Scientific Discovery Within Data Streams - Ambient Intelligence for Scientific Discovery - page 73

Fig. 5. The ThemeRiver Visual Metaphor.

disappears in the months of January and February 1960 but reappears in March
1960. The entire river may 'dry up' for a period of time if the composite currents
all disappear. This is possible because a river can effectively represent only a
subset of themes from a document collection, typically a few dozen selected
themes. In context of the concept presented in this paper, these may represent
the major clusters from a stream or the most significant events that are being
queried for change.

3.2 Perceptual Considerations

During the perception process humans do not organize individual, low-level,
sensed elements but sense more complete 'packages' that represent objects or
patterns. In a recent topic, Hoffman [5] presents a compelling discussion of how
our perceptual processes identify curves and silhouettes, recognize parts, and
group them together into objects. Numerous aspects of the image influence our
ability to perceive these parts and objects, including similarity, continuity, sym-
metry, proximity, and closure. For example, it is easier to perceive objects that
are bounded by continuous curves than objects that contain abrupt changes [6].
Smooth, continuous curves bound a theme current in the ThemeRiver visu-
alization. A theme current is assigned a single color for the entire length of the
river. The smooth bounds and distinct color help the user track and compare a
current's behavior along the river. At a glance the user can see the pattern of
the current as an object - where it bulges, where it shrinks, and where it remains
unchanged. We naturally associate the size (area) of the object with strength; a
larger area indicates more strength, while a smaller area indicates less strength.
The absence of the object indicates no use or strength at that time.