Documentation/input/multi-touch-protocol.rst

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
19 The protocol is divided into two types, depending on the capabilities of the
20 hardware. For devices handling anonymous contacts (type A), the protocol
21 describes how to send the raw data for all contacts to the receiver. For
22 devices capable of tracking identifiable contacts (type B), the protocol
33 events. Only the ABS_MT events are recognized as part of a contact
35 applications, the MT protocol can be implemented on top of the ST protocol
39 input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
40 event, which instructs the receiver to accept the data for the current
44 input_mt_slot(), with a slot as argument, at the beginning of each packet.
45 This generates an ABS_MT_SLOT event, which instructs the receiver to
46 prepare for updates of the given slot.
48 All drivers mark the end of a multi-touch transfer by calling the usual
49 input_sync() function. This instructs the receiver to act upon events
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
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 hardware or computed from
57 the raw data [#f5]_.
59 For type A devices, the kernel driver should generate an arbitrary
60 enumeration of the full set of anonymous contacts currently on the
61 surface. The order in which the packets appear in the event stream is not
64 For type B devices, the kernel driver should associate a slot with each
65 identified contact, and use that slot to propagate changes for the contact.
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
72 end.  Upon receiving an MT event, one simply updates the appropriate
73 attribute of the current slot.
75 Some devices identify and/or track more contacts than they can report to the
77 contact that is reported by the hardware.  Whenever the identity of the
78 contact associated with a slot changes, the driver should invalidate that
79 slot by changing its ABS_MT_TRACKING_ID.  If the hardware signals that it is
80 tracking more contacts than it is currently reporting, the driver should use
81 a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
82 being tracked by the hardware at that moment.  The driver should do this by
83 explicitly sending the corresponding BTN_TOOL_*TAP event and setting
85 The driver should only advertise as many slots as the hardware can report.
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 absinfo for the ABS_MT_SLOT axis.
90 The minimum value of the ABS_MT_SLOT axis must be 0.
106 The sequence after moving one of the contacts looks exactly the same; the
110 Here is the sequence after lifting the first contact::
117 And here is the sequence after lifting the second contact::
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 may be omitted. Otherwise, the
124 last SYN_REPORT will be dropped by the input core, resulting in no
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::
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 contact 45, thereby
159 Finally, here is the sequence after lifting the second contact::
169 A set of ABS_MT events with the desired properties is defined. The events
170 are divided into categories, to allow for partial implementation.  The
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 be used to provide the size
174 of the contact area and approaching tool, respectively.
176 The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
177 looking through a window at someone gently holding a finger against the
178 glass.  You will see two regions, one inner region consisting of the part
179 of the finger actually touching the glass, and one outer region formed by
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 approaching finger (b) is
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 the person pressing the finger
184 harder against the glass. The touch region will increase, and in general,
185 the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
186 than unity, is related to the contact pressure. For pressure-based devices,
187 ABS_MT_PRESSURE may be used to provide the pressure on the contact area
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
214 regions can be described by adding the MINOR parameters, such that MAJOR
215 and MINOR are the major and minor axis of an ellipse. The orientation of
216 the touch ellipse can be described with the ORIENTATION parameter, and the
217 direction of the finger ellipse is given by the vector (a - b).
219 For type A devices, further specification of the touch shape is possible
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, the ABS_MT_TRACKING_ID event
226 In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
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 times Y resolution, the largest
237     possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.
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
245     tool. This should be understood as the size of the tool itself. The
246     orientation of the contact and the approaching tool are assumed to be the
250     The length, in surface units, of the minor axis of the approaching
253     The above four values can be used to derive additional information about
254     the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
255     the notion of pressure. The fingers of the hand and the palm all have
259     The pressure, in arbitrary units, on the contact area. May be used instead
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
268     The distance, in surface units, between the contact and the surface. Zero
269     distance means the contact is touching the surface. A positive number means
270     the contact is hovering above the surface.
273     The orientation of the touching ellipse. The value should describe a signed
274     quarter of a revolution clockwise around the touch center. The signed value
276     the Y axis (north) of the surface, a negative value when the ellipse is
277     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 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.
283     degree orientation, the reported orientation must exceed the range max to
287     Orientation can be omitted if the touch area is circular, or if the
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
294     The surface X coordinate of the center of the touching ellipse.
297     The surface Y coordinate of the center of the touching ellipse.
300     The surface X coordinate of the center of the approaching tool. Omit if
301     the device cannot distinguish between the intended touch point and the
305     The surface Y coordinate of the center of the approaching tool. Omit if the
306     device cannot distinguish between the intended touch point and the tool
309     The four position values can be used to separate the position of the touch
310     from the position of the tool. If both positions are present, the major
311     tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
312     aligned with the touch axes.
315     The type of approaching tool. A lot of kernel drivers cannot distinguish
316     between different tool types, such as a finger or a pen. In such cases, the
317     event should be omitted. The protocol currently mainly supports
321     change over time while still touching the device, because the firmware may
325     The BLOB_ID groups several packets together into one arbitrarily shaped
326     contact. The sequence of points forms a polygon which defines the shape of
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
332     The TRACKING_ID identifies an initiated contact throughout its life cycle
333     [#f5]_. The value range of the TRACKING_ID should be large enough to ensure
342 The flora of different hardware unavoidably leads to some devices fitting
343 better to the MT protocol than others. To simplify and unify the mapping,
347 cannot be obtained. Assuming X and Y are the lengths of the sides of the
348 touching rectangle, here is a simple formula that retains the most
355 The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
356 the device can distinguish between a finger along the Y axis (0) and a
357 finger along the X axis (1).
359 For Win8 devices with both T and C coordinates, the position mapping is::
366 Unfortunately, there is not enough information to specify both the touching
367 ellipse and the tool ellipse, so one has to resort to approximations.  One
376 Rationale: We have no information about the orientation of the touching
377 ellipse, so approximate it with an inscribed circle instead. The tool
378 ellipse should align with the vector (T - C), so the diameter must
379 increase with distance(T, C). Finally, assume that the touch diameter is
380 equal to the tool thickness, and we arrive at the formulas above.
385 The process of finger tracking, i.e., to assign a unique trackingID to each
386 initiated contact on the surface, is a Euclidean Bipartite Matching
387 problem.  At each event synchronization, the set of actual contacts is
388 matched to the set of contacts from the previous synchronization. A full
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,
405 In order to stay compatible with existing applications, the data reported
409 subsequent events of the same type refer to different fingers.
411 .. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
412 .. [#f2] The list can of course be extended.
413 .. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
414 .. [#f4] See the section on event computation.
415 .. [#f5] See the section on finger tracking.