the button in touch-sens-
ing input provides three
states (nontouch, touch,
and press). The additional
state allows more precise
control of the system. For
example, the toolbox of a
GUI application automati-
cally picks up more tools
when a user moves a cursor
to a toolbar region with a
finger touch of the button.
Pressure is another use-
fulinput parameter for
organic interaction. We
intuitively use and control
pressure for natural com-
munication (such as when
shaking hands). With a
simple pressure sensor
(such as a force-sensitive
resister) embedded in a
regular mouse or touch- Figure 3. PreSense 2D input
pad, the device easily device enhanced with pressure
sensors. Users add pressure to
^{senses finger pressure by} control analog parameters (such
measuring the pressure as scaling) and specify “positive”
^{sensor’s resister values. and} “^negative” ^{pressures by} changing the size of the finger

PreSense [ 8] is a touch- contact area on the touchpad and pressure-sensing input ^{surface. PreSense can be} _{device that uses finger} combined with tactile feedback _{to emulate a discrete button} pressure, as well as finger press with “click” sensation. position (see Figure 3). It consists of a capacitive touchpad, force-sensitive resister pressure sensor, and actuator for tactile feedback. It also recognizes finger contact by measuring the capacitive change on the touchpad surface. Combining pressure sensing and tactile feedback, it also emulates a variety of buttons (such as one-level and two-level) by setting thresholds to pressure parameters. For example, a user can “soft press” the target to select it and “hard press” it to display a pop-up menu.

Analog pressure sensing enables users to control continuous parameters (such as the scale of the displayed image). The finger contact area is used to distinguish between scaling directions (scale-up and scale-down). By changing the position of the finger slightly, one can control both zooming-in and zoom-ing-out with one finger (see Figure 3b).

Pressure is useful for explicit parameter control (such as scaling) while offering the possibility of sensing the user’s implicit or emotional state. When a user is, say, frustrated with the system, his or her mouse button pressure might change from the normal state, and the system would be able to react to that frustration.

Finger input with pressure, combined with tactile feedback, is the most common form of natural interaction. Like Shiatsu (Japanese finger-pressure therapy), users of PreSense feel and control the performance of computer systems directly.

RESEARCH ISSUES

Because organic user interfaces represent such a new and emerging research field, many related research challenges and issues require further study. In what follows, I outline four of them:

Interaction techniques for OUIs. GUIs have a long history and incorporate a large number of interaction techniques. When the mouse was invented by Douglas Englebart at Stanford Research Institute in 1963, it was used only to point at on-screen objects. Development of mouse-based interaction techniques (such as pop-up menus and scrollbars) followed. The current level of development of organic user interfaces is the equivalent of where the mouse was when first invented. For multi-touch interaction, only a simple set of techniques (such as zooming) has been introduced, though many more should be possible; the interaction techniques explored in [ 4] may be candidates.

Stone(Tool) vs. skin. It is also interesting and worthwhile to consider the similarities and differences between tangible UIs and organic UIs. Although these two types of UIs overlap in many ways, the conceptual differences are clear. Tangible UI systems often use multiple physical objects as tools for manipulation; each object is graspable so users are able to use physical manipulation. Because these objects often have a concrete meaning (called physical icons, or “phicons”) in the application, many tangible systems are domain-specific (tuned for a particular application). For organic UI systems, users directly interact with possibly curved interactive surfaces (such as walls, tables, and electronic paper) with no intermediate objects. Interactions are more generic and less application-oriented. This situation may be compared to real-world interaction. In the real world, we use physical instruments (tools) to manipulate something but prefer direct contact for human-to-human com-