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Input: document the MT event slot protocol
This patch adds documentation for the ABS_MT_SLOT event and gives examples of how to use the event slot protocol. Reviewed-by: Ping Cheng <pingc@wacom.com> Signed-off-by: Henrik Rydberg <rydberg@euromail.se> Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
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Henrik Rydberg
Dmitry Torokhov
parent
40d007e7df
commit
72c8a94a58
@ -6,31 +6,149 @@ Multi-touch (MT) Protocol
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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 devices, a way to
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report detailed finger data to user space is needed. This document
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describes the multi-touch (MT) protocol which allows kernel drivers to
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report details for an arbitrary number of fingers.
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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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Usage
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-----
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Protocol Usage
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--------------
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Anonymous finger details are sent sequentially as separate packets of ABS
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events. Only the ABS_MT events are recognized as part of a finger
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packet. The end of a packet is marked by calling the input_mt_sync()
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function, which generates a SYN_MT_REPORT event. This instructs the
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receiver to accept the data for the current finger and prepare to receive
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another. The end of a multi-touch transfer is marked by calling the usual
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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
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set of events/packets.
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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 [5].
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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 [3].
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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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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 fingers to be tracked. If the device supports it, the
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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 finger, respectively.
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of the contact area and approaching contact, 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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@ -41,56 +159,26 @@ ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
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ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
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against the glass. The inner region will increase, and in general, the
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ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
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unity, is related to the finger pressure. For pressure-based devices,
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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.
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In addition to the MAJOR parameters, the oval shape of the finger can be
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In addition to the MAJOR parameters, the oval shape of the contact can be
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described by adding the MINOR parameters, such that MAJOR and MINOR are the
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major and minor axis of an ellipse. Finally, the orientation of the oval
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shape can be describe with the ORIENTATION parameter.
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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. Devices with more granular information
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contact or a pen or something else. Devices with more granular information
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may specify general shapes as blobs, i.e., as a sequence of rectangular
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shapes grouped together by an ABS_MT_BLOB_ID. Finally, for the few devices
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that currently support it, the ABS_MT_TRACKING_ID event may be used to
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report finger tracking from hardware [5].
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report contact tracking from hardware [5].
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Here is what a minimal event sequence for a two-finger touch would look
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like:
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ABS_MT_POSITION_X
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ABS_MT_POSITION_Y
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SYN_MT_REPORT
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ABS_MT_POSITION_X
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ABS_MT_POSITION_Y
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SYN_MT_REPORT
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SYN_REPORT
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Here is the sequence after lifting one of the fingers:
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ABS_MT_POSITION_X
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ABS_MT_POSITION_Y
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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 remaining finger:
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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-finger event reaching userland.
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Event Semantics
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---------------
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The word "contact" is used to describe a tool which is in direct contact
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with the surface. A finger, a pen or a rubber all classify as contacts.
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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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@ -157,15 +245,16 @@ MT_TOOL_PEN [2].
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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. This is a low-level anonymous grouping, and should not be confused
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with the high-level trackingID [5]. Most kernel drivers will not have blob
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capability, and can safely omit the event.
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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 [5]. 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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[5]. There are currently only a few devices that support it, so this event
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should normally be omitted.
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[5]. This event is mandatory for type B devices. The value range of the
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TRACKING_ID should be large enough to ensure unique identification of a
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contact maintained over an extended period of time.
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Event Computation
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@ -192,20 +281,11 @@ finger along the X axis (1).
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Finger Tracking
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---------------
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The kernel driver should generate an arbitrary enumeration of the set of
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anonymous contacts currently on the surface. The order in which the packets
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appear in the event stream is not important.
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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 left to user space; preferably the
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multi-touch X driver [3]. In that driver, the trackingID stays the same and
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unique until the contact vanishes (when the finger leaves the surface). The
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problem of assigning a set of anonymous fingers to a set of identified
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fingers is a euclidian bipartite matching problem at each event update, and
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relies on a sufficiently rapid update rate.
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There are a few devices that support trackingID in hardware. User space can
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make use of these native identifiers to reduce bandwidth and cpu usage.
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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 [3].
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Gestures
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