Chapter 2 Principles of Stereoscopic Depth Perception and ...

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2. Principles of Stereoscopic Depth Perception and Reproduction Figure 2.3: Panum’s fusional area. Points within Panum’s area are fused perceptually into a single image. Points that are closer or farther produce double images of crossed or uncrossed disparity (figure adapted from Palmer (1999), page 209). 56
2.2. Human depth perception provide the same information to the visual system as spatially separated views (Rogers & Graham 1982). 2.2.2 Accommodation and vergence In normal vision, the oculomotor mechanisms (accommodation and vergence) work in concert with stereopsis. Thus, when viewing an object the eyes converge on it, so that the disparities providing depth information about the object and its environment fall within Panum’s fusional area. The eye’s lens automatically focuses (accommodation) on the currently fixated object, making it stand out against its surroundings. Thus, double images in front or behind the plane of fixation tend to be out of focus and will ’disappear’ in increasing optical blur. Accommodation and vergence operate in a closely coupled fashion, which implies that under natural viewing conditions all objects which are in focus are also fixated upon, thus bringing them within the limits of fusion. However, most of the currently available stereoscopic display techniques do not support the linkage between accommodation and vergence, forcing the observer to focus at a fixed distance (i.e., on the screen plane where the image is sharpest) irrespective of the fixation distance (i.e., where the object is located in depth according to the disparity information). This de-coupling of accommodation and vergence has been suggested as a potential cause of visual strain. This topic will be addressed in more detail in Chapter 4. 2.2.3 Individual differences The mechanism that underlies stereopsis is extremely precise, but also somewhat fragile. Visual disorders in early childhood, even if only temporary, may result in stereoblindness, which is estimated to affect 5-10% of the population. Richards (1970) did a survey of 150 members of the student community at MIT and found that 4 % of the population was unable to use the depth cue offered by disparity and 10% had great difficulty deciding the direction of a hidden Julesz figure (i.e., a random-dot stereogram) relative to the background. More recent studies have shown that the performance of observers on tests of stereoanomaly depends to a large extent on the duration the observer is allowed to look at the test figures. The proportion of the population unable to perform a depth discrimination task decreases with increasing display duration (Patterson & Fox 1984, Tam & 57
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Chapter 2 Principles of Stereoscopic Depth Perception and ...

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