TOUCH_MINOR := min(X, Y)
350	   ABS_MT_ORIENTATION := bool(X > Y)
351	
352	The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
353	the device can distinguish between a finger along the Y axis (0) and a
354	finger along the X axis (1).
355	
356	For win8 devices with both T and C coordinates, the position mapping is
357	
358	   ABS_MT_POSITION_X := T_X
359	   ABS_MT_POSITION_Y := T_Y
360	   ABS_MT_TOOL_X := C_X
361	   ABS_MT_TOOL_X := C_Y
362	
363	Unfortunately, there is not enough information to specify both the touching
364	ellipse and the tool ellipse, so one has to resort to approximations.  One
365	simple scheme, which is compatible with earlier usage, is:
366	
367	   ABS_MT_TOUCH_MAJOR := min(X, Y)
368	   ABS_MT_TOUCH_MINOR := <not used>
369	   ABS_MT_ORIENTATION := <not used>
370	   ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
371	   ABS_MT_WIDTH_MINOR := min(X, Y)
372	
373	Rationale: We have no information about the orientation of the touching
374	ellipse, so approximate it with an inscribed circle instead. The tool
375	ellipse should align with the the vector (T - C), so the diameter must
376	increase with distance(T, C). Finally, assume that the touch diameter is
377	equal to the tool thickness, and we arrive at the formulas above.
378	
379	Finger Tracking
380	---------------
381	
382	The process of finger tracking, i.e., to assign a unique trackingID to each
383	initiated contact on the surface, is a Euclidian Bipartite Matching
384	problem.  At each event synchronization, the set of actual contacts is
385	matched to the set of contacts from the previous synchronization. A full
386	implementation can be found in [3].
387	
388	
389	Gestures
390	--------
391	
392	In the specific application of creating gesture events, the TOUCH and WIDTH
393	parameters can be used to, e.g., approximate finger pressure or distinguish
394	between index finger and thumb. With the addition of the MINOR parameters,
395	one can also distinguish between a sweeping finger and a pointing finger,
396	and with ORIENTATION, one can detect twisting of fingers.
397	
398	
399	Notes
400	-----
401	
402	In order to stay compatible with existing applications, the data reported
403	in a finger packet must not be recognized as single-touch events.
404	
405	For type A devices, all finger data bypasses input filtering, since
406	subsequent events of the same type refer to different fingers.
407	
408	For example usage of the type A protocol, see the bcm5974 driver. For
409	example usage of the type B protocol, see the hid-egalax driver.
410	
411	[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
412	[2] The list can of course be extended.
413	[3] The mtdev project: http://bitmath.org/code/mtdev/.
414	[4] See the section on event computation.
415	[5] See the section on finger tracking.
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