The AR Workbench: A Complete Co-located Reach-in Mirror-Based ...

Marlon Richert, Master’s Thesis, Friday, 23 February, 2007
conjunction. Finally, the VoidPythonMap executes the actual call to the Interaction
that created the callback structure.
Figure 17 Interaction callback structure in the case of two Pivots (object
diagram).
Figure 18 then shows a model of the generic case. Let us call this structure a
callback unit. Then, you can combine several of these units to construct a callback
structure like the one shown in Figure 17 but then for an arbitrary number of pivots
as follows: Create a callback unit for all Pivots except one. Next, chain the units
together by substituting each unit’s MapBind for the undefined Bind on the right side
of the figure. Next, replace the only remaining undefined Bind with an EditorBind
and link the only remaining Pivot to in the same way that the other Pivots in Figure
17 and Figure 18 connect to an EditorBind. Finally, link the only remaining MapBind
to a VoidPythonMap, as shown in Figure 17. Thus, all Pivots link together in a logical
conjunction that executes the code in the VoidPythonMap whenever it evaluates to
True.
Table 7 shows the code that implements the Interaction constructor, which
creates the callback structure. It takes away many repetitive tasks the application
programmer would normally have to perform and automates them. In addition,
these tasks are quite complicated to perform and prone to error. Automating them
reduces the risk of errors, reduces the amount of domain knowledge required and
frees up the programmer’s time for implementing the actual ways in which the
virtual objects will interact by implementing subclasses of Role.
An explanation of the code in Table 7 is in order. Figure 19 shows the first part of
the Interaction system. Every frame cycle—that is, every time before it draws a
frame on screen—IPF tells all of its Binds to update. Each Bind checks whether its
inputs depend on the output of another bind. If so, it waits for the Binds it depends
on to update their output first before updating itself. Thus, in the Interaction system,
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