2 THE INTERNATIONALJOURNAL OF VIRTUALREALITY Vol. I, No. 2 STEREOSCOPIC IMAGING and anaglyph cassettes and videodiscs appear occasionally. Unfortunately, this technique, like those TECHNOLOGY: employing Pulfrichor prism glasses, is hopeless for high A Review of Patents and the Literature quality or comfortable viewing with video. It is better with computer displays. James Butterfield broadcasted Michael Starks side-by-side stereo images for viewing with prism 3DTV Corporation glasses in Mexico in the 1950s. He was one of many to San Rafael, California make stereo systems with dual cameras viewed through a binocular stereoscope. He was alsoone of thefirst to use Editor:Hal Thwaites polarized glasses to view anaglyphs by placing dichroic polarizers on the face of the CRT, an idea later refined ABSTRACT by Benton2. Polarized glasses for use with dual cross polarized CRTs as well as interdigitated images on a This article contains comments on the nature of single display covered with crossed polarizers were stereo perception as it relates to stereo video displays as proposed many times in the patent literature. However, well as a discussion of interfaces for stereo graphics and theproblem of manufacture prevented commercialization the interaction of stereo video and stereo graphics. A until Faris applied lithography to create the number of areas of interest are briefly reviewed and micropolarizer arrays [Faris]. This may be a viable accompanied by extensive citations from the patent and alternative to field sequential techniques where flat technical literature. These include single camera (70 screens or projection are involved, but for CRTs it has references) and dual camera (100 references) the same problem of aligning pixels with the optical stereoscopy, compatible 3D recording and transmission elements through a thick glass surface as the lenticular (57 references), head mounted displays (85 references), technique. Field sequential deviceswere described in the field sequential stereo (285 references), and patent literature many times without a commercially autostereoscopic systems including lenticular ( 64 viable product appearinguntil the 1980's. references), parallax barrier (22 references), Shmakov, working in St. Petersburg, Russia, stereoptiplexer (17 references), integral imaging (24 devoted much time to this field during the 1940s and references), direction selective mirrors, lenses or screens 1950s and wrote thefirst text on thesubject in 1953 but (26 references), volumetric displays (133 references), it only appeared in 1958 [Shmakov]. The proceedings of holovision (13 references), stereoendoscopy (14 the SPIE [Merritt and Fisher] and several symposia references), and stereosculpting (15 references). Also [Hamasaki-1992] are the best recent sources. The discussed are interfaces for stereo graphics and the literature on stereoscopic video is large and the patent interactionof stereovideo andstereo graphics. literature vast. The present review will concentrate on the , field sequential technique, since it is currently 1. History dominant, and is likely to remain so well into the 21st century. Stereoscopic television was a goal of the earliest experimenters with this new medium. Electronics 2. Stereovision and Electronic Displays pioneers such as Hammond, Logie Baird, Lee DeForest, Zworykin and others described 3DTV devices in their Stereo vision evolved hundreds of millions of years early patents1 . Logie Baird seems to have been the first ago in invertebrates as a critical survival mechanism. to actually build working devices. The first commercial The first definitive demonstration of stereovision in device may have been Dumont's dual CRT system that insects was recently accomplishedby a Swiss researcher appears to have been sold in the 1950s. Experiments who glued tiny prisms to the eyes of a praying mantis, which then missed its prey by precisely the calculated with anaglyph ( colored eyeglasses) video were numerous and broadcasts were done at least as early as 1953. amount. Humans have become so genetically degenerate Anaglyph broadcasts continue to be made sporadically that serious visual problems including loss of stereo perception are common. The vast majority have good 1 US Patents 1725710; 2107464; 2163749; British 266564, depth perception but sophisticated tests show wide 292365, 321441, 552582, 557837, 562168, 573008. As was variations. The individual variations in stereo-vision done here, references to patents will be given in the footnotes, with the countryabbreviation first (Those abbreviations not should be of vital concern in the creation and use of obvious are WO -- also known as WIPO -- an international patent filed in Switzerland, EP, a European patent, and GB, a patent of .Great Britain.) 