1 .. include:: <isonum.txt> 1 .. include:: <isonum.txt>
2 2
3 ========================= 3 =========================
4 Multi-touch (MT) Protocol 4 Multi-touch (MT) Protocol
5 ========================= 5 =========================
6 6
7 :Copyright: |copy| 2009-2010 Henrik Rydberg< 7 :Copyright: |copy| 2009-2010 Henrik Rydberg <rydberg@euromail.se>
8 8
9 9
10 Introduction 10 Introduction
11 ------------ 11 ------------
12 12
13 In order to utilize the full power of the new 13 In order to utilize the full power of the new multi-touch and multi-user
14 devices, a way to report detailed data from mu 14 devices, a way to report detailed data from multiple contacts, i.e.,
15 objects in direct contact with the device surf 15 objects in direct contact with the device surface, is needed. This
16 document describes the multi-touch (MT) protoc 16 document describes the multi-touch (MT) protocol which allows kernel
17 drivers to report details for an arbitrary num 17 drivers to report details for an arbitrary number of contacts.
18 18
19 The protocol is divided into two types, depend 19 The protocol is divided into two types, depending on the capabilities of the
20 hardware. For devices handling anonymous conta 20 hardware. For devices handling anonymous contacts (type A), the protocol
21 describes how to send the raw data for all con 21 describes how to send the raw data for all contacts to the receiver. For
22 devices capable of tracking identifiable conta 22 devices capable of tracking identifiable contacts (type B), the protocol
23 describes how to send updates for individual c 23 describes how to send updates for individual contacts via event slots.
24 24
25 .. note:: 25 .. note::
26 MT protocol type A is obsolete, all kernel 26 MT protocol type A is obsolete, all kernel drivers have been
27 converted to use type B. 27 converted to use type B.
28 28
29 Protocol Usage 29 Protocol Usage
30 -------------- 30 --------------
31 31
32 Contact details are sent sequentially as separ 32 Contact details are sent sequentially as separate packets of ABS_MT
33 events. Only the ABS_MT events are recognized 33 events. Only the ABS_MT events are recognized as part of a contact
34 packet. Since these events are ignored by curr 34 packet. Since these events are ignored by current single-touch (ST)
35 applications, the MT protocol can be implement 35 applications, the MT protocol can be implemented on top of the ST protocol
36 in an existing driver. 36 in an existing driver.
37 37
38 Drivers for type A devices separate contact pa 38 Drivers for type A devices separate contact packets by calling
39 input_mt_sync() at the end of each packet. Thi 39 input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
40 event, which instructs the receiver to accept 40 event, which instructs the receiver to accept the data for the current
41 contact and prepare to receive another. 41 contact and prepare to receive another.
42 42
43 Drivers for type B devices separate contact pa 43 Drivers for type B devices separate contact packets by calling
44 input_mt_slot(), with a slot as argument, at t 44 input_mt_slot(), with a slot as argument, at the beginning of each packet.
45 This generates an ABS_MT_SLOT event, which ins 45 This generates an ABS_MT_SLOT event, which instructs the receiver to
46 prepare for updates of the given slot. 46 prepare for updates of the given slot.
47 47
48 All drivers mark the end of a multi-touch tran 48 All drivers mark the end of a multi-touch transfer by calling the usual
49 input_sync() function. This instructs the rece 49 input_sync() function. This instructs the receiver to act upon events
50 accumulated since last EV_SYN/SYN_REPORT and p 50 accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
51 of events/packets. 51 of events/packets.
52 52
53 The main difference between the stateless type 53 The main difference between the stateless type A protocol and the stateful
54 type B slot protocol lies in the usage of iden 54 type B slot protocol lies in the usage of identifiable contacts to reduce
55 the amount of data sent to userspace. The slot 55 the amount of data sent to userspace. The slot protocol requires the use of
56 the ABS_MT_TRACKING_ID, either provided by the 56 the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
57 the raw data [#f5]_. 57 the raw data [#f5]_.
