416 lines
18 KiB
ReStructuredText
416 lines
18 KiB
ReStructuredText
.. include:: <isonum.txt>
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=========================
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Multi-touch (MT) Protocol
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=========================
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:Copyright: |copy| 2009-2010 Henrik Rydberg <rydberg@euromail.se>
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Introduction
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------------
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In order to utilize the full power of the new multi-touch and multi-user
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devices, a way to report detailed data from multiple contacts, i.e.,
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objects in direct contact with the device surface, is needed. This
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document describes the multi-touch (MT) protocol which allows kernel
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drivers to report details for an arbitrary number of contacts.
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The protocol is divided into two types, depending on the capabilities of the
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hardware. For devices handling anonymous contacts (type A), the protocol
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describes how to send the raw data for all contacts to the receiver. For
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devices capable of tracking identifiable contacts (type B), the protocol
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describes how to send updates for individual contacts via event slots.
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.. note::
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MT protocol type A is obsolete, all kernel drivers have been
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converted to use type B.
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Protocol Usage
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--------------
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Contact details are sent sequentially as separate packets of ABS_MT
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events. Only the ABS_MT events are recognized as part of a contact
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packet. Since these events are ignored by current single-touch (ST)
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applications, the MT protocol can be implemented on top of the ST protocol
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in an existing driver.
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Drivers for type A devices separate contact packets by calling
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input_mt_sync() at the end of each packet. This generates a SYN_MT_REPORT
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event, which instructs the receiver to accept the data for the current
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contact and prepare to receive another.
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Drivers for type B devices separate contact packets by calling
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input_mt_slot(), with a slot as argument, at the beginning of each packet.
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This generates an ABS_MT_SLOT event, which instructs the receiver to
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prepare for updates of the given slot.
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All drivers mark the end of a multi-touch transfer by calling the usual
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input_sync() function. This instructs the receiver to act upon events
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accumulated since last EV_SYN/SYN_REPORT and prepare to receive a new set
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of events/packets.
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The main difference between the stateless type A protocol and the stateful
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type B slot protocol lies in the usage of identifiable contacts to reduce
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the amount of data sent to userspace. The slot protocol requires the use of
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the ABS_MT_TRACKING_ID, either provided by the hardware or computed from
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the raw data [#f5]_.
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For type A devices, the kernel driver should generate an arbitrary
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enumeration of the full set of anonymous contacts currently on the
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surface. The order in which the packets appear in the event stream is not
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important. Event filtering and finger tracking is left to user space [#f3]_.
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For type B devices, the kernel driver should associate a slot with each
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identified contact, and use that slot to propagate changes for the contact.
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Creation, replacement and destruction of contacts is achieved by modifying
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the ABS_MT_TRACKING_ID of the associated slot. A non-negative tracking id
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is interpreted as a contact, and the value -1 denotes an unused slot. A
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tracking id not previously present is considered new, and a tracking id no
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longer present is considered removed. Since only changes are propagated,
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the full state of each initiated contact has to reside in the receiving
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end. Upon receiving an MT event, one simply updates the appropriate
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attribute of the current slot.
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Some devices identify and/or track more contacts than they can report to the
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driver. A driver for such a device should associate one type B slot with each
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contact that is reported by the hardware. Whenever the identity of the
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contact associated with a slot changes, the driver should invalidate that
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slot by changing its ABS_MT_TRACKING_ID. If the hardware signals that it is
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tracking more contacts than it is currently reporting, the driver should use
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a BTN_TOOL_*TAP event to inform userspace of the total number of contacts
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being tracked by the hardware at that moment. The driver should do this by
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explicitly sending the corresponding BTN_TOOL_*TAP event and setting
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use_count to false when calling input_mt_report_pointer_emulation().
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The driver should only advertise as many slots as the hardware can report.
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Userspace can detect that a driver can report more total contacts than slots
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by noting that the largest supported BTN_TOOL_*TAP event is larger than the
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total number of type B slots reported in the absinfo for the ABS_MT_SLOT axis.
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The minimum value of the ABS_MT_SLOT axis must be 0.
