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19.3 DISPLAY-RELATIVE MAPPING

DRM is a dynamic coordinate mapping technique that controls a target object according to display relative inputs, such as from the mouse or arrow keys. In terms of source and target spaces, DRM generally uses the view itself as the source space node and the actuator's target transform group is used as the target space node.

At a conceptual level, the DRM process can be summarized as follows. When the user moves the drag cursor left in the display, the intuitive drag mapper interprets that motion as occurring in 3D in the plane of the view's display. In turn, the source drag mapper obtains the view's local-to-Vworld transform, and transforms the local display movement into the equivalent motion expressed in absolute world space coordinates. The mapper then obtains the actuator target's local-to-Vworld transform, inverts it, and then transforms the world space movement into an equivalent motion in the target's local coordinate space. If, for example, the target actuator performs geometric translation, then the target object will always appear to move left in the display regard-less of the position or direction of the view.

Table 19.3 summarizes the classes associated with DRM intuitive mapping. Figure 19. 4 illustrates the relationship of these classes. The next example will demonstrate how to configure these and the rest of the intuitive mapping building blocks for DRM.

Table 19.3 Summary of classes related to DRM intuitive mapping

Class/Interface	Description	Input Events	Output Events
IntutiveDragMapper	A coordinate mapper for input drag to source drag conversion. Uses a plugin to perform a specific type of coordinate mapping	InputDragTarget	SourceDragTarget
IntuitiveDragMapperPlugin	The abstract base class for intuitive drag mapper plugins
DrmPlugin	An intuitive drag mapper plugin that maps the 2D input drag to the Z= 0 plane of the source space

19.3.1 DrmDragPlugin class

For support of DRM, the framework provides the DrmDragPlugin subclass of IntuitiveDragMapperPlugin. The plug-in has very little to do. Because in DRM the intent is for the user's control inputs to be interpreted relative to the display space, the plug-in simply maps the 2D drag input to the Z= 0 plane of the 3D source space. In other words, for DRM, the source space is the view space with the input drag confined to its display plane. (The intuitive drag mapper plug-in will play a more prominent role in WRM than it does in DRM.)

The constructor for DrmDragPlugin gives you a choice for how the source node is selected. Either you can let the input event's source display define it, in which case the source display's ViewingPlatform is used, or you can set it explicitly, such as with a node in the same space as the view. In any case, the source node is passed along with the mapper's output SourceDragTarget event for use by the source drag Mapper.

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