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+Multi-touch (MT) Protocol
+-------------------------
+	Copyright (C) 2009-2010	Henrik Rydberg <rydberg@euromail.se>
+
+
+Introduction
+------------
+
+In order to utilize the full power of the new multi-touch and multi-user
+devices, a way to report detailed data from multiple contacts, i.e.,
+objects in direct contact with the device surface, is needed.  This
+document describes the multi-touch (MT) protocol which allows kernel
+drivers to report details for an arbitrary number of contacts.
+
+The protocol is divided into two types, depending on the capabilities of the
+hardware. For devices handling anonymous contacts (type A), the protocol
+describes how to send the raw data for all contacts to the receiver. For
+devices capable of tracking identifiable contacts (type B), the protocol
+describes how to send updates for individual contacts via event slots.
+
+
+Protocol Usage
+--------------
+
+Contact details are sent sequentially as separate packets of ABS_MT
+events. Only the ABS_MT events are recognized as part of a contact
+packet. Since these events are ignored by current single-touch (ST)
+applications, the MT protocol can be implemented on top of the ST protocol
+in an existing driver.
+
+Drivers for type A devices separate contact packets by calling
+input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
+event, which instructs the receiver to accept the data for the current
+contact and prepare to receive another.
+
+Drivers for type B devices separate contact packets by calling
+input_mt_slot(), with a slot as argument, at the beginning of each packet.
+This generates an ABS_MT_SLOT event, which instructs the receiver to
+prepare for updates of the given slot.
+
+All drivers mark the end of a multi-touch transfer by calling the usual
+input_sync() function. This instructs the receiver to act upon events
+accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
+of events/packets.
+
+The main difference between the stateless type A protocol and the stateful
+type B slot protocol lies in the usage of identifiable contacts to reduce
+the amount of data sent to userspace. The slot protocol requires the use of
+the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
+the raw data [5].
+
+For type A devices, the kernel driver should generate an arbitrary
+enumeration of the full set of anonymous contacts currently on the
+surface. The order in which the packets appear in the event stream is not
+important.  Event filtering and finger tracking is left to user space [3].
+
+For type B devices, the kernel driver should associate a slot with each
+identified contact, and use that slot to propagate changes for the contact.
+Creation, replacement and destruction of contacts is achieved by modifying
+the ABS_MT_TRACKING_ID of the associated slot.  A non-negative tracking id
+is interpreted as a contact, and the value -1 denotes an unused slot.  A
+tracking id not previously present is considered new, and a tracking id no
+longer present is considered removed.  Since only changes are propagated,
+the full state of each initiated contact has to reside in the receiving
+end.  Upon receiving an MT event, one simply updates the appropriate
+attribute of the current slot.
+
+Some devices identify and/or track more contacts than they can report to the
+driver.  A driver for such a device should associate one type B slot with each
+contact that is reported by the hardware.  Whenever the identity of the
+contact associated with a slot changes, the driver should invalidate that
+slot by changing its ABS_MT_TRACKING_ID.  If the hardware signals that it is
+tracking more contacts than it is currently reporting, the driver should use
+a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
+being tracked by the hardware at that moment.  The driver should do this by
+explicitly sending the corresponding BTN_TOOL_*TAP event and setting
+use_count to false when calling input_mt_report_pointer_emulation().
+The driver should only advertise as many slots as the hardware can report.
+Userspace can detect that a driver can report more total contacts than slots
+by noting that the largest supported BTN_TOOL_*TAP event is larger than the
+total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
+
+The minimum value of the ABS_MT_SLOT axis must be 0.
+
+Protocol Example A
+------------------
+
+Here is what a minimal event sequence for a two-contact touch would look
+like for a type A device:
+
+   ABS_MT_POSITION_X x[0]
+   ABS_MT_POSITION_Y y[0]
+   SYN_MT_REPORT
+   ABS_MT_POSITION_X x[1]
+   ABS_MT_POSITION_Y y[1]
+   SYN_MT_REPORT
+   SYN_REPORT
+
+The sequence after moving one of the contacts looks exactly the same; the
+raw data for all present contacts are sent between every synchronization
+with SYN_REPORT.
+
+Here is the sequence after lifting the first contact:
+
+   ABS_MT_POSITION_X x[1]
+   ABS_MT_POSITION_Y y[1]
+   SYN_MT_REPORT
+   SYN_REPORT
+
+And here is the sequence after lifting the second contact:
+
+   SYN_MT_REPORT
+   SYN_REPORT
+
+If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
+ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
+last SYN_REPORT will be dropped by the input core, resulting in no
+zero-contact event reaching userland.
