Input: multitouch - augment event semantics documentation
Through the collaboration to adapt the N-trig and Stantum HID drivers to the MT protocol, some semantic clarifications to the protocol have been made. This patch adds them to the MT documentation. Signed-off-by: Henrik Rydberg <rydberg@euromail.se> Tested-by: Stéphane Chatty <chatty@enac.fr> Signed-off-by: Dmitry Torokhov <dtor@mail.ru>hifive-unleashed-5.1
Henrik Rydberg
Dmitry Torokhov
parent
df391e0eda
commit
f9fcfc3b46
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@ -18,8 +18,12 @@ Usage
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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. The end of multi-touch
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transfer is marked by calling the usual input_sync() function.
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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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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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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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@ -27,11 +31,26 @@ minimum set consists of ABS_MT_TOUCH_MAJOR, ABS_MT_POSITION_X and
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ABS_MT_POSITION_Y, which allows for multiple fingers to be tracked. If the
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device supports it, the ABS_MT_WIDTH_MAJOR may be used to provide the size
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of the approaching finger. Anisotropy and direction may be specified with
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ABS_MT_TOUCH_MINOR, ABS_MT_WIDTH_MINOR and ABS_MT_ORIENTATION. Devices with
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more granular information may specify general shapes as blobs, i.e., as a
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sequence of rectangular shapes grouped together by an
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ABS_MT_BLOB_ID. Finally, the ABS_MT_TOOL_TYPE may be used to specify
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whether the touching tool is a finger or a pen or something else.
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ABS_MT_TOUCH_MINOR, ABS_MT_WIDTH_MINOR and ABS_MT_ORIENTATION. The
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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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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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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_TOUCH_MAJOR
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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_TOUCH_MAJOR
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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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Event Semantics
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@ -44,24 +63,24 @@ 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.
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possible value of ABS_MT_TOUCH_MAJOR is sqrt(X^2 + Y^2), the diagonal [4].
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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.
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contact is circular, this event can be omitted [4].
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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.
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same [4].
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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.
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tool. Omit if circular [4].
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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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@ -70,14 +89,17 @@ different characteristic widths [1].
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ABS_MT_ORIENTATION
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The orientation of the ellipse. The value should describe half a revolution
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clockwise around the touch center. The scale of the value is arbitrary, but
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zero should be returned for an ellipse aligned along the Y axis of the
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surface. As an example, an index finger placed straight onto the axis could
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return zero orientation, something negative when twisted to the left, and
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something positive when twisted to the right. This value can be omitted if
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the touching object is circular, or if the information is not available in
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the kernel driver.
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The orientation of the ellipse. The value should describe a signed quarter
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of a revolution clockwise around the touch center. The signed value range
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is arbitrary, but zero should be returned for a finger aligned along the Y
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axis of the surface, a negative value when finger is turned to the left, and
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a positive value when finger turned to the right. When completely aligned with
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the X axis, the range max should be returned. Orientation can be omitted
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if the touching object is circular, or if the information is not available
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in the kernel driver. Partial orientation support is possible if the device
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can distinguish between the two axis, but not (uniquely) any values in
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between. In such cases, the range of ABS_MT_ORIENTATION should be [0, 1]
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[4].
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ABS_MT_POSITION_X
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@ -98,8 +120,35 @@ 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 contactID, explained below. Most kernel drivers will
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not have this capability, and can safely omit the event.
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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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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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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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Finger Tracking
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@ -109,14 +158,18 @@ 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 contactID to each
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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 contactID stays the same and
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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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Notes
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-----
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@ -136,5 +189,7 @@ could be used to derive tilt.
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time of writing (April 2009), the MT protocol is not yet merged, and the
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prototype implements finger matching, basic mouse support and two-finger
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scrolling. The project aims at improving the quality of current multi-touch
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functionality available in the synaptics X driver, and in addition
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functionality available in the Synaptics X driver, and in addition
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implement more advanced gestures.
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[4] See the section on event computation.
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[5] See the section on finger tracking.
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