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Protocol Example A
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------------------
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Here is what a minimal event sequence for a two-contact touch would look
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like for a type A device::
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ABS_MT_POSITION_X x[0]
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ABS_MT_POSITION_Y y[0]
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SYN_MT_REPORT
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ABS_MT_POSITION_X x[1]
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ABS_MT_POSITION_Y y[1]
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SYN_MT_REPORT
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SYN_REPORT
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The sequence after moving one of the contacts looks exactly the same; the
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raw data for all present contacts are sent between every synchronization
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with SYN_REPORT.
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Here is the sequence after lifting the first contact::
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ABS_MT_POSITION_X x[1]
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ABS_MT_POSITION_Y y[1]
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SYN_MT_REPORT
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SYN_REPORT
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And here is the sequence after lifting the second contact::
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SYN_MT_REPORT
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SYN_REPORT
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If the driver reports one of BTN_TOUCH or ABS_PRESSURE in addition to the
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ABS_MT events, the last SYN_MT_REPORT event may be omitted. Otherwise, the
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last SYN_REPORT will be dropped by the input core, resulting in no
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zero-contact event reaching userland.
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Protocol Example B
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------------------
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Here is what a minimal event sequence for a two-contact touch would look
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like for a type B device::
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ABS_MT_SLOT 0
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ABS_MT_TRACKING_ID 45
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ABS_MT_POSITION_X x[0]
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ABS_MT_POSITION_Y y[0]
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ABS_MT_SLOT 1
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ABS_MT_TRACKING_ID 46
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ABS_MT_POSITION_X x[1]
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ABS_MT_POSITION_Y y[1]
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SYN_REPORT
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Here is the sequence after moving contact 45 in the x direction::
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ABS_MT_SLOT 0
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ABS_MT_POSITION_X x[0]
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SYN_REPORT
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Here is the sequence after lifting the contact in slot 0::
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ABS_MT_TRACKING_ID -1
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SYN_REPORT
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The slot being modified is already 0, so the ABS_MT_SLOT is omitted. The
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message removes the association of slot 0 with contact 45, thereby
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destroying contact 45 and freeing slot 0 to be reused for another contact.
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Finally, here is the sequence after lifting the second contact::
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ABS_MT_SLOT 1
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ABS_MT_TRACKING_ID -1
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SYN_REPORT
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Event Usage
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-----------
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A set of ABS_MT events with the desired properties is defined. The events
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are divided into categories, to allow for partial implementation. The
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minimum set consists of ABS_MT_POSITION_X and ABS_MT_POSITION_Y, which
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allows for multiple contacts to be tracked. If the device supports it, the
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ABS_MT_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
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of the contact area and approaching tool, respectively.
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The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
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looking through a window at someone gently holding a finger against the
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glass. You will see two regions, one inner region consisting of the part
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of the finger actually touching the glass, and one outer region formed by
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the perimeter of the finger. The center of the touching region (a) is
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ABS_MT_POSITION_X/Y and the center of the approaching finger (b) is
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ABS_MT_TOOL_X/Y. The touch diameter is ABS_MT_TOUCH_MAJOR and the finger
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diameter is ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger
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harder against the glass. The touch region will increase, and in general,
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the ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller
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than unity, is related to the contact pressure. For pressure-based devices,
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ABS_MT_PRESSURE may be used to provide the pressure on the contact area
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instead. Devices capable of contact hovering can use ABS_MT_DISTANCE to
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indicate the distance between the contact and the surface.
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::
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Linux MT Win8
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__________ _______________________
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/ \ | |
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/ \ | |
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/ ____ \ | |
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/ / \ \ | |
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\ \ a \ \ | a |
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\ \____/ \ | |
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\ \ | |
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\ b \ | b |
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\ \ | |
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\ \ | |
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\ \ | |
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\ / | |
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\ / | |
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\ / | |
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\__________/ |_______________________|
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In addition to the MAJOR parameters, the oval shape of the touch and finger
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regions can be described by adding the MINOR parameters, such that MAJOR
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and MINOR are the major and minor axis of an ellipse. The orientation of
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the touch ellipse can be described with the ORIENTATION parameter, and the
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direction of the finger ellipse is given by the vector (a - b).