58 58
59 For type A devices, the kernel driver should g 59 For type A devices, the kernel driver should generate an arbitrary
60 enumeration of the full set of anonymous conta 60 enumeration of the full set of anonymous contacts currently on the
61 surface. The order in which the packets appear 61 surface. The order in which the packets appear in the event stream is not
62 important. Event filtering and finger trackin 62 important. Event filtering and finger tracking is left to user space [#f3]_.
63 63
64 For type B devices, the kernel driver should a 64 For type B devices, the kernel driver should associate a slot with each
65 identified contact, and use that slot to propa 65 identified contact, and use that slot to propagate changes for the contact.
66 Creation, replacement and destruction of conta 66 Creation, replacement and destruction of contacts is achieved by modifying
67 the ABS_MT_TRACKING_ID of the associated slot. 67 the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
68 is interpreted as a contact, and the value -1 68 is interpreted as a contact, and the value -1 denotes an unused slot. A
69 tracking id not previously present is consider 69 tracking id not previously present is considered new, and a tracking id no
70 longer present is considered removed. Since o 70 longer present is considered removed. Since only changes are propagated,
71 the full state of each initiated contact has t 71 the full state of each initiated contact has to reside in the receiving
72 end. Upon receiving an MT event, one simply u 72 end. Upon receiving an MT event, one simply updates the appropriate
73 attribute of the current slot. 73 attribute of the current slot.
74 74
75 Some devices identify and/or track more contac 75 Some devices identify and/or track more contacts than they can report to the
76 driver. A driver for such a device should ass 76 driver. A driver for such a device should associate one type B slot with each
77 contact that is reported by the hardware. Whe 77 contact that is reported by the hardware. Whenever the identity of the
78 contact associated with a slot changes, the dr 78 contact associated with a slot changes, the driver should invalidate that
79 slot by changing its ABS_MT_TRACKING_ID. If t 79 slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
80 tracking more contacts than it is currently re 80 tracking more contacts than it is currently reporting, the driver should use
81 a BTN_TOOL_*TAP event to inform userspace of t 81 a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
82 being tracked by the hardware at that moment. 82 being tracked by the hardware at that moment. The driver should do this by
83 explicitly sending the corresponding BTN_TOOL_ 83 explicitly sending the corresponding BTN_TOOL_*TAP event and setting
84 use_count to false when calling input_mt_repor 84 use_count to false when calling input_mt_report_pointer_emulation().
85 The driver should only advertise as many slots 85 The driver should only advertise as many slots as the hardware can report.
86 Userspace can detect that a driver can report 86 Userspace can detect that a driver can report more total contacts than slots
87 by noting that the largest supported BTN_TOOL_ 87 by noting that the largest supported BTN_TOOL_*TAP event is larger than the
88 total number of type B slots reported in the a 88 total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
89 89
90 The minimum value of the ABS_MT_SLOT axis must 90 The minimum value of the ABS_MT_SLOT axis must be 0.
91 91
92 Protocol Example A 92 Protocol Example A
93 ------------------ 93 ------------------
94 94
95 Here is what a minimal event sequence for a tw 95 Here is what a minimal event sequence for a two-contact touch would look
96 like for a type A device:: 96 like for a type A device::
97 97
98 ABS_MT_POSITION_X x[0] 98 ABS_MT_POSITION_X x[0]
99 ABS_MT_POSITION_Y y[0] 99 ABS_MT_POSITION_Y y[0]
100 SYN_MT_REPORT 100 SYN_MT_REPORT
101 ABS_MT_POSITION_X x[1] 101 ABS_MT_POSITION_X x[1]
102 ABS_MT_POSITION_Y y[1] 102 ABS_MT_POSITION_Y y[1]
103 SYN_MT_REPORT 103 SYN_MT_REPORT
104 SYN_REPORT 104 SYN_REPORT
105 105
106 The sequence after moving one of the contacts 106 The sequence after moving one of the contacts looks exactly the same; the
107 raw data for all present contacts are sent bet 107 raw data for all present contacts are sent between every synchronization
108 with SYN_REPORT. 108 with SYN_REPORT.
