Input: document the multi-touch (MT) protocol
This patchs adds documentation for the multi-touch protocol to Documentation/input/. [randy.dunlap@oracle.com: grammar fixes] Signed-off-by: Henrik Rydberg <rydberg@euromail.se> Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
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Henrik Rydberg
Dmitry Torokhov
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Multi-touch (MT) Protocol
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-------------------------
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Copyright (C) 2009 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 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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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. The end of multi-touch
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transfer is marked by calling the usual input_sync() function.
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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_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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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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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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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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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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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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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 [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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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_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 supports MT_TOOL_FINGER and
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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 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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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 contactID 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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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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Notes
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-----
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In order to stay compatible with existing applications, the data
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reported in a finger packet must not be recognized as single-touch
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events. In addition, all finger data must bypass input filtering,
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since subsequent events of the same type refer to different fingers.
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The first kernel driver to utilize the MT protocol is the bcm5974 driver,
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where examples can be found.
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[1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
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difference between the contact position and the approaching tool position
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could be used to derive tilt.
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[2] The list can of course be extended.
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[3] The multi-touch X driver is currently in the prototyping stage. At the
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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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implement more advanced gestures.
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