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For type A devices, further specification of the touch shape is possible
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via ABS_MT_BLOB_ID.
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The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
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finger or a pen or something else. Finally, the ABS_MT_TRACKING_ID event
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may be used to track identified contacts over time [#f5]_.
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In the type B protocol, ABS_MT_TOOL_TYPE and ABS_MT_TRACKING_ID are
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implicitly handled by input core; drivers should instead call
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input_mt_report_slot_state().
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Event Semantics
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---------------
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ABS_MT_TOUCH_MAJOR
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The length of the major axis of the contact. The length should be given in
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surface units. If the surface has an X times Y resolution, the largest
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possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [#f4]_.
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ABS_MT_TOUCH_MINOR
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The length, in surface units, of the minor axis of the contact. If the
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contact is circular, this event can be omitted [#f4]_.
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ABS_MT_WIDTH_MAJOR
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The length, in surface units, of the major axis of the approaching
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tool. This should be understood as the size of the tool itself. The
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orientation of the contact and the approaching tool are assumed to be the
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same [#f4]_.
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ABS_MT_WIDTH_MINOR
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The length, in surface units, of the minor axis of the approaching
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tool. Omit if circular [#f4]_.
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The above four values can be used to derive additional information about
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the contact. The ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR approximates
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the notion of pressure. The fingers of the hand and the palm all have
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different characteristic widths.
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ABS_MT_PRESSURE
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The pressure, in arbitrary units, on the contact area. May be used instead
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of TOUCH and WIDTH for pressure-based devices or any device with a spatial
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signal intensity distribution.
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If the resolution is zero, the pressure data is in arbitrary units.
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If the resolution is non-zero, the pressure data is in units/gram. See
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:ref:`input-event-codes` for details.
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ABS_MT_DISTANCE
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The distance, in surface units, between the contact and the surface. Zero
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distance means the contact is touching the surface. A positive number means
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the contact is hovering above the surface.
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ABS_MT_ORIENTATION
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The orientation of the touching ellipse. The value should describe a signed
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quarter of a revolution clockwise around the touch center. The signed value
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range is arbitrary, but zero should be returned for an ellipse aligned with
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the Y axis (north) of the surface, a negative value when the ellipse is
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turned to the left, and a positive value when the ellipse is turned to the
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right. When aligned with the X axis in the positive direction, the range
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max should be returned; when aligned with the X axis in the negative
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direction, the range -max should be returned.
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Touch ellipses are symmetrical by default. For devices capable of true 360
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degree orientation, the reported orientation must exceed the range max to
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indicate more than a quarter of a revolution. For an upside-down finger,
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range max * 2 should be returned.
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Orientation can be omitted if the touch area is circular, or if the
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information is not available in the kernel driver. Partial orientation
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support is possible if the device can distinguish between the two axes, but
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not (uniquely) any values in between. In such cases, the range of
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ABS_MT_ORIENTATION should be [0, 1] [#f4]_.
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ABS_MT_POSITION_X
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The surface X coordinate of the center of the touching ellipse.
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ABS_MT_POSITION_Y
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The surface Y coordinate of the center of the touching ellipse.
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ABS_MT_TOOL_X
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The surface X coordinate of the center of the approaching tool. Omit if
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the device cannot distinguish between the intended touch point and the
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tool itself.
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ABS_MT_TOOL_Y
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The surface Y coordinate of the center of the approaching tool. Omit if the
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device cannot distinguish between the intended touch point and the tool
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itself.
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The four position values can be used to separate the position of the touch
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from the position of the tool. If both positions are present, the major
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tool axis points towards the touch point [#f1]_. Otherwise, the tool axes are
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aligned with the touch axes.
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ABS_MT_TOOL_TYPE
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The type of approaching tool. A lot of kernel drivers cannot distinguish
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between different tool types, such as a finger or a pen. In such cases, the
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event should be omitted. The protocol currently mainly supports
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MT_TOOL_FINGER, MT_TOOL_PEN, and MT_TOOL_PALM [#f2]_.
