T he Phantom haptic interface is a device that lets you literally feel virtual objects with your hands. It sits conveniently on your desktop next to your computer, looking a bit like a miniature desk lamp (see Figure 1 ). The Phantom haptic interface has a stylus grip or a fingertip thimble with which users can reach into virtual worlds to touch and interact with 3D objects. With hundreds of Phantom systems now in use throughout the world, physical interaction with virtual environments (VEs) is

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... rapidly becoming a reality. Touch, or haptic interaction is one of the most fundamental ways in which people perceive and effect changes in the world around them. Our very understanding of the geometry and physics of the world begins by touching and physically interacting with objects in our environment. Touch interaction differs fundamentally from all our other sensory modalities in that it is intrinsically bilateral. We exchange energy between ourselves and the physical world as we push on it and it pushes back on us. In this exchange, information and intent are conveyed in a physically direct and cognitively primal way. Our ability to express ourselves in musical performance, painting, sculpting, and gymnastics depends on physically performing the task and learning from the interactions. Yet, in the world of computers, our primary mode of receiving information is still visual and depends on information abstractions such as words, diagrams, and pictures. Worse, to enter information into a computer, we are restricted to typing on a keyboard and clicking and dragging a mouse. Given the importance of touch in our lives, it is ironic that a convenient and truly bilateral means for haptic interaction with information has been, until recently, unavailable to computer users. Mechanical devices that allow haptic interaction with remote and virtual objects have been around for decades, though unsatisfactory for widespread use. Early remote manipulation systems were used for handling hazardous substances as far back as the 1940s, when the danger of working with nuclear materials necessitated developing remote manipulation devices. Today's surge of haptic research and commercial activity grew from the early efforts of designers who built the "master" input devices needed to control remote manipulators. In the 1960s, Knoll at Bell Labs was perhaps the first to demonstrate touching virtual shapes with a computer-controlled haptic interface. Since then, numerous devices have been built for haptic interaction, based on the recognition that adding haptics to graphic images significantly improves humancomputer interactions. In 1993, haptic interaction with computers took a significant step forward with the development of the Phantom haptic interface. This simple device has spawned a new field analogous to computer graphicscomputer haptics-defined as the discipline concerned with the techniques and processes associated with generating and displaying synthesized haptic stimuli to the human user. Inspired by our previous work in interpreting robot touch sensor information and study of human touch perception, the Phantom interface permits users to feel the forces of interaction they would encounter while touching objects with the end of a stylus or the tip of their finger. The resulting sensations prove startling, and many first-time users are quite surprised at the compelling sense of physical presence they encounter when touching virtual objects. To appreciate why the Phantom system succeeded where others failed, you need to understand the nature and functioning of the human haptic system. How and what do humans feel? We use our hands to explore and manipulate objects in our environment. Unique among our sensory modalities, the haptic system relies on action to stimulate perception and vice versa. While exploring an object, we actively scan our fingers across its surface and squeeze or palpate it to sense its physical properties. To manipulate an object, we apply forces to move it, while simultaneously sensing the results of these actions. Thus, in almost all of the hand's activities, either to extract information from or to alter the environment, we use both the sensory and motor parts of our haptic system. Correspondingly, a haptic interface needs to sense our motor actions and display appropriate haptic "images." Keyboards and mice convey very restricted motor actions to the computer. Instrumented gloves sense more degrees of freedom, but with less precision. These passive interfaces, however, cannot display any synthesized haptic images to the user. What should this haptic image consist of? In the real world, whenever we touch an object, it imposes forces