Input: update multi-touch protocol documentation

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>

Documentation/input/multi-touch-protocol.txt

@@ -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
 -----