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For type B devices, this event is handled by input core; drivers should
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instead use input_mt_report_slot_state(). A contact's ABS_MT_TOOL_TYPE may
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change over time while still touching the device, because the firmware may
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not be able to determine which tool is being used when it first appears.
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ABS_MT_BLOB_ID
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The BLOB_ID groups several packets together into one arbitrarily shaped
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contact. The sequence of points forms a polygon which defines the shape of
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the contact. This is a low-level anonymous grouping for type A devices, and
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should not be confused with the high-level trackingID [#f5]_. Most type A
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devices do not have blob capability, so drivers can safely omit this event.
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ABS_MT_TRACKING_ID
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The TRACKING_ID identifies an initiated contact throughout its life cycle
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[#f5]_. The value range of the TRACKING_ID should be large enough to ensure
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unique identification of a contact maintained over an extended period of
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time. For type B devices, this event is handled by input core; drivers
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should instead use input_mt_report_slot_state().
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Event Computation
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-----------------
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The flora of different hardware unavoidably leads to some devices fitting
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better to the MT protocol than others. To simplify and unify the mapping,
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this section gives recipes for how to compute certain events.
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For devices reporting contacts as rectangular shapes, signed orientation
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cannot be obtained. Assuming X and Y are the lengths of the sides of the
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touching rectangle, here is a simple formula that retains the most
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information possible::
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ABS_MT_TOUCH_MAJOR := max(X, Y)
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ABS_MT_TOUCH_MINOR := min(X, Y)
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ABS_MT_ORIENTATION := bool(X > Y)
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The range of ABS_MT_ORIENTATION should be set to [0, 1], to indicate that
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the device can distinguish between a finger along the Y axis (0) and a
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finger along the X axis (1).
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For Win8 devices with both T and C coordinates, the position mapping is::
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ABS_MT_POSITION_X := T_X
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ABS_MT_POSITION_Y := T_Y
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ABS_MT_TOOL_X := C_X
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ABS_MT_TOOL_Y := C_Y
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Unfortunately, there is not enough information to specify both the touching
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ellipse and the tool ellipse, so one has to resort to approximations. One
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simple scheme, which is compatible with earlier usage, is::
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ABS_MT_TOUCH_MAJOR := min(X, Y)
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ABS_MT_TOUCH_MINOR := <not used>
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ABS_MT_ORIENTATION := <not used>
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ABS_MT_WIDTH_MAJOR := min(X, Y) + distance(T, C)
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ABS_MT_WIDTH_MINOR := min(X, Y)
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Rationale: We have no information about the orientation of the touching
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ellipse, so approximate it with an inscribed circle instead. The tool
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ellipse should align with the vector (T - C), so the diameter must
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increase with distance(T, C). Finally, assume that the touch diameter is
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equal to the tool thickness, and we arrive at the formulas above.
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Finger Tracking
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---------------
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The process of finger tracking, i.e., to assign a unique trackingID to each
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initiated contact on the surface, is a Euclidian Bipartite Matching
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problem. At each event synchronization, the set of actual contacts is
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matched to the set of contacts from the previous synchronization. A full
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implementation can be found in [#f3]_.
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Gestures
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--------
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In the specific application of creating gesture events, the TOUCH and WIDTH
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parameters can be used to, e.g., approximate finger pressure or distinguish
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between index finger and thumb. With the addition of the MINOR parameters,
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one can also distinguish between a sweeping finger and a pointing finger,
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and with ORIENTATION, one can detect twisting of fingers.
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Notes
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-----
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In order to stay compatible with existing applications, the data reported
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in a finger packet must not be recognized as single-touch events.
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For type A devices, all finger data bypasses input filtering, since
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subsequent events of the same type refer to different fingers.
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.. [#f1] Also, the difference (TOOL_X - POSITION_X) can be used to model tilt.
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.. [#f2] The list can of course be extended.
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.. [#f3] The mtdev project: http://bitmath.org/code/mtdev/.
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.. [#f4] See the section on event computation.
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.. [#f5] See the section on finger tracking.
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