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Introduction
------------
This document describes how the gestures are extracted from multi-finger
actions. The process is divided into gestural transformations, gesture
recognition, and gesture instantiation.
Gestural Transformations
------------------------
All two-finger transformations are extracted. These are all exact, in the
sense that continuously transforming the original finger positions, frame
per frame, will exactly follow the actual finger positions.
In addition to two-finger transformations, a global gesture is also
extracted. The rotation and scaling is taken from the contact pair with the
longest distance between contacts. This approximates the behavior of a
region, such that complex transformations could, in principle, be happening
inside the region, but at a distance, the transformation will look like it
was performed with two fingers.
At each time step, a gestural transformation is goverened by rotation,
scaling and translation. The point around which rotation and scaling is
performed is called the pivot. To form as natural gestures as possible,
this point always lies within the area formed by the contacts themselves.
It is placed at the point which, after rotation and scaling, leaves the
transformed contacts as close to the actual positions as possible. The
translation, or drag, is always a fraction of the movement of the center
point. The fraction is called moveness, and is related to the distance
between the pivot and the center point. When the pivot is at the center
point, the moveness is one and the drag is the same as the center movement.
When the pivot is at one of the contacts, as in rotation around a finger,
the moveness is zero, and consequently the drag is zero. The relations
between the pivot, the moveness and the drag are detailed in the document
pivot.txt.
Gesture Recognition
-------------------
Each gestural transformation can give rise to one or several gesture
primitives. Based on the rotation, scaling, moveness and drag values, the
gesture primitives drag, pinch and rotate are recognized. The onset of a
gesture primitive is governed by a threshold and a timeout. Basically, if
performed distinctly enough, the gesture will be triggered, and will remain
active until a finger is lifted or added.
Gesture Instantiation
---------------------
Given a set of detected gesture primitives, only some will trigger actual
gesture events. First, the set of available primitives are matched against
available listeners. Primitives not listened for are dropped. The remaining
set is arranged according to priority. A pointer gesture has lower priority
than a two-finger gesture, which has lower priority than a tap, for
instance. As long as the gesture primitives of higher priority are expected
but not activated, all gestures are held back. Once one of the highest
priority primitives are activated, all primitives of lower priority are
cancelled, and events are emitted.
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