ABSTRACTBased mainly on a series of studies the author conducted at the University of Toronto, this article reviews the usability of various 6 degrees of freedom (6 DOF) input devices for 3D user interfaces. The following issues are covered in the article: the multiple aspects of input device usability (performance measures), mouse based 6 DOF interaction, mouse modifications for 3D interfaces, free-moving isotonic 6 DOF devices, desktop isometric and elastic 6 DOF devices,armature-based 6 DOF devices, position vs. rate control, and the form factors of 6 DOF control handle. These issues are treated at an introductory and practical level, with pointers to more technical and theoretical references.
As three-dimensional (3D) graphics moves to the core of many mainstream computer systems and applications, the search for usable input devices for 3D object manipulation becomes both an academic inquiry and a practical concern. In the case of the 2D Graphical User Interface (GUI), the computer mouse established itself very early and quickly replaced the light pen as the de facto standard input device (See [Perry, 1989] for a review of mouse history). In the case of 3D interfaces, however, there is still not an obvious winner suitable for all applications. Primarily based on the author's own research, this article offers a few perspectives on the usability of various input devices for 3D interface. This article does not intend to present a comprehensive literature review or a series of experimental studies in a methodical manner. Rather, it intends to be introductory and practical. Interested readers are encouraged to examine more technical details in the papers referenced.
In order to be able to manipulate 3D objects, one generally needs at least six degrees of freedom (6 DOF), three for X, Y, and Z translation and three for 3D rotation. The difficulty in establishing a standard 6 DOF device is twofold. First, there are engineering challenges in terms of sensor technologies, manufacturing cost, and designer's creativity. It is highly likely that the most elegant 6 DOF device has still not been designed. Second, and perhaps more importantly, even if we could easily make any device we like to, there is only a very limited knowledge about what properties a good 6 DOF controller should have. Given the long history of human factors study on input control devices, dated back to World War II [Orlansky, 1949], "One would expect the relationship of the hand to the controlled element, being at the one time both an input and output, to be a fruitful area for research", but the reality is that little is well understood [Burrows, 1965]. Burrows pointed out that the reluctance to conduct research in this area is understandable in view of the immensity of the possible interactions among the many dimensions of control feel.
This is not to say that there isn't any intellectual guidance to 6 DOF input device design. Motivated by the manual control problems in vehicles, air crafts and other dynamic complex machines, the topic of "manual control and tracking" has been extensively studied (see [Poulton, 1974] for a summary). However, system dynamics resulted from mass, spring, viscosity, transmission delay etc in these systems, soon dominated the area. The study of input control device properties (e.g. [Bahrick, Bennett, & Fitts, 1955] ) quickly gave way to mathematical control theory modeling of man-machine systems. The more general body of knowledge on human motor control and learning (see [Schmidt, 1988] for example), while offers many insights, rarely provide direct design guidelines. One recent review of the scattered literature related to input device design is provided in [Zhai, 1995].
The sixth aspect of input device usability is the ease of device acquisition. This is often an overlooked aspect of input device usability. Although a mouse is less dexterous than a pen like input device (a stylus), the fact that a mouse can be more easily acquired is one important reason that made it the dominant 2 DOF input device. Many factors, such as the distance to the computer keyboard home row (ASDFGHJKL keys), contribute to the ease of device acquisition. One of them is the device location persistence when released. With a mouse or a trackball, when released by the hand (in order to type something, for example) it stays in position. This is not true with a stylus.
With these measures in mind, the remainder of the paper examines a few common classes of 6DOF input devices.
Researchers soon moved away from the 6 slider implementation to more complex mapping techniques. One such technique is enclosing the manipulated 3D object with a virtual sphere [Chen, Mountford, & Sellen, 1988]. Some of these techniques in fact have be widely used in 3D graphics software such as VRML browsers and CAD packages. A recent study [Hinckley, 1997] found the mouse mapping techniques still inferior to integrated 6 DOF devices.
