Michael McKenna Three Dimensional Visual Media Laboratory Display Massachusetts Institute of Technology SyStCttlS fOP Vlftlial EnVÍl"OniTientS Cambridge, Massachusetts 02137 David Zeltzer Media Laboratory Research of Electronics Laboratory Abstract Massachusetts Institute of Technology Cambridge, Massachusetts 02139 This paper surveys three-dimensional (3D) visual display technology as it relates to real- time, interactive systems—or virtual environment systems. Five major 3D display types are examined: stereoscopic, lenticular, parallax barrier, slice-stacking, and holographic displays. The major characteristics of each display type are examined, i.e. spatial resolu- tion, depth resolution, field of view, viewing zone, bandwidth, etc. In addition, the cor- responding parameters of the human visual systems are described. The different display systems, as well as the human visual system, are compared in tabular form. I Introduction Our sense of visual "depth" is often taken for granted until we encounter a situation in which various depth cues are missing—as when we view a sup- posedly "realistically" rendered, "three-dimensional" image on the face of an ordinary CRT. Yet for many tasks, we can show that the presence or absence of 3D depth cues has important effects on human performance (McWhorter, Hodges, & Rodriguez, 1990; Liu, Stark, & Hirose, 1992). A number of three- dimensional display technologies have been developed, however, and for those virtual environment or teleoperator applications in which depth perception must be supported, it is important to provide the appropriate three-dimen- sional display. This paper is a survey of three-dimensional display techniques, with a de- scription and analysis of five major types of three-dimensional imaging systems: stereoscopic displays, lenticulars, parallax barriers, slice-stacking displays, and holography. These systems are analyzed using a set of criteria to allow quantita- tive comparisons among example displays, and the analyses are geared toward display systems for virtual environments (VEs). This paper does not cover as- pects of VE technology outside of visual displays, except as they relate to the display requirements. For example, there are rendering issues, such as the fidel- ity of lighting models, which are not directly addressed. Also, the input devices necessary to interact with a VE are not discussed—with the exception of head- tracking, required for viewpoint-dependent imaging using stereoscopic dis- plays. There are also a number of other, nonvisual, output devices of use to VEs, such as force-displays and acoustic output, which must be left to other surveys. It is assumed that the reader is familiar with two-dimensional display tech- nology for computer graphic imagery, such as CRT (cathode ray tube) displays raster "frames" and Presence. Volume I. Number 4. Fall 1992 (including calligraphic and displays) and the concepts of e 1993 The Massachusetts Institute of Technology interlaced "fields." Good references on raster CRT displays are Conrac (1985) McKenna and Zeltzer 421 Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/pres.1992.1.4.421 by guest on 29 September 2021 422 PRESENCE: VOLUME I, NUMBER 4 and Foley, van Dam, Feiner, and Hughes (1990), which of 3D display systems. We will first discuss general visual also includes a thorough review of rendering techniques. cues such as brightness and color, as well as the temporal It is important to analyze the task to be accomplished and spatial resolution characteristics of the human visual in order to match the technology to the problem in a system. We will conclude the section with a discussion of cost-effective manner. For example, some tasks may re- depth perception and depth cues. Some of the criteria quire high-resolution displays, or wide-angle views, but are more important for certain kinds of display systems not stereo viewing. The criteria used in this paper should than for others. These differences will be discussed as be able to guide researchers in their selection of an ap- each display system is described. propriate display system for their display requirements. In Section 4, tables are given that compare the at- In addition, it may be possible to develop a general dis- tributes of a set of examples of the different display sys- play type, useful to a wide range of VE tasks. tems and the human visual system, based on the criteria The next section presents the criteria used to examine given below. In this section many of the criteria are ac- the 3D display systems, and relates each of the criteria to companied by a short description, which develops its an aspect of the human visual system. Section 3 provides corresponding table entry. a functional description and analysis of five three-dimen- sional Section 4 a display systems. gives quantitative 2.1 Visual Cues and Display Attributes comparison of the different display systems, in a tabular form. Finally, conclusions are presented in Section 5. 