Two of Data Explorer's mechanisms to control execution flow through a visual program are the Switch and Route modules. Switch allows you to switch between one or more inputs to drive a single output; Route is the inverse of Switch, having a single input that can be routed to zero, one, or more than one output. Switch is typically used to choose between different paths in a visualization program; for example, to pass an imported data field through either the Glyph module or through Isosurface, depending on user choice or characteristics of the data field itself. Route is typically used to turn off portions of the visualization program; for example, to turn off WriteImage or Export, or to prevent rendering to an image window unless the user chooses to create an image. Switch can be thought of as turning off portions of the visual program logically above Switch; Route can be thought of as turning off portions of the visual program logically below Route. Note that while Route turns off modules that receive its unused outputs, the Collect module is an exception: it runs even if some of its inputs have been turned off by Route.
Figure 19 shows an example of a Switch module controlling whether an Isosurface or a MapToPlane is passed to Image. In a simple data-flow execution model, this Switch module will be executed when its inputs are available (i.e., the results of the Isosurface and MapToPlane modules, and the value of the selector). On execution, the Switch module chooses whether to pass the Isosurface or MapToPlane result to the output based on the selection input to Switch. In the case of a pure data-flow model both the Isosurface and MapToPlane modules execute before the decision as to which will actually be used is known. Since both operations can be computationally expensive, the execution of both of them is very inefficient.
Again, this problem is handled in Data Explorer within the simple data-flow execution module by an examination of the graph prior to execution. If the selection value comes from an external source (e.g., an interactor) and is known a priori, the selection may be performed by a simple transformation of the graph: excising the Switch module altogether, and substituting arcs from the selected source (either Isosurface or MapToPlane) to each of the modules that, in the original network, received the result of the Switch module. This leaves the unselected module dangling. It and any of its ancestors that are therefore made unnecessary will not be executed.
A different procedure is used if the controlling value is not static (e.g., if it is determined elsewhere in the network), as shown in Figure 20. Suppose either an isosurface or a set of vector glyphs is selected depending on whether the data are scalar or vector. The determination of the type of the data is made using the Inquire module (i.e. at run time). In this case, the selection value for the Switch module cannot be determined before the execution of the graph. Instead, the graph must be evaluated in stages: 1) determine the selection value, 2) determine the necessary input to the Switch module and 3) evaluate the remainder of the graph. Since dynamic inputs may themselves be descended from other non-static inputs (e.g., computed in the network), this process may have to be performed repeatedly.
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