Figure: The user environment for SURVIVE.
The simplest mapping is, of course, the direct one: map interface device features directly (one-to-one) into the control space of some application. Usually a small amount of filtering will be required, and possibly it's desirable to use non-linear mappings, but otherwise interface outputs feed directly into application inputs.
SURVIVE (Simulated Urban Recreational Violence Interactive Virtual Environment) is an entertainment application that uses a direct mapping. SURVIVE allows the user to interact with a 3D game environment using the IVE space. Figure 7 shows a user in SURVIVE. The gestural interpretation provided by the vision system (Section 2.2) is mapped into the game controls for the popular id Software game Doom.
The user holds a large (two-handed) toy gun, and moves around the IVE stage. Position on the stage is fed into Doom's directional velocity controls. The hand position features are used to drive Doom's rotational velocity control. The results of a matched-filter on and audio input stream provide control over weapon changes and firing. This direct mapping, given the application, may be called ``user-as-joystick''.
Although simplistic, this mapping has some very important features: low lag, intuitive control strategy, and a control abstraction well suited to the task. The mapping requires little post-processing of the interface features, so it adds very little lag to the interface. Since many games have velocity-control interfaces, people adapt quickly to the control strategy because it meshes with their expectations about the game.
Finally, it's insightful to contrast the SURVIVE interface with the standard keyboard Doom interface. The task in Doom is navigating through a virtual environment. This is usually accomplished by pressing keys on a keyboard. Changing the direction of travel is as easy as picking up one finger and pressing down another. Split-second decisions become split-second actions. The SURVIVE interface is much less forgiving. Movement of the virtual body is linked to the movement the real body. A change of virtual direction actually requires a movement in that direction, maybe several feet of movement. This leads to a much more engrossing, visceral experience of the game.
Interestingly, even when people use the keyboard interface, they tend to move their heads, and sometime their whole body, while playing the game. SURVIVE capitalizes on this natural link between visual and visceral experience to create a more immersive, if more physically demanding, experience.