2.1.1 Field of View. Thefield ofview (VOW) of a display measures the angle subtended by the viewing surface from a given observer location. FOV and spatial 2 Criteria for Display Systems resolution are related since a change in the FOV of a dis- play (i.e., enlarging the viewing surface) requires either a The perception of distance is a complex phenome- change in the size or number of pixels. non, involving many mechanisms of the eye as well as The human eye has a very wide visual field. The static the brain. There are many cues, or patterns of stimuli, visualfield is the FOV that is instantaneously seen when that provide us with information about the depth and the eyes are looking straight ahead—over 120° vertically shape of objects in the real world, as well as objects pre- for a single eye, and approximately 180° horizontally for sented to us in images. None of the display systems dis- both eyes, with a 120° overlap between the two eyes. cussed in this paper supports every depth cue, and the Because the FOV is limited by the occluding cheeks, various systems have different shortcomings. Certain brows, and the nose, however, it has a rather irregular cues are better suited to certain tasks or imaging require- shape. The addition of head, neck, and body movements ments, thus certain displays may be more appropriate for allows a full 360° of visual coverage; head and eye move- certain tasks. ments combined can exceed velocities of hundreds of To compare different types of display systems, a set of degrees per second (Rolfe & Staples, 1989). criteria was developed that, in general, relates an aspect of the human visual system to a corresponding aspect of Field ofview. The horizontal x vertical static visual the display system. A few of the criteria arise from tech- field for two eyes is 180° x 120°. Throughout this sur- nological limits, rather than a human visual characteris- vey we will consider a typical workstation display to tic. Using the criteria, quantitative comparisons of dif- measure 33 x 26 cm. When viewed at a "comfortable ferent displays can be made. However, it can be difficult viewing distance" of 46 cm, the display will subtend a to quantitatively compare different types of display sys- horizontal x vertical FOV of roughly 40° x 32°. tems, because display attributes can vary widely within a given display system type. 2.1.2 Spatial Resolution. One of the most com- This section reviews the major output characteristics mon measurements made of 2D displays is spatial résolu- Downloaded from http://www.mitpressjournals.org/doi/pdf/10.1162/pres.1992.1.4.421 by guest on 29 September 2021 McKenna and Zeltzer 423 tion. With CRT technology, resolution is typically mea- viewing distance of 46 cm, would require a currently sured by the number of pixels that can be displayed in unattainable resolution of 4800 x 3840 pixels. the horizontal and vertical directions. However, resolu- Rates. an tion also measures the size of the pixels: the pitch of a 2.1.3 Refresh and Update To display pixel tells how "wide" or "tall" each pixel is, and the an- apparently stable image, most electronic displays need to gular resolution of a pixel gives the visual angle that the repeatedly redraw or refresh the imagery on the display rate is the pixel subtends, from a particular viewing location. surface many times per second. The refresh at a redraws The Photoreceptors in the human eye are most densely frequency which display its imagery. critical is the threshold above packed in a central area of the retina called thefovea, and fusion frequency (CFF) acuity falls off sharply outside of this region. Measures of which a refreshed image appears steady; displays that refresh below the CFF will to flicker. The CFF is visual acuity, or the spatial resolution of the eye, usually appear on a number of refer to the foveal FOV, which subtends approximately strongly dependent factors, including the of the the ambient 1-2° of the visual field (Davson, 1980). Our eyes are in brightness display, illumination, and the size and location in the visual field of the stimu- constant motion, however, giving the illusion that we lus. For most in room a re- perceive the entire visual field at this foveal resolution. applications average light, fresh rate of 60 Hz will appear flicker-free, and many There are many ways to characterize the manner in workstations refresh their at 60 Hz or which the human eye can resolve detail. Some tests mea- graphics displays To reduce the cost of the refresh while sure visual resolution in terms of response to spatial fre- higher. circuitry still appearing to display flicker free imagery, consumer quencies, such as patterns of light and dark bars or sine TV sets in the United States are interlaced.