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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
16 document describes the multi-touch (MT) protocol which allows kernel
30 --------------
34 packet. Since these events are ignored by current single-touch (ST)
48 All drivers mark the end of a multi-touch transfer by calling the usual
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
93 ------------------
95 Here is what a minimal event sequence for a two-contact touch would look
99    ABS_MT_POSITION_Y y[0]
102    ABS_MT_POSITION_Y y[1]
113    ABS_MT_POSITION_Y y[1]
125 zero-contact event reaching userland.
129 ------------------
131 Here is what a minimal event sequence for a two-contact touch would look
137    ABS_MT_POSITION_Y y[0]
141    ABS_MT_POSITION_Y y[1]
152    ABS_MT_TRACKING_ID -1
162    ABS_MT_TRACKING_ID -1
167 -----------
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
186 than unity, is related to the contact pressure. For pressure-based devices,
217 direction of the finger ellipse is given by the vector (a - b).
232 ---------------
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]_.
260     of TOUCH and WIDTH for pressure-based devices or any device with a spatial
264     If the resolution is non-zero, the pressure data is in units/gram. See
265     :ref:`input-event-codes` for details.
276     the Y axis (north) of the surface, a negative value when the ellipse is
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
284     indicate more than a quarter of a revolution. For an upside-down finger,
285     range max * 2 should be returned.
297     The surface Y coordinate of the center of the touching ellipse.
305     The surface Y coordinate of the center of the approaching tool. Omit if 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 -----------------
347 cannot be obtained. Assuming X and Y are the lengths of the sides of the
351    ABS_MT_TOUCH_MAJOR := max(X, Y)
352    ABS_MT_TOUCH_MINOR := min(X, Y)
353    ABS_MT_ORIENTATION := bool(X > Y)
356 the device can distinguish between a finger along the Y axis (0) and a
370    ABS_MT_TOUCH_MAJOR := min(X, Y)
373    ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
374    ABS_MT_WIDTH_MINOR := min(X, Y)
378 ellipse should align with the vector (T - C), so the diameter must
383 ---------------
393 --------
403 -----
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.