Part Two

Project Analysis

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2.2 Technical Survey
2.2.1 Autosteroscopic 3D

Abstract
Three classes of autostereoscopic displays are described: re-imaging displays, volumetric displays, and parallax displays. Re-imaging displays re-project an existing three-dimensional object to a new location or depth. Volumetric displays illuminate

points in a spatial volume. Parallax displays emit directionally-varying image information into the viewing zone. Parallax displays are the most common autostereoscopic displays and are most compatible with computer graphics. Different display technologies of the three types are described. Computer graphics techniques useful for three-dimensional image generation are outlined.


Introduction
After many years of relative obscurity, three-dimensional displays have recently become both increasingly popular and practical in the computer graphics community. This interest can be attributed to many factors. In our daily lives we are surrounded by synthetic computer graphic images in print and on television, and can now even generate similar images on personal computers in our home. We also have holograms on our credit cards and lenticular displays on our cereal boxes. And has it really been so many years since we first saw Princess Leia projected into thin air in the Star Wars motion picture? In fact, the general public has been excited about three-dimensional images since the days when stereoscopes graced every mantelpiece at the turn of the century, through the 3D movie craze of the early 1950's, the wonder of holography in the 1960's, and the new frontier of virtual reality today. With each new technology or movie, the excitement seems to grow.


Developments in the computer graphics industry have also done their part to make spatial images more practical and accessible. In the business of computer graphics, the computational power now exists for desktop workstations to generate stereoscopic image pairs for interactive display. At the high end of the computational power spectrum, the same advances that permit intricate object databases to be interactively manipulated and animated also permit large amounts of image data to be rendered for high quality three-dimensional displays. Finally, there seems to be a general realization in the research and scientific community that the two-dimensional projections of three-dimensional scenes traditionally referred to as "three-dimensional computer graphics" are insufficient for inspection, navigation, and comprehension of some types of multivariate data. For these databases, the oft-neglected human depth cues of stereos, motion parallax, and to a lesser extent ocular accommodation are essential for image understanding.


The broad field of virtual reality has driven the computer and optics industries to produce better stereoscopic helmet- or boom-mounted displays, as well as the associated software and hardware to render scenes at rates and qualities needed to produce the illusion of reality. However, most journeys into virtual reality are currently solitary and encumbered ones: user often wear helmets or other devices that present the three-dimensional world to them, and only to them. Presenting a three-dimensional image to a casual passerby, a group of collaborators, or an audience requires a different technology: autostereoscopic displays.


Autosteroscopic Display
Autostereoscopic displays present a spatial image to a viewer without the use of glasses, goggles, or other viewing aids. Autostereoscopic displays are appealing because they offer the best approximation to the optical characteristics of a real object. As a result, though, there is much misunderstanding and misinformation by those who would oversell the capabilities of a particular technology. This paper will try to outline the strengths and practical limitations of the different technologies by classifying them into broad categories.
Our current understanding of physics does not include a practical way of forcing photons to change direction in the absence of an optical medium. Thus, a fundamental and general statement can be made about all spatial displays, whatever its particular technology. This paper will refer to this requirement as the projection constraint:
A display medium or element must always lie along a line of sight between the viewer and all parts of a spatial image.
Photons must originate in, or be redirected by, some material. The material can be behind, in front of, or within the space of the image, but it must be present. All claims to the contrary violate what we understand about the world. Figure 1 shows the possible relationships between the image and the display. A corollary to this constraint is the observation that air, water, or smoke are, in general, very poor display media. Images appearing "in mid-air", called aerial images, will invariably have originated not in the air from some other medium. Technologies lavished with claims of mid-air projection should always be scrutinized with regard to the fundamental laws of physics .


A specific and practical result of the projection constraint is that no matter where a spatial image appears with respect to its display, the image will be clipped by the display's physical boundaries. If for instance, an image appears in front of its display, a sufficient translation of the viewer will cause part or all of the object to intersect and "fall off" the edge of the display. This condition, known as a window violation, is particularly disturbing for aerial images .Figure 2 illustrates a window violation.

Figure 2. A window violation.
Physically realizable autostereoscopic displays can be classified into three broad categories: re-imaging displays, volumetric displays, and parallax displays. Re-imaging displays capture and re-radiate the light from a three-dimensional object, perhaps to a new location in space. Volumetric displays span a volume of space, allowing individual parts of the space to be illuminated. Finally, parallax displays are surfaces that radiate light of directionally-varying intensity. Displays of each type have been used in commercial display systems, and each has inherent strengths and weaknesses.

2.2.2 Existing Tool for Autosterescopic--DTI
We cannot talk about autosteroscopic without referring to DTI. Dimension Technologies Inc. (DTI) has developed and patented a unique method for generating three-dimensional images by use of stereo pairs. The results of its work have been commercialized, and an innovative autosteroscopic display, the Virtual Window™, was introduced.