109 109
110 Here is the sequence after lifting the first c 110 Here is the sequence after lifting the first contact::
111 111
112 ABS_MT_POSITION_X x[1] 112 ABS_MT_POSITION_X x[1]
113 ABS_MT_POSITION_Y y[1] 113 ABS_MT_POSITION_Y y[1]
114 SYN_MT_REPORT 114 SYN_MT_REPORT
115 SYN_REPORT 115 SYN_REPORT
116 116
117 And here is the sequence after lifting the sec 117 And here is the sequence after lifting the second contact::
118 118
119 SYN_MT_REPORT 119 SYN_MT_REPORT
120 SYN_REPORT 120 SYN_REPORT
121 121
122 If the driver reports one of BTN_TOUCH or ABS_ 122 If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
123 ABS_MT events, the last SYN_MT_REPORT event ma 123 ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
124 last SYN_REPORT will be dropped by the input c 124 last SYN_REPORT will be dropped by the input core, resulting in no
125 zero-contact event reaching userland. 125 zero-contact event reaching userland.
126 126
127 127
128 Protocol Example B 128 Protocol Example B
129 ------------------ 129 ------------------
130 130
131 Here is what a minimal event sequence for a tw 131 Here is what a minimal event sequence for a two-contact touch would look
132 like for a type B device:: 132 like for a type B device::
133 133
134 ABS_MT_SLOT 0 134 ABS_MT_SLOT 0
135 ABS_MT_TRACKING_ID 45 135 ABS_MT_TRACKING_ID 45
136 ABS_MT_POSITION_X x[0] 136 ABS_MT_POSITION_X x[0]
137 ABS_MT_POSITION_Y y[0] 137 ABS_MT_POSITION_Y y[0]
138 ABS_MT_SLOT 1 138 ABS_MT_SLOT 1
139 ABS_MT_TRACKING_ID 46 139 ABS_MT_TRACKING_ID 46
140 ABS_MT_POSITION_X x[1] 140 ABS_MT_POSITION_X x[1]
141 ABS_MT_POSITION_Y y[1] 141 ABS_MT_POSITION_Y y[1]
142 SYN_REPORT 142 SYN_REPORT
143 143
144 Here is the sequence after moving contact 45 i 144 Here is the sequence after moving contact 45 in the x direction::
145 145
146 ABS_MT_SLOT 0 146 ABS_MT_SLOT 0
147 ABS_MT_POSITION_X x[0] 147 ABS_MT_POSITION_X x[0]
148 SYN_REPORT 148 SYN_REPORT
149 149
150 Here is the sequence after lifting the contact 150 Here is the sequence after lifting the contact in slot 0::
151 151
152 ABS_MT_TRACKING_ID -1 152 ABS_MT_TRACKING_ID -1
153 SYN_REPORT 153 SYN_REPORT
154 154
155 The slot being modified is already 0, so the A 155 The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
156 message removes the association of slot 0 with 156 message removes the association of slot 0 with contact 45, thereby
157 destroying contact 45 and freeing slot 0 to be 157 destroying contact 45 and freeing slot 0 to be reused for another contact.
158 158
159 Finally, here is the sequence after lifting th 159 Finally, here is the sequence after lifting the second contact::
160 160
161 ABS_MT_SLOT 1 161 ABS_MT_SLOT 1
162 ABS_MT_TRACKING_ID -1 162 ABS_MT_TRACKING_ID -1
163 SYN_REPORT 163 SYN_REPORT
164 164
165 165
166 Event Usage 166 Event Usage
167 ----------- 167 -----------
168 168
169 A set of ABS_MT events with the desired proper 169 A set of ABS_MT events with the desired properties is defined. The events
170 are divided into categories, to allow for part 170 are divided into categories, to allow for partial implementation. The
171 minimum set consists of ABS_MT_POSITION_X and 171 minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
172 allows for multiple contacts to be tracked. I 172 allows for multiple contacts to be tracked. If the device supports it, the
173 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may 173 ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
174 of the contact area and approaching tool, resp 174 of the contact area and approaching tool, respectively.
