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Multi-touch (MT) Protocol
-------------------------
Copyright (C) 2009 Henrik Rydberg <rydberg@euromail.se>
Introduction
------------
In order to utilize the full power of the new multi-touch devices, a way to
report detailed finger data to user space is needed. This document
describes the multi-touch (MT) protocol which allows kernel drivers to
report details for an arbitrary number of fingers.
Usage
-----
Anonymous finger details are sent sequentially as separate packets of ABS
events. Only the ABS_MT events are recognized as part of a finger
packet. The end of a packet is marked by calling the input_mt_sync()
function, which generates a SYN_MT_REPORT event. The end of multi-touch
transfer is marked by calling the usual input_sync() function.
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_TOUCH_MAJOR, ABS_MT_POSITION_X and
ABS_MT_POSITION_Y, which allows for multiple fingers to be tracked. If the
device supports it, the ABS_MT_WIDTH_MAJOR may be used to provide the size
of the approaching finger. Anisotropy and direction may be specified with
ABS_MT_TOUCH_MINOR, ABS_MT_WIDTH_MINOR and ABS_MT_ORIENTATION. Devices with
more granular information may specify general shapes as blobs, i.e., as a
sequence of rectangular shapes grouped together by an
ABS_MT_BLOB_ID. Finally, the ABS_MT_TOOL_TYPE may be used to specify
whether the touching tool is a finger or a pen or something else.
Event Semantics
---------------
The word "contact" is used to describe a tool which is in direct contact
with the surface. A finger, a pen or a rubber all classify as contacts.
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.
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.
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.
ABS_MT_WIDTH_MINOR
The length, in surface units, of the minor axis of the approaching
tool. Omit if circular.
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 [1].
ABS_MT_ORIENTATION
The orientation of the ellipse. The value should describe half a revolution
clockwise around the touch center. The scale of the value is arbitrary, but
zero should be returned for an ellipse aligned along the Y axis of the
surface. As an example, an index finger placed straight onto the axis could
return zero orientation, something negative when twisted to the left, and
something positive when twisted to the right. This value can be omitted if
the touching object is circular, or if the information is not available in
the kernel driver.
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_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 and
MT_TOOL_PEN [2].
ABS_MT_BLOB_ID
The BLOB_ID groups several packets together into one arbitrarily shaped
contact. This is a low-level anonymous grouping, and should not be confused
with the high-level contactID, explained below. Most kernel drivers will
not have this capability, and can safely omit the event.
Finger Tracking
---------------
The kernel driver should generate an arbitrary enumeration of the set of
anonymous contacts currently on the surface. The order in which the packets
appear in the event stream is not important.
The process of finger tracking, i.e., to assign a unique contactID to each
initiated contact on the surface, is left to user space; preferably the
multi-touch X driver [3]. In that driver, the contactID stays the same and
unique until the contact vanishes (when the finger leaves the surface). The
problem of assigning a set of anonymous fingers to a set of identified
fingers is a euclidian bipartite matching problem at each event update, and
relies on a sufficiently rapid update rate.
Notes
-----
In order to stay compatible with existing applications, the data
reported in a finger packet must not be recognized as single-touch
events. In addition, all finger data must bypass input filtering,
since subsequent events of the same type refer to different fingers.
The first kernel driver to utilize the MT protocol is the bcm5974 driver,
where examples can be found.
[1] With the extension ABS_MT_APPROACH_X and ABS_MT_APPROACH_Y, the
difference between the contact position and the approaching tool position
could be used to derive tilt.
[2] The list can of course be extended.
[3] The multi-touch X driver is currently in the prototyping stage. At the
time of writing (April 2009), the MT protocol is not yet merged, and the
prototype implements finger matching, basic mouse support and two-finger
scrolling. The project aims at improving the quality of current multi-touch
functionality available in the synaptics X driver, and in addition
implement more advanced gestures.
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