Unlike other stereoscopic displays, DTI unit generates vivid, full-color three-dimensional images that can be viewed without the need to wear special eyeglasses. This feature makes the use of the autosteroscopic displays very convenient and is particularly important in commercial applications.
The principle of autostereoscopic image presentation is frequently used in three-dimensional postcards and large advertising displays that are intended to enable the observer to perceive depth by looking at a two-dimensional picture. A stereo pair (i.e., a pair of images corresponding, respectively, to the views through the left and right eyes) are interlaced in alternate columns in a two-dimensional image. A special optical device, called the "lenticular lens," is placed in front of the interlaced image or, in the case of a postcard, bonded directly to the front surface. The lenticular lens is an array of very narrow vertical cylindrical lenslets spaced to correspond to the columns of the interlaced stereo pair. In this manner, the appropriate images of the stereo pair are directed to the proper eyes thus generating a three-dimensional image.
DTI has applied the same principle to its autostereoscopic displays, which contain liquid-crystal displays (LCDs) that are viewed by observers. To generate three-dimensional images, the LCD presents left and right halves of a stereo pair on alternate columns of pixels at a rate of 60 frames per second. The left image appears on the odd columns and the right image appears on the even columns. If the LCD in use has 1,024 columns and 768 rows of pixels, each complete stereoscopic image consists of 512 columns and 768 rows.
Both halves of a stereo pair are displayed simultaneously and directed to the corresponding eyes. This is accomplished with a special illumination plate located behind the LCD and employing a lenticular lens of the type mentioned above. Using light from compact, intense light sources, the illumination plate optically generates a lattice of very thin, very bright, uniformly spaced vertical light lines. The lines are precisely spaced with respect to pixel columns of the LCD, and, because of the parallax inherent in binocular vision, the left eye sees all of these lines through the odd columns of the LCD, while the right eye sees them through the even columns, thus enabling the observer to perceive the image in three dimensions. This arrangement, exclusive to DTI, is called "parallax illumination."
When the halves of the stereo pair are made to correspond to the scene perspective that would naturally be seen by the respective eyes, a vivid illusion of three-dimensionality is created. The objects seem to come out of the screen, giving the impression of an open window through which objects can protrude or retreat to the background, hence, the name Virtual Window™. In addition, the parallax illumination system is designed such that it can generate in the same display, at a flick of a switch, both the stereoscopic and non stereoscopic images ?the latter at double the resolution.


The displays are compatible with computer workstations, including PC and Power Mac platforms, and accept real-time inputs through multiplexers in National Television Systems Committee (NTSC) and PAL formats from pairs of video cameras.
It is possible to produce displays that enable several people to view in stereo at the same time. The displays are light in weight and are available at moderate cost.
Efforts continue to further enhance the Virtual Window™ displays to obtain greater resolution, and to provide for generation of hologram like imagery, in which objects can be observed from different perspectives, and, most importantly, in developing applications. For scientific applications, some areas of interest include the display of multidimensional graphs and tables, molecular structures, turbulent flows, biological and artificial structures, and images obtained by use of stereo microscopes. Other applications include remote control of vehicles and robots, inspection of luggage and parcels, quality assurance in the production of semiconductor devices and other miniature structures, aircraft and spacecraft cockpit displays, interpretation of aerial photography, medical imaging including endoscopy, and, last but not least, such consumer products as video games and three-dimensional television.

more information about DTI, please visit http://www.dti3d.com/


2.2.3 Other Possible Applications and Problems need to be solved
Applications of the autostereoscopic display include:

  • Scientific Visualization;
  • Medical Imaging;
  • Tele-presence ;
  • Gaming;
  • CRT Based systems;

These are possible applications of autostereoscopic 3D in commercial use in the near future. But, since autostereoscopic 3D is a new technology, many bottlenecks restrict its development and currently, the most urgent problems of autostereoscopic 3D should be solved are:

  • Need For Flexible Viewing Technology. Now, when a user sit in front of a DTI monitor, he needs to adjust his eyes position to get an ideal visual effect. That means the monitor does not have any auto eye tracking technology. It is impossible for a viewer to keep the same position for long time without any motion, so auto-adjusting technology is much urgent for auto stereoscopic 3D researchers and commercial developers;
  • Low Resolution. Compared with normal CRT or LCD, when viewing autostereoscopic 3D images, DTI monitor seems to sacrifice the image's resolution for autostereoscopic visual effect. But what end-users and customers need is autostereoscopic 3D effect plus high clarity. The technical dilemma of 3D effect and high resolution should be given a perfect compromise by researchers;
  • Pricing Problem. This is a more commercial than technical problem but personally I do not think this is a serious problem that tackle autostereoscopic 3D development because the prices of autostereoscopic 3D terminals, such as DTI monitor, PDA or mobile phones are going down steadily and more and more people are capable of affording such a device.

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