175 175
176 The TOUCH and WIDTH parameters have a geometri 176 The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
177 looking through a window at someone gently hol 177 looking through a window at someone gently holding a finger against the
178 glass. You will see two regions, one inner re 178 glass. You will see two regions, one inner region consisting of the part
179 of the finger actually touching the glass, and 179 of the finger actually touching the glass, and one outer region formed by
180 the perimeter of the finger. The center of the 180 the perimeter of the finger. The center of the touching region (a) is
181 ABS_MT_POSITION_X/Y and the center of the appr 181 ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
182 ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_ 182 ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
183 diameter is ABS_MT_WIDTH_MAJOR. Now imagine th 183 diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
184 harder against the glass. The touch region wil 184 harder against the glass. The touch region will increase, and in general,
185 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MA 185 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
186 than unity, is related to the contact pressure 186 than unity, is related to the contact pressure. For pressure-based devices,
187 ABS_MT_PRESSURE may be used to provide the pre 187 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
188 instead. Devices capable of contact hovering c 188 instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
189 indicate the distance between the contact and 189 indicate the distance between the contact and the surface.
190 190
191 :: 191 ::
192 192
193 193
194 Linux MT 194 Linux MT Win8
195 __________ ______ 195 __________ _______________________
196 / \ | 196 / \ | |
197 / \ | 197 / \ | |
198 / ____ \ | 198 / ____ \ | |
199 / / \ \ | 199 / / \ \ | |
200 \ \ a \ \ | 200 \ \ a \ \ | a |
201 \ \____/ \ | 201 \ \____/ \ | |
202 \ \ | 202 \ \ | |
203 \ b \ | 203 \ b \ | b |
204 \ \ | 204 \ \ | |
205 \ \ | 205 \ \ | |
206 \ \ | 206 \ \ | |
207 \ / | 207 \ / | |
208 \ / | 208 \ / | |
209 \ / | 209 \ / | |
210 \__________/ |______ 210 \__________/ |_______________________|
211 211
212 212
213 In addition to the MAJOR parameters, the oval 213 In addition to the MAJOR parameters, the oval shape of the touch and finger
214 regions can be described by adding the MINOR p 214 regions can be described by adding the MINOR parameters, such that MAJOR
215 and MINOR are the major and minor axis of an e 215 and MINOR are the major and minor axis of an ellipse. The orientation of
216 the touch ellipse can be described with the OR 216 the touch ellipse can be described with the ORIENTATION parameter, and the
217 direction of the finger ellipse is given by th 217 direction of the finger ellipse is given by the vector (a - b).
218 218
219 For type A devices, further specification of t 219 For type A devices, further specification of the touch shape is possible
220 via ABS_MT_BLOB_ID. 220 via ABS_MT_BLOB_ID.
221 221
222 The ABS_MT_TOOL_TYPE may be used to specify wh 222 The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
223 finger or a pen or something else. Finally, th 223 finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
224 may be used to track identified contacts over 224 may be used to track identified contacts over time [#f5]_.
225 225
226 In the type B protocol, ABS_MT_TOOL_TYPE and A 226 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
227 implicitly handled by input core; drivers shou 227 implicitly handled by input core; drivers should instead call
228 input_mt_report_slot_state(). 228 input_mt_report_slot_state().
229 229
230 230
231 Event Semantics 231 Event Semantics
232 --------------- 232 ---------------
233 233
234 ABS_MT_TOUCH_MAJOR 234 ABS_MT_TOUCH_MAJOR
235 The length of the major axis of the contac 235 The length of the major axis of the contact. The length should be given in
236 surface units. If the surface has an X tim 236 surface units. If the surface has an X times Y resolution, the largest
237 possible value of ABS_MT_TOUCH_MAJOR is sq 237 possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.
238 238
239 ABS_MT_TOUCH_MINOR 239 ABS_MT_TOUCH_MINOR
240 The length, in surface units, of the minor 240 The length, in surface units, of the minor axis of the contact. If the
241 contact is circular, this event can be omi 241 contact is circular, this event can be omitted [#f4]_.
242 242
243 ABS_MT_WIDTH_MAJOR 243 ABS_MT_WIDTH_MAJOR
244 The length, in surface units, of the major 244 The length, in surface units, of the major axis of the approaching
245 tool. This should be understood as the siz 245 tool. This should be understood as the size of the tool itself. The
246 orientation of the contact and the approac 246 orientation of the contact and the approaching tool are assumed to be the
247 same [#f4]_. 247 same [#f4]_.