+
+
+Protocol Example B
+------------------
+
+Here is what a minimal event sequence for a two-contact touch would look
+like for a type B device:
+
+   ABS_MT_SLOT 0
+   ABS_MT_TRACKING_ID 45
+   ABS_MT_POSITION_X x[0]
+   ABS_MT_POSITION_Y y[0]
+   ABS_MT_SLOT 1
+   ABS_MT_TRACKING_ID 46
+   ABS_MT_POSITION_X x[1]
+   ABS_MT_POSITION_Y y[1]
+   SYN_REPORT
+
+Here is the sequence after moving contact 45 in the x direction:
+
+   ABS_MT_SLOT 0
+   ABS_MT_POSITION_X x[0]
+   SYN_REPORT
+
+Here is the sequence after lifting the contact in slot 0:
+
+   ABS_MT_TRACKING_ID -1
+   SYN_REPORT
+
+The slot being modified is already 0, so the ABS_MT_SLOT is omitted.  The
+message removes the association of slot 0 with contact 45, thereby
+destroying contact 45 and freeing slot 0 to be reused for another contact.
+
+Finally, here is the sequence after lifting the second contact:
+
+   ABS_MT_SLOT 1
+   ABS_MT_TRACKING_ID -1
+   SYN_REPORT
+
+
+Event Usage
+-----------
+
+A set of ABS_MT events with the desired properties is defined. The events
+are divided into categories, to allow for partial implementation.  The
+minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
+allows for multiple contacts to be tracked.  If the device supports it, the
+ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
+of the contact area and approaching tool, respectively.
+
+The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
+looking through a window at someone gently holding a finger against the
+glass.  You will see two regions, one inner region consisting of the part
+of the finger actually touching the glass, and one outer region formed by
+the perimeter of the finger. The center of the touching region (a) is
+ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
+ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
+diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
+harder against the glass. The touch region will increase, and in general,
+the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
+than unity, is related to the contact pressure. For pressure-based devices,
+ABS_MT_PRESSURE may be used to provide the pressure on the contact area
+instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
+indicate the distance between the contact and the surface.
+
+
+	  Linux MT                               Win8
+         __________                     _______________________
+        /          \                   |                       |
+       /            \                  |                       |
+      /     ____     \                 |                       |
+     /     /    \     \                |                       |
+     \     \  a  \     \               |       a               |
+      \     \____/      \              |                       |
+       \                 \             |                       |
+        \        b        \            |           b           |
+         \                 \           |                       |
+          \                 \          |                       |
+           \                 \         |                       |
+            \                /         |                       |
+             \              /          |                       |
+              \            /           |                       |
+               \__________/            |_______________________|
+
+
+In addition to the MAJOR parameters, the oval shape of the touch and finger
+regions can be described by adding the MINOR parameters, such that MAJOR
+and MINOR are the major and minor axis of an ellipse. The orientation of
+the touch ellipse can be described with the ORIENTATION parameter, and the
+direction of the finger ellipse is given by the vector (a - b).
+
+For type A devices, further specification of the touch shape is possible
+via ABS_MT_BLOB_ID.
+
+The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
+finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
+may be used to track identified contacts over time [5].
+
+In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
+implicitly handled by input core; drivers should instead call
+input_mt_report_slot_state().
+
+
+Event Semantics
+---------------
+
+ABS_MT_TOUCH_MAJOR
+
+The length of the major axis of the contact. The length should be given in
+surface units. If the surface has an X times Y resolution, the largest
+possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
+
+ABS_MT_TOUCH_MINOR
+
+The length, in surface units, of the minor axis of the contact. If the
+contact is circular, this event can be omitted [4].
+
+ABS_MT_WIDTH_MAJOR
+
+The length, in surface units, of the major axis of the approaching
+tool. This should be understood as the size of the tool itself. The
+orientation of the contact and the approaching tool are assumed to be the
+same [4].
+
+ABS_MT_WIDTH_MINOR
+
+The length, in surface units, of the minor axis of the approaching
+tool. Omit if circular [4].
+
+The above four values can be used to derive additional information about
+the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
+the notion of pressure. The fingers of the hand and the palm all have
+different characteristic widths.
+
+ABS_MT_PRESSURE
+
+The pressure, in arbitrary units, on the contact area. May be used instead
+of TOUCH and WIDTH for pressure-based devices or any device with a spatial
+signal intensity distribution.
+
+ABS_MT_DISTANCE
+
+The distance, in surface units, between the contact and the surface. Zero
+distance means the contact is touching the surface. A positive number means
+the contact is hovering above the surface.
+
+ABS_MT_ORIENTATION
+
+The orientation of the touching ellipse. The value should describe a signed
+quarter of a revolution clockwise around the touch center. The signed value
+range is arbitrary, but zero should be returned for an ellipse aligned with
+the Y axis of the surface, a negative value when the ellipse is turned to
+the left, and a positive value when the ellipse is turned to the
+right. When completely aligned with the X axis, the range max should be
+returned.
+
+Touch ellipsis are symmetrical by default. For devices capable of true 360
+degree orientation, the reported orientation must exceed the range max to
+indicate more than a quarter of a revolution. For an upside-down finger,
+range max * 2 should be returned.
+
+Orientation can be omitted if the touch area is circular, or if the
+information is not available in the kernel driver. Partial orientation
+support is possible if the device can distinguish between the two axis, but
+not (uniquely) any values in between. In such cases, the range of
+ABS_MT_ORIENTATION should be [0, 1] [4].