Since dedicated physical degrees of freedom are provided for the third dimension, these modified mice should outperform a conventional mouse that operates by means of simple mode switching. (One common mouse switching technique, for example, is that pressing and holding a mouse button down switches vertical mouse cursor motion to motion in the depth dimension). It was shown that in a 3D positioning task, the rockin' mouse was 30% faster in comparison to a standard mouse tablet [Balakrishnan, 1997]. On the other hand, since the depth dimension is operated by a behavior and muscle groups different from those of the x-y mouse motion, it can be difficult to produce simultaneous, coordinated motion with either the roller mouse or the Rockin' mouse.
Figure 1. A mouse with a roller for 3D input [Venolia, 1993]
Figure 3. A sample of free moving 6 DOF devices - "flying mice". (a) The "Bat", designed by C. Ware [Ware, 1990] (b) The Cricket(tm), manufactured by Digital Image Design Inc., New York, NY, USA (c) The MITS Glove, designed by the author, consists of a Bird(tm) tracker and a clutch.The advantages of these "flying mice" devices are:
However, there are many disadvantages to this class of devices:
Figure 4. Trial completion time in a 6 DOF docking task. The Free moving device was faster, particularly in early stage (From [Zhai, 1998])
Figure 5. Coordination as measured by inefficiency in free moving position control vs. in elastic rate control. Significantly longer movement was "wasted" with the free moving device. See [Zhai, 1998] for details.
Figure 6. The Fingerball. This device uses the same sensor as in the glove in Figure 3 (C). However, the different form factor made it possible to manipulate it with fingers, in addition to wrist and arm, leading to better performance [Zhai, 1996]
Figure 7. Performance difference between the Fingerball and the Glove in a 6 DOF docking task [Zhai, 1996]
Figure 8. A sample of desktop 6 DOF input devices. A sample of input devices for 6 DOF manipulation;. (a) The Spaceball(tm) is an isometric device manufactured by Spaceball Technologies Inc., Boston, MA, USA. (b) The SpaceMaster(tm) is an elastic device with a small range of movement (5 mm in translation and 15° in rotation), manufactured by BASYS GmbH, Erlangen, Germany. (c) The Space Mouse(tm) is an elastic device with slight movement (5 mm in translation and 4° in rotation). It is initially designed by DLR, the German aerospace research establishment, manufactured by Space Control Company, Malching, Germany and marketed by Logitech, Fremont, CA, USA.
Figure 9. Two input device design dimensions: transfer function vs. controller resistance Figure 10. Isotonic rate control and Isometric position control tend to produce poor performance [Zhai & Milgram, 1993a] [Zhai, 1995]
On the other hand, isometric rate control devices may have the following disadvantages:
Figure 11. Coordination as quantified by movement inefficiency. The Free moving position control device "wasted" more movement. Adpated from Figure 10 in [Zhai, 1998]
Figure 12. A prototype of a 6 DOF Elastic General-purpose Grip (EGG) [Zhai, 1995] [Zhai, 1993]
When optimized, the appropriate amount of elasticity does improve a user's performance. This is particularly true at the early learning stages (Figures 13 & 14).
Figure 13. Time performance difference between an elastic and an isometric rate controller in a 6 DOF docking task [Zhai, 1995] [Zhai, 1993]
Figure 14. Tracking error with an elastic vs. isometric 6 DOF controller task [Zhai, 1995] [Zhai & Milgram, 1993a]
While strong in their intended, specialized purpose, their use is limited. They can do an excellent job for some types of character posing, but their applicability to interaction in general is limited, at least in this "puppet" type configuration.
One option that can bring armatures much closer to the generality seen with the other devices discussed occurs if they are configured as a single arm. This can be done with either the DID or Puppet Works devices, and is the standard configuration for other armatures, such as Immersion Corp.'s MicroScribe illustrated in Figure 15 [Immersion_corp].
In this configuration, the armature is actually a hybrid between a flying-mouse type of device and a desktop device. It is like a desktop device in that it is typically mounted on the desktop, and consumes a small footprint. On the other hand, it is like a flying mouse in that manipulating the end point of the arm in space provides the desired 6DOF position data. Conceptually, these are near isotonic - with exceptional singularity positions - position control device like a flying mouse and thus share many of the pros and cons of isotonic position control discussed earlier. In addition, this approach has the following particular advantages:
Figure 15. A single arm multi-DOF armature input device [Immersion Corp, ]
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