248 248
249 ABS_MT_WIDTH_MINOR 249 ABS_MT_WIDTH_MINOR
250 The length, in surface units, of the minor 250 The length, in surface units, of the minor axis of the approaching
251 tool. Omit if circular [#f4]_. 251 tool. Omit if circular [#f4]_.
252 252
253 The above four values can be used to deriv 253 The above four values can be used to derive additional information about
254 the contact. The ratio ABS_MT_TOUCH_MAJOR 254 the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
255 the notion of pressure. The fingers of the 255 the notion of pressure. The fingers of the hand and the palm all have
256 different characteristic widths. 256 different characteristic widths.
257 257
258 ABS_MT_PRESSURE 258 ABS_MT_PRESSURE
259 The pressure, in arbitrary units, on the c 259 The pressure, in arbitrary units, on the contact area. May be used instead
260 of TOUCH and WIDTH for pressure-based devi 260 of TOUCH and WIDTH for pressure-based devices or any device with a spatial
261 signal intensity distribution. 261 signal intensity distribution.
262 262
263 If the resolution is zero, the pressure da <<
264 If the resolution is non-zero, the pressur <<
265 :ref:`input-event-codes` for details. <<
266 <<
267 ABS_MT_DISTANCE 263 ABS_MT_DISTANCE
268 The distance, in surface units, between th 264 The distance, in surface units, between the contact and the surface. Zero
269 distance means the contact is touching the 265 distance means the contact is touching the surface. A positive number means
270 the contact is hovering above the surface. 266 the contact is hovering above the surface.
271 267
272 ABS_MT_ORIENTATION 268 ABS_MT_ORIENTATION
273 The orientation of the touching ellipse. T 269 The orientation of the touching ellipse. The value should describe a signed
274 quarter of a revolution clockwise around t 270 quarter of a revolution clockwise around the touch center. The signed value
275 range is arbitrary, but zero should be ret 271 range is arbitrary, but zero should be returned for an ellipse aligned with
276 the Y axis (north) of the surface, a negat 272 the Y axis (north) of the surface, a negative value when the ellipse is
277 turned to the left, and a positive value w 273 turned to the left, and a positive value when the ellipse is turned to the
278 right. When aligned with the X axis in the 274 right. When aligned with the X axis in the positive direction, the range
279 max should be returned; when aligned with 275 max should be returned; when aligned with the X axis in the negative
280 direction, the range -max should be return 276 direction, the range -max should be returned.
281 277
282 Touch ellipses are symmetrical by default. !! 278 Touch ellipsis are symmetrical by default. For devices capable of true 360
283 degree orientation, the reported orientati 279 degree orientation, the reported orientation must exceed the range max to
284 indicate more than a quarter of a revoluti 280 indicate more than a quarter of a revolution. For an upside-down finger,
285 range max * 2 should be returned. 281 range max * 2 should be returned.
286 282
287 Orientation can be omitted if the touch ar 283 Orientation can be omitted if the touch area is circular, or if the
288 information is not available in the kernel 284 information is not available in the kernel driver. Partial orientation
289 support is possible if the device can dist !! 285 support is possible if the device can distinguish between the two axis, but
290 not (uniquely) any values in between. In s 286 not (uniquely) any values in between. In such cases, the range of
291 ABS_MT_ORIENTATION should be [0, 1] [#f4]_ 287 ABS_MT_ORIENTATION should be [0, 1] [#f4]_.
292 288
293 ABS_MT_POSITION_X 289 ABS_MT_POSITION_X
294 The surface X coordinate of the center of 290 The surface X coordinate of the center of the touching ellipse.
295 291
296 ABS_MT_POSITION_Y 292 ABS_MT_POSITION_Y
297 The surface Y coordinate of the center of 293 The surface Y coordinate of the center of the touching ellipse.
298 294
299 ABS_MT_TOOL_X 295 ABS_MT_TOOL_X
300 The surface X coordinate of the center of 296 The surface X coordinate of the center of the approaching tool. Omit if
301 the device cannot distinguish between the 297 the device cannot distinguish between the intended touch point and the
302 tool itself. 298 tool itself.