2 us 4431265 Fall1995 THE INTERNATIONALJOURNAL OF VIRTUALREALITY 3 stereo systems, but are usually completely ignored. As that many strabismic (cross eyed) subjects have some with all other physiological systems, stereovision may depth perception due to a type of field sequential improve rapidly with use, both short term and long tem1. activation of the optic pathways by the reticular Repeated use of a stereo display can lead to more rapid activatingsystem in thebrain stem. fusion and greater comfort. Except for a few persons who practice frequently with a wide variety of stereo 3. Flicker and Asymmetrical Illumination displays and images, it is not possible to evaluate a stereo display systemor image by casual examination. As with Another common scapegoat for inadequate any otherparameter, a randomly selected individual may hardware,software, and lack of stereo trainingis flicker, be several standard deviations from the mean in eith(:r which is most noticeable in standard frequency ( e.g. 60 direction. Perhaps 10 percent who have severe problems Hz) field sequential systems. Flicker has been the with stereo under any conditions. Another IO percent subject of a great deal of research, nearly all of it qualifyas stereo prodigies due to their rapid, prolonged, monoscopic. It varies with many factors, especially and comfortable fusion of images which may be screen brightness and screen size. We must distinguish unpleasant or impossible for the average person, or due the flicker due to ambient illumination ("room flicker") to their other abilities such as making very fine depth from theflicker due to thedisplay ("image flicker"). In detemiinations. Variation with ageis to be expected alS addition, the image may flicker due to high luminosity is a circadian rhythm. Evaluation by a battery of users areas or to low rates of update. The image may still withknown stereovision abilitiesusing the hardware and flicker even at 120 Hz screenrefresh if theimage is not softwareexactly as it will be employedby theend user is updated in the proper way [Woods]. Decreasing the essential. This should include frequency and duration c,f level of ambientillumination in the room can reduce the use, similar imagery, ambient illumination, and viewing room flicker to imperceptible levels. Reducing screen distance, as well as exactlythe same monitor. The latt(:r luminosity with brightness and contrast controls will is necessary since in the dominant field sequential reduce image flickerto low or imperceptiblelevels. This technique the exact hues and saturations of the images, may reduce the contrast excessively, for some level of contrast and brightness, and thedifferent persistences of image flickeris common in 60 Hz displays. A white wall various phosphors are very important. Also, the same will have a noticeable flicker which is exacerbated if a hardware and software may yield dramatically different man in a dark shirt is in front of it and even more results if the color of figure and background are altered. noticeableif he has a lot of horizontal parallax.The same Long persistence green phosphors are a common conditions will tend to give considerable crosstalk with problem. Screen size and viewing distance, horizontal passive glasses or even autostereoscopic systems. and vertical parallax, binocular asymmetri(:s However, when the image lacks high luminosity areas (illuminationetc.), and nonstereo depth cues arecritical. flickerand crosstalkmay be nearly imperceptible even at Most stereo displays and images are created and used fairly high brightness. This condition occurs frequently with little attention to these factors, even when highly in naturalsubjects andcan be avoided much of thetime if skilled personnel are involved. A vital component of a one has thefield sequential display in mind. stereo project should be a stereoscopist having extensive It occurred to me a decade ago that one could experience with many systems and images. This is eliminate flickerby color coding thetwo images in every seldom considered necessary, resulting in defects in field and viewing them with field sequential anaglyphic hardware, software, viewing conditions, and viewers, as shutters. Field orframe I would contain the right image well as less-than-optimal images that are regarded as on thegreen phosphor and the left on the blue and red. natural limitations of electronic stereoscopy, field Field 2 the would contain the reverse, and the viewer sequentialinput, or head mounteddisplays. would wear dichroic LCD shutters to filter out image of It is even said that these are unnatural ways to looik the other eye. Each eye would receive 60 images a at images (as though2D CRT'S, photos, and books grew second. The coding could also be done with 3 color on trees!).