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1 | Multi-touch (MT) Protocol |
2 | ------------------------- | |
3 | Copyright (C) 2009 Henrik Rydberg <rydberg@euromail.se> | |
4 | ||
5 | ||
6 | Introduction | |
7 | ------------ | |
8 | ||
9 | In order to utilize the full power of the new multi-touch devices, a way to | |
10 | report detailed finger data to user space is needed. This document | |
11 | describes the multi-touch (MT) protocol which allows kernel drivers to | |
12 | report details for an arbitrary number of fingers. | |
13 | ||
14 | ||
15 | Usage | |
16 | ----- | |
17 | ||
18 | Anonymous finger details are sent sequentially as separate packets of ABS | |
19 | events. Only the ABS_MT events are recognized as part of a finger | |
20 | packet. The end of a packet is marked by calling the input_mt_sync() | |
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21 | function, which generates a SYN_MT_REPORT event. This instructs the |
22 | receiver to accept the data for the current finger and prepare to receive | |
23 | another. The end of a multi-touch transfer is marked by calling the usual | |
24 | input_sync() function. This instructs the receiver to act upon events | |
25 | accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new | |
26 | set of events/packets. | |
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27 | |
28 | A set of ABS_MT events with the desired properties is defined. The events | |
29 | are divided into categories, to allow for partial implementation. The | |
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30 | minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which |
31 | allows for multiple fingers to be tracked. If the device supports it, the | |
32 | ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size | |
33 | of the contact area and approaching finger, respectively. | |
34 | ||
35 | The TOUCH and WIDTH parameters have a geometrical interpretation; imagine | |
36 | looking through a window at someone gently holding a finger against the | |
37 | glass. You will see two regions, one inner region consisting of the part | |
38 | of the finger actually touching the glass, and one outer region formed by | |
39 | the perimeter of the finger. The diameter of the inner region is the | |
40 | ABS_MT_TOUCH_MAJOR, the diameter of the outer region is | |
41 | ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder | |
42 | against the glass. The inner region will increase, and in general, the | |
43 | ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than | |
44 | unity, is related to the finger pressure. For pressure-based devices, | |
45 | ABS_MT_PRESSURE may be used to provide the pressure on the contact area | |
46 | instead. | |
47 | ||
48 | In addition to the MAJOR parameters, the oval shape of the finger can be | |
49 | described by adding the MINOR parameters, such that MAJOR and MINOR are the | |
50 | major and minor axis of an ellipse. Finally, the orientation of the oval | |
51 | shape can be describe with the ORIENTATION parameter. | |
52 | ||
53 | The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a | |
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54 | finger or a pen or something else. Devices with more granular information |
55 | may specify general shapes as blobs, i.e., as a sequence of rectangular | |
56 | shapes grouped together by an ABS_MT_BLOB_ID. Finally, for the few devices | |
57 | that currently support it, the ABS_MT_TRACKING_ID event may be used to | |
58 | report finger tracking from hardware [5]. | |
59 | ||
60 | Here is what a minimal event sequence for a two-finger touch would look | |
61 | like: | |
62 | ||
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63 | ABS_MT_POSITION_X |
64 | ABS_MT_POSITION_Y | |
65 | SYN_MT_REPORT | |
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66 | ABS_MT_POSITION_X |
67 | ABS_MT_POSITION_Y | |
68 | SYN_MT_REPORT | |
69 | SYN_REPORT | |
eacaad01 | 70 | |
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71 | Here is the sequence after lifting one of the fingers: |
72 | ||
73 | ABS_MT_POSITION_X | |
74 | ABS_MT_POSITION_Y | |
75 | SYN_MT_REPORT | |
76 | SYN_REPORT | |
77 | ||
78 | And here is the sequence after lifting the remaining finger: | |
79 | ||
80 | SYN_MT_REPORT | |
81 | SYN_REPORT | |
82 | ||
83 | If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the | |
84 | ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the | |
85 | last SYN_REPORT will be dropped by the input core, resulting in no | |
86 | zero-finger event reaching userland. | |
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87 | |
88 | Event Semantics | |
89 | --------------- | |
90 | ||
91 | The word "contact" is used to describe a tool which is in direct contact | |
92 | with the surface. A finger, a pen or a rubber all classify as contacts. | |
93 | ||
94 | ABS_MT_TOUCH_MAJOR | |
95 | ||
96 | The length of the major axis of the contact. The length should be given in | |
97 | surface units. If the surface has an X times Y resolution, the largest | |
f9fcfc3b | 98 | possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4]. |
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99 | |
100 | ABS_MT_TOUCH_MINOR | |
101 | ||
102 | The length, in surface units, of the minor axis of the contact. If the | |
f9fcfc3b | 103 | contact is circular, this event can be omitted [4]. |
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104 | |
105 | ABS_MT_WIDTH_MAJOR | |
106 | ||
107 | The length, in surface units, of the major axis of the approaching | |
108 | tool. This should be understood as the size of the tool itself. The | |
109 | orientation of the contact and the approaching tool are assumed to be the | |
f9fcfc3b | 110 | same [4]. |
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111 | |
112 | ABS_MT_WIDTH_MINOR | |
113 | ||
114 | The length, in surface units, of the minor axis of the approaching | |
f9fcfc3b | 115 | tool. Omit if circular [4]. |
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116 | |
117 | The above four values can be used to derive additional information about | |
118 | the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates | |
119 | the notion of pressure. The fingers of the hand and the palm all have | |
120 | different characteristic widths [1]. | |