303 299
304 ABS_MT_TOOL_Y 300 ABS_MT_TOOL_Y
305 The surface Y coordinate of the center of 301 The surface Y coordinate of the center of the approaching tool. Omit if the
306 device cannot distinguish between the inte 302 device cannot distinguish between the intended touch point and the tool
307 itself. 303 itself.
308 304
309 The four position values can be used to se 305 The four position values can be used to separate the position of the touch
310 from the position of the tool. If both pos 306 from the position of the tool. If both positions are present, the major
311 tool axis points towards the touch point [ 307 tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
312 aligned with the touch axes. 308 aligned with the touch axes.
313 309
314 ABS_MT_TOOL_TYPE 310 ABS_MT_TOOL_TYPE
315 The type of approaching tool. A lot of ker 311 The type of approaching tool. A lot of kernel drivers cannot distinguish
316 between different tool types, such as a fi 312 between different tool types, such as a finger or a pen. In such cases, the
317 event should be omitted. The protocol curr 313 event should be omitted. The protocol currently mainly supports
318 MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_P 314 MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_.
319 For type B devices, this event is handled 315 For type B devices, this event is handled by input core; drivers should
320 instead use input_mt_report_slot_state(). 316 instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may
321 change over time while still touching the 317 change over time while still touching the device, because the firmware may
322 not be able to determine which tool is bei 318 not be able to determine which tool is being used when it first appears.
323 319
324 ABS_MT_BLOB_ID 320 ABS_MT_BLOB_ID
325 The BLOB_ID groups several packets togethe 321 The BLOB_ID groups several packets together into one arbitrarily shaped
326 contact. The sequence of points forms a po 322 contact. The sequence of points forms a polygon which defines the shape of
327 the contact. This is a low-level anonymous 323 the contact. This is a low-level anonymous grouping for type A devices, and
328 should not be confused with the high-level 324 should not be confused with the high-level trackingID [#f5]_. Most type A
329 devices do not have blob capability, so dr 325 devices do not have blob capability, so drivers can safely omit this event.
330 326
331 ABS_MT_TRACKING_ID 327 ABS_MT_TRACKING_ID
332 The TRACKING_ID identifies an initiated co 328 The TRACKING_ID identifies an initiated contact throughout its life cycle
333 [#f5]_. The value range of the TRACKING_ID 329 [#f5]_. The value range of the TRACKING_ID should be large enough to ensure
334 unique identification of a contact maintai 330 unique identification of a contact maintained over an extended period of
335 time. For type B devices, this event is ha 331 time. For type B devices, this event is handled by input core; drivers
336 should instead use input_mt_report_slot_st 332 should instead use input_mt_report_slot_state().
337 333
338 334
339 Event Computation 335 Event Computation
340 ----------------- 336 -----------------
341 337
342 The flora of different hardware unavoidably le 338 The flora of different hardware unavoidably leads to some devices fitting
343 better to the MT protocol than others. To simp 339 better to the MT protocol than others. To simplify and unify the mapping,
344 this section gives recipes for how to compute 340 this section gives recipes for how to compute certain events.
345 341
346 For devices reporting contacts as rectangular 342 For devices reporting contacts as rectangular shapes, signed orientation
347 cannot be obtained. Assuming X and Y are the l 343 cannot be obtained. Assuming X and Y are the lengths of the sides of the
348 touching rectangle, here is a simple formula t 344 touching rectangle, here is a simple formula that retains the most
349 information possible:: 345 information possible::
350 346
351 ABS_MT_TOUCH_MAJOR := max(X, Y) 347 ABS_MT_TOUCH_MAJOR := max(X, Y)
352 ABS_MT_TOUCH_MINOR := min(X, Y) 348 ABS_MT_TOUCH_MINOR := min(X, Y)
353 ABS_MT_ORIENTATION := bool(X > Y) 349 ABS_MT_ORIENTATION := bool(X > Y)
354 350
355 The range of ABS_MT_ORIENTATION should be set 351 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
356 the device can distinguish between a finger al 352 the device can distinguish between a finger along the Y axis (0) and a
357 finger along the X axis (1). 353 finger along the X axis (1).
