2.1 Spatiotemporal components of gesture: Phase

The form of all gestures can be categorized physically according to their temporal and spatial characteristics. As illustrated in Figure 1, a distinction must be made between source and the target of the gesture and the intermediate trajectory between those two points. Those gestures that can be described with regard to all three aspects are dynamic gestures. Static gestures, in contrast, have only a source and a target point, which are identical. The gestural phases referred to here are form-oriented temporal phases and are distinct from the function-orienetd gestural phases such as preparation, stroke and retraction described by McNeill (1992). The latter are phases with an inherent functional interpretation, whereas the temporal phases source, trajectory and target proposed here describe only the form of gestures. The source and target of gestures can be further described regarding their location in space, whereas the form of the trajectory can be specified in terms of directionality and shape.

2.1.1 Source and Target: Location

As described above, some gestures have one (static gestures), others have two points in space (source and target of dynamic gestures), which can be described according to the position of the relevant limb or body part relative to the speaker-centered coordinate system. It is also possible, in principle, to describe those points in space using a speaker-independent absolute spatial reference system. For a special subcase of gesture, deictic gestures, this may be relevant.

2.1.2 Trajectory: Directionality and shape

Whereas for static gestures it will suffice to define them by their location in space, dynamic gestures can be further described according to their trajectory along the axes up-down, backward-forward, and right-left, relative to the speaker. All axes are seen from the speaker's perspective so that a backward movement of the hands or arms means `towards the body' and a forward movement means `away from the body'. Gestures are carried out in all three dimensions. If necessary, information from two axes can be combined, as, for example, when raising one's hand in an upward/forward direction. Absolute spatial coordinates are not generally necessary for the description of e.g hand gestures, but are necessary for the description of position and posture. In addition, trajectories can be classified into different shapes such as circles, squares, or simple trajectories such as straight lines.