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3.4.3 Mapping Between Syntax and Semantics
Having defined the semantic and the syntactic level of interaction, we now focus
on the intersection between both levels. We analyze how syntactic core interactions
can be mapped to semantic activities. The number of activities is larger than the
number of core interactions. For this reason, the model must rely on some kind of
multiplexing [8] between a smaller number of core interactions and a larger number
of semantic activities.
We distinguish the following four types of multiplexing, which are used to map
core interactions to activities. A syntactic interaction that performs a specific activity
might use one or more of these types of multiplexing between core interactions and
semantic activities. Figure 3.8 illustrates the four types of multiplexing.
Annotating
Lin
Gestural multiplexing
Device
multiplexing
Compositional
multiplexing
Spatial multiplexing
Inking
Clicking
Fig. 3.8
Types of multiplexing between core interactions and semantic activities
Compositional Multiplexing
A key finding of our field work was that in tradi-
tional paper practice, users often combine several core interactions to perform a sin-
gle conceptual activity. The user for instance attaches an adhesive indexing sticker
onto a document page and then writes a label on this sticker. We call this compo-
sitional multiplexing and judge it to be - together with spatial multiplexing - most
important for interaction in PPUIs. This principle enables users to draw upon a small
repository of simple core interactions. These act as flexible building blocks which
are used and reused for multiple conceptual activities.
Spatial Multiplexing
In traditional practice of working with paper documents, dif-
ferent sheets of paper or different areas on one sheet have different functions. For
example, writing a label on an index sticker possibly has another meaning than writ-
ing the same label on a document page. Applied to our model of PPUIs this implies
that PPUIs should comprise different types of page areas that serve different pur-
