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The purpose of stereo imagery is the creation of the perception (in the human viewer) of a 3-D world, for purposes such as entertainment, visualization, education, and allowing people to better learn about and control the world about them. Human perception of 3-D is based upon a combination of many cues from the senses, as well as internal mental templates and expectations. In a stereo presentation, if some of the 3-D cues are inconsistent with others, the perceptual system receives conflicting information, and seeks to find a consistent interpretation. In cases of severe conflict, 3-D perception of the scene may be totally disrupted or highly inaccurate. Even if the user is able to perceive a consistent 3-D view, the effort required to resolve conflicts may reduce the sense of realism and the enjoyment of viewing, and may contribute to fatigue, eyestrain, and headache. Many experienced users of stereo imagery have learned to ignore a certain degree of image inconsistency, and can derive considerable pleasure from viewing even significantly inconsistent stereo images. Unfortunately, novice users generally do not have this ability, and while they may not be able to verbally explain what's wrong with a stereo image, they may comment that it gives them a headache, or that something's not quite right about it, or that they have trouble seeing the depth. Worse, a novice user may conclude that the problems with this image are characteristic of all stereo images, and decide (and tell others) that stereo is not worthwhile. Since the field of display can greatly benefit from the development of a large user base (to provide money to support manufacturers and future technology development, and to draw the interest of content providers), it is important that every effort be made to make sure that novice users have a pleasant viewing experience, with stereo views that have a high degree of consistency in their 3-D cues. Even experienced users have varying degrees of tolerance for inconsistencies, so any improvement in stereo realism increases the number of people who can enjoy it. Despite the importance of providing consistent 3-D cues, no existing display system can do a perfect job of displaying any significant variety of stereo images, nor will such a perfect display be created in the next several decades. It is therefore important to look at the sources of 3-D cue inconsistencies in terms of the severity of impact on the viewing experience, and the effort required to minimize the effect of each inconsistency.
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