358 354
359 For Win8 devices with both T and C coordinates !! 355 For win8 devices with both T and C coordinates, the position mapping is::
360 356
361 ABS_MT_POSITION_X := T_X 357 ABS_MT_POSITION_X := T_X
362 ABS_MT_POSITION_Y := T_Y 358 ABS_MT_POSITION_Y := T_Y
363 ABS_MT_TOOL_X := C_X 359 ABS_MT_TOOL_X := C_X
364 ABS_MT_TOOL_Y := C_Y 360 ABS_MT_TOOL_Y := C_Y
365 361
366 Unfortunately, there is not enough information 362 Unfortunately, there is not enough information to specify both the touching
367 ellipse and the tool ellipse, so one has to re 363 ellipse and the tool ellipse, so one has to resort to approximations. One
368 simple scheme, which is compatible with earlie 364 simple scheme, which is compatible with earlier usage, is::
369 365
370 ABS_MT_TOUCH_MAJOR := min(X, Y) 366 ABS_MT_TOUCH_MAJOR := min(X, Y)
371 ABS_MT_TOUCH_MINOR := <not used> 367 ABS_MT_TOUCH_MINOR := <not used>
372 ABS_MT_ORIENTATION := <not used> 368 ABS_MT_ORIENTATION := <not used>
373 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance( 369 ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
374 ABS_MT_WIDTH_MINOR := min(X, Y) 370 ABS_MT_WIDTH_MINOR := min(X, Y)
375 371
376 Rationale: We have no information about the or 372 Rationale: We have no information about the orientation of the touching
377 ellipse, so approximate it with an inscribed c 373 ellipse, so approximate it with an inscribed circle instead. The tool
378 ellipse should align with the vector (T - C), 374 ellipse should align with the vector (T - C), so the diameter must
379 increase with distance(T, C). Finally, assume 375 increase with distance(T, C). Finally, assume that the touch diameter is
380 equal to the tool thickness, and we arrive at 376 equal to the tool thickness, and we arrive at the formulas above.
381 377
382 Finger Tracking 378 Finger Tracking
383 --------------- 379 ---------------
384 380
385 The process of finger tracking, i.e., to assig 381 The process of finger tracking, i.e., to assign a unique trackingID to each
386 initiated contact on the surface, is a Euclide !! 382 initiated contact on the surface, is a Euclidian Bipartite Matching
387 problem. At each event synchronization, the s 383 problem. At each event synchronization, the set of actual contacts is
388 matched to the set of contacts from the previo 384 matched to the set of contacts from the previous synchronization. A full
389 implementation can be found in [#f3]_. 385 implementation can be found in [#f3]_.
390 386
391 387
392 Gestures 388 Gestures
393 -------- 389 --------
394 390
395 In the specific application of creating gestur 391 In the specific application of creating gesture events, the TOUCH and WIDTH
396 parameters can be used to, e.g., approximate f 392 parameters can be used to, e.g., approximate finger pressure or distinguish
397 between index finger and thumb. With the addit 393 between index finger and thumb. With the addition of the MINOR parameters,
398 one can also distinguish between a sweeping fi 394 one can also distinguish between a sweeping finger and a pointing finger,
399 and with ORIENTATION, one can detect twisting 395 and with ORIENTATION, one can detect twisting of fingers.
400 396
401 397
402 Notes 398 Notes
403 ----- 399 -----
404 400
405 In order to stay compatible with existing appl 401 In order to stay compatible with existing applications, the data reported
406 in a finger packet must not be recognized as s 402 in a finger packet must not be recognized as single-touch events.
407 403
408 For type A devices, all finger data bypasses i 404 For type A devices, all finger data bypasses input filtering, since
409 subsequent events of the same type refer to di 405 subsequent events of the same type refer to different fingers.
410 406
411 .. [#f1] Also, the difference (TOOL_X - POSITI 407 .. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
412 .. [#f2] The list can of course be extended. 408 .. [#f2] The list can of course be extended.
413 .. [#f3] The mtdev project: http://bitmath.org 409 .. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
414 .. [#f4] See the section on event computation. 410 .. [#f4] See the section on event computation.
415 .. [#f5] See the section on finger tracking. 411 .. [#f5] See the section on finger tracking.
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