121 | ||
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122 | ABS_MT_PRESSURE |
123 | ||
124 | The pressure, in arbitrary units, on the contact area. May be used instead | |
125 | of TOUCH and WIDTH for pressure-based devices or any device with a spatial | |
126 | signal intensity distribution. | |
127 | ||
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128 | ABS_MT_ORIENTATION |
129 | ||
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130 | The orientation of the ellipse. The value should describe a signed quarter |
131 | of a revolution clockwise around the touch center. The signed value range | |
132 | is arbitrary, but zero should be returned for a finger aligned along the Y | |
133 | axis of the surface, a negative value when finger is turned to the left, and | |
134 | a positive value when finger turned to the right. When completely aligned with | |
135 | the X axis, the range max should be returned. Orientation can be omitted | |
136 | if the touching object is circular, or if the information is not available | |
137 | in the kernel driver. Partial orientation support is possible if the device | |
138 | can distinguish between the two axis, but not (uniquely) any values in | |
139 | between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1] | |
140 | [4]. | |
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141 | |
142 | ABS_MT_POSITION_X | |
143 | ||
144 | The surface X coordinate of the center of the touching ellipse. | |
145 | ||
146 | ABS_MT_POSITION_Y | |
147 | ||
148 | The surface Y coordinate of the center of the touching ellipse. | |
149 | ||
150 | ABS_MT_TOOL_TYPE | |
151 | ||
152 | The type of approaching tool. A lot of kernel drivers cannot distinguish | |
153 | between different tool types, such as a finger or a pen. In such cases, the | |
154 | event should be omitted. The protocol currently supports MT_TOOL_FINGER and | |
155 | MT_TOOL_PEN [2]. | |
156 | ||
157 | ABS_MT_BLOB_ID | |
158 | ||
159 | The BLOB_ID groups several packets together into one arbitrarily shaped | |
160 | contact. This is a low-level anonymous grouping, and should not be confused | |
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161 | with the high-level trackingID [5]. Most kernel drivers will not have blob |
162 | capability, and can safely omit the event. | |
163 | ||
164 | ABS_MT_TRACKING_ID | |
165 | ||
166 | The TRACKING_ID identifies an initiated contact throughout its life cycle | |
167 | [5]. There are currently only a few devices that support it, so this event | |
168 | should normally be omitted. | |
169 | ||
170 | ||
171 | Event Computation | |
172 | ----------------- | |
173 | ||
174 | The flora of different hardware unavoidably leads to some devices fitting | |
175 | better to the MT protocol than others. To simplify and unify the mapping, | |
176 | this section gives recipes for how to compute certain events. | |
177 | ||
178 | For devices reporting contacts as rectangular shapes, signed orientation | |
179 | cannot be obtained. Assuming X and Y are the lengths of the sides of the | |
180 | touching rectangle, here is a simple formula that retains the most | |
181 | information possible: | |
182 | ||
183 | ABS_MT_TOUCH_MAJOR := max(X, Y) | |
184 | ABS_MT_TOUCH_MINOR := min(X, Y) | |
185 | ABS_MT_ORIENTATION := bool(X > Y) | |
186 | ||
187 | The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that | |
188 | the device can distinguish between a finger along the Y axis (0) and a | |
189 | finger along the X axis (1). | |
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190 | |
191 | ||
192 | Finger Tracking | |
193 | --------------- | |
194 | ||
195 | The kernel driver should generate an arbitrary enumeration of the set of | |
196 | anonymous contacts currently on the surface. The order in which the packets | |
197 | appear in the event stream is not important. | |
198 | ||
f9fcfc3b | 199 | The process of finger tracking, i.e., to assign a unique trackingID to each |
eacaad01 | 200 | initiated contact on the surface, is left to user space; preferably the |
f9fcfc3b | 201 | multi-touch X driver [3]. In that driver, the trackingID stays the same and |
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202 | unique until the contact vanishes (when the finger leaves the surface). The |
203 | problem of assigning a set of anonymous fingers to a set of identified | |
204 | fingers is a euclidian bipartite matching problem at each event update, and | |
205 | relies on a sufficiently rapid update rate. | |
206 | ||
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207 | There are a few devices that support trackingID in hardware. User space can |
208 | make use of these native identifiers to reduce bandwidth and cpu usage. | |
209 | ||
210 | ||
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211 | Gestures |
212 | -------- | |
213 | ||
214 | In the specific application of creating gesture events, the TOUCH and WIDTH | |
215 | parameters can be used to, e.g., approximate finger pressure or distinguish | |
216 | between index finger and thumb. With the addition of the MINOR parameters, | |
217 | one can also distinguish between a sweeping finger and a pointing finger, | |
218 | and with ORIENTATION, one can detect twisting of fingers. | |
219 | ||
220 | ||
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221 | Notes |
222 | ----- | |
223 | ||
224 | In order to stay compatible with existing applications, the data | |
225 | reported in a finger packet must not be recognized as single-touch | |
226 | events. In addition, all finger data must bypass input filtering, | |
227 | since subsequent events of the same type refer to different fingers. | |
228 | ||
229 | The first kernel driver to utilize the MT protocol is the bcm5974 driver, | |
230 | where examples can be found. | |
231 | ||
232 | [1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the | |
233 | difference between the contact position and the approaching tool position | |
234 | could be used to derive tilt. | |
235 | [2] The list can of course be extended. | |
13bad37b | 236 | [3] Multitouch X driver project: http://bitmath.org/code/multitouch/. |
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237 | [4] See the section on event computation. |
238 | [5] See the section on finger tracking. |