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Input: update multi-touch protocol documentation

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This patch documents a new ABS_MT parameter and adds further text to
clarify some points around the MT protocol.

Requested-by: Yoonyoung Shim <jy0922.shim@samsung.com>
Requested-by: Mika Kuoppala <mika.kuoppala@nokia.com>
Requested-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
This commit is contained in:
Henrik Rydberg 2010-01-28 22:28:28 -08:00 committed by Dmitry Torokhov
parent cb6ecf6f7a
commit f6bdc2303d
1 changed files with 40 additions and 8 deletions
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48
Documentation/input/multi-touch-protocol.txt
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@ -27,12 +27,30 @@ set of events/packets.
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. The
ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
minimum set consists of 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_TOUCH_MAJOR and ABS_MT_WIDTH_MAJOR may be used to provide the size
of the contact area and approaching finger, respectively.
The TOUCH and WIDTH parameters have a geometrical interpretation; imagine
looking through a window at someone gently holding a finger against the
glass. You will see two regions, one inner region consisting of the part
of the finger actually touching the glass, and one outer region formed by
the perimeter of the finger. The diameter of the inner region is the
ABS_MT_TOUCH_MAJOR, the diameter of the outer region is
ABS_MT_WIDTH_MAJOR. Now imagine the person pressing the finger harder
against the glass. The inner region will increase, and in general, the
ratio ABS_MT_TOUCH_MAJOR / ABS_MT_WIDTH_MAJOR, which is always smaller than
unity, is related to the finger pressure. For pressure-based devices,
ABS_MT_PRESSURE may be used to provide the pressure on the contact area
instead.
In addition to the MAJOR parameters, the oval shape of the finger can be
described by adding the MINOR parameters, such that MAJOR and MINOR are the
major and minor axis of an ellipse. Finally, the orientation of the oval
shape can be describe with the ORIENTATION parameter.
The ABS_MT_TOOL_TYPE may be used to specify whether the touching tool is a
finger or a pen or something else. 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, for the few devices
@ -42,11 +60,9 @@ report finger tracking from hardware [5].
Here is what a minimal event sequence for a two-finger touch would look
like:
ABS_MT_TOUCH_MAJOR
ABS_MT_POSITION_X
ABS_MT_POSITION_Y
SYN_MT_REPORT
ABS_MT_TOUCH_MAJOR
ABS_MT_POSITION_X
ABS_MT_POSITION_Y
SYN_MT_REPORT
@ -87,6 +103,12 @@ 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_PRESSURE
The pressure, in arbitrary units, on the contact area. May be used instead
of TOUCH and WIDTH for pressure-based devices or any device with a spatial
signal intensity distribution.
ABS_MT_ORIENTATION
The orientation of the ellipse. The value should describe a signed quarter
@ -170,6 +192,16 @@ There are a few devices that support trackingID in hardware. User space can
make use of these native identifiers to reduce bandwidth and cpu usage.
Gestures
--------
In the specific application of creating gesture events, the TOUCH and WIDTH
parameters can be used to, e.g., approximate finger pressure or distinguish
between index finger and thumb. With the addition of the MINOR parameters,
one can also distinguish between a sweeping finger and a pointing finger,
and with ORIENTATION, one can detect twisting of fingers.
Notes
-----