Documentation/input/multi-touch-protocol.rst

4 Multi-touch (MT) Protocol
7 :Copyright: |copy| 2009-2010	Henrik Rydberg <rydberg@euromail.se>
11 ------------
13 In order to utilize the full power of the new multi-touch and multi-user
15 objects in direct contact with the device surface, is needed.  This
16 document describes the multi-touch (MT) protocol which allows kernel
30 --------------
34 packet. Since these events are ignored by current single-touch (ST)
36 in an existing driver.
48 All drivers mark the end of a multi-touch transfer by calling the usual
53 The main difference between the stateless type A protocol and the stateful
54 type B slot protocol lies in the usage of identifiable contacts to reduce
61 surface. The order in which the packets appear in the event stream is not
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 denotes an unused slot.  A
71 the full state of each initiated contact has to reside in the receiving
88 total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
93 ------------------
95 Here is what a minimal event sequence for a two-contact touch would look
107 raw data for all present contacts are sent between every synchronization
122 If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
124 last SYN_REPORT will be dropped by the input core, resulting in no
125 zero-contact event reaching userland.
129 ------------------
131 Here is what a minimal event sequence for a two-contact touch would look
144 Here is the sequence after moving contact 45 in the x direction::
150 Here is the sequence after lifting the contact in slot 0::
152    ABS_MT_TRACKING_ID -1
162    ABS_MT_TRACKING_ID -1
167 -----------
184 harder against the glass. The touch region will increase, and in general,
186 than unity, is related to the contact pressure. For pressure-based devices,
189 indicate the distance between the contact and the surface.
213 In addition to the MAJOR parameters, the oval shape of the touch and finger
217 direction of the finger ellipse is given by the vector (a - b).
226 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
232 ---------------
235     The length of the major axis of the contact. The length should be given in
240     The length, in surface units, of the minor axis of the contact. If the
244     The length, in surface units, of the major axis of the approaching
250     The length, in surface units, of the minor axis of the approaching
259     The pressure, in arbitrary units, on the contact area. May be used instead
260     of TOUCH and WIDTH for pressure-based devices or any device with a spatial
263     If the resolution is zero, the pressure data is in arbitrary units.
264     If the resolution is non-zero, the pressure data is in units/gram. See
265     :ref:`input-event-codes` for details.
268     The distance, in surface units, between the contact and the surface. Zero
278     right. When aligned with the X axis in the positive direction, the range
279     max should be returned; when aligned with the X axis in the negative
280     direction, the range -max should be returned.
284     indicate more than a quarter of a revolution. For an upside-down finger,
288     information is not available in the kernel driver. Partial orientation
289     support is possible if the device can distinguish between the two axes, but
290     not (uniquely) any values in between. In such cases, the range of
301     the device cannot distinguish between the intended touch point and the
306     device cannot distinguish between the intended touch point and the tool
316     between different tool types, such as a finger or a pen. In such cases, the
327     the contact. This is a low-level anonymous grouping for type A devices, and
328     should not be confused with the high-level trackingID [#f5]_. Most type A
340 -----------------
356 the device can distinguish between a finger along the Y axis (0) and a
378 ellipse should align with the vector (T - C), so the diameter must
383 ---------------
389 implementation can be found in [#f3]_.
393 --------
395 In the specific application of creating gesture events, the TOUCH and WIDTH
397 between index finger and thumb. With the addition of the MINOR parameters,
398 one can also distinguish between a sweeping finger and a pointing finger,
403 -----
405 In order to stay compatible with existing applications, the data reported
406 in a finger packet must not be recognized as single-touch events.
411 .. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.