+
+ABS_MT_POSITION_X
+
+The surface X coordinate of the center of the touching ellipse.
+
+ABS_MT_POSITION_Y
+
+The surface Y coordinate of the center of the touching ellipse.
+
+ABS_MT_TOOL_X
+
+The surface X coordinate of the center of the approaching tool. Omit if
+the device cannot distinguish between the intended touch point and the
+tool itself.
+
+ABS_MT_TOOL_Y
+
+The surface Y coordinate of the center of the approaching tool. Omit if the
+device cannot distinguish between the intended touch point and the tool
+itself.
+
+The four position values can be used to separate the position of the touch
+from the position of the tool. If both positions are present, the major
+tool axis points towards the touch point [1]. Otherwise, the tool axes are
+aligned with the touch axes.
+
+ABS_MT_TOOL_TYPE
+
+The type of approaching tool. A lot of kernel drivers cannot distinguish
+between different tool types, such as a finger or a pen. In such cases, the
+event should be omitted. The protocol currently supports MT_TOOL_FINGER,
+MT_TOOL_PEN, and MT_TOOL_PALM [2]. For type B devices, this event is handled
+by input core; drivers should instead use input_mt_report_slot_state().
+A contact's ABS_MT_TOOL_TYPE may change over time while still touching the
+device, because the firmware may not be able to determine which tool is being
+used when it first appears.
+
+ABS_MT_BLOB_ID
+
+The BLOB_ID groups several packets together into one arbitrarily shaped
+contact. The sequence of points forms a polygon which defines the shape of
+the contact. This is a low-level anonymous grouping for type A devices, and
+should not be confused with the high-level trackingID [5]. Most type A
+devices do not have blob capability, so drivers can safely omit this event.
+
+ABS_MT_TRACKING_ID
+
+The TRACKING_ID identifies an initiated contact throughout its life cycle
+[5]. The value range of the TRACKING_ID should be large enough to ensure
+unique identification of a contact maintained over an extended period of
+time. For type B devices, this event is handled by input core; drivers
+should instead use input_mt_report_slot_state().
+
+
+Event Computation
+-----------------
+
+The flora of different hardware unavoidably leads to some devices fitting
+better to the MT protocol than others. To simplify and unify the mapping,
+this section gives recipes for how to compute certain events.
+
+For devices reporting contacts as rectangular shapes, signed orientation
+cannot be obtained. Assuming X and Y are the lengths of the sides of the
+touching rectangle, here is a simple formula that retains the most
+information possible:
+
+   ABS_MT_TOUCH_MAJOR := max(X, Y)
+   ABS_MT_TOUCH_MINOR := min(X, Y)
+   ABS_MT_ORIENTATION := bool(X > Y)
+
+The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
+the device can distinguish between a finger along the Y axis (0) and a
+finger along the X axis (1).
+
+For win8 devices with both T and C coordinates, the position mapping is
+
+   ABS_MT_POSITION_X := T_X
+   ABS_MT_POSITION_Y := T_Y
+   ABS_MT_TOOL_X := C_X
+   ABS_MT_TOOL_X := C_Y
+
+Unfortunately, there is not enough information to specify both the touching
+ellipse and the tool ellipse, so one has to resort to approximations.  One
+simple scheme, which is compatible with earlier usage, is:
+
+   ABS_MT_TOUCH_MAJOR := min(X, Y)
+   ABS_MT_TOUCH_MINOR := <not used>
+   ABS_MT_ORIENTATION := <not used>
+   ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
+   ABS_MT_WIDTH_MINOR := min(X, Y)
+
+Rationale: We have no information about the orientation of the touching
+ellipse, so approximate it with an inscribed circle instead. The tool
+ellipse should align with the vector (T - C), so the diameter must
+increase with distance(T, C). Finally, assume that the touch diameter is
+equal to the tool thickness, and we arrive at the formulas above.
+
+Finger Tracking
+---------------
+
+The process of finger tracking, i.e., to assign a unique trackingID to each
+initiated contact on the surface, is a Euclidian Bipartite Matching
+problem.  At each event synchronization, the set of actual contacts is
+matched to the set of contacts from the previous synchronization. A full
+implementation can be found in [3].
+
+
+Gestures
+--------
+
+In the specific application of creating gesture events, the TOUCH and WIDTH
+parameters can be used to, e.g., approximate finger pressure or distinguish
+between index finger and thumb. With the addition of the MINOR parameters,
+one can also distinguish between a sweeping finger and a pointing finger,
+and with ORIENTATION, one can detect twisting of fingers.
+
+
+Notes
+-----
+
+In order to stay compatible with existing applications, the data reported
+in a finger packet must not be recognized as single-touch events.
+
+For type A devices, all finger data bypasses input filtering, since
+subsequent events of the same type refer to different fingers.
+
+For example usage of the type A protocol, see the bcm5974 driver. For
+example usage of the type B protocol, see the hid-egalax driver.
+
+[1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
+[2] The list can of course be extended.
+[3] The mtdev project: http://bitmath.org/code/mtdev/.
+[4] See the section on event computation.
+[5] See the section on finger tracking.