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Figure 13.5 Difference between natural vision and stereoscopic restitution
a stimulus of two seconds in positive parallax. The conditions are more restrictive in
case of quick movements of objects and if the spatial frequencies of images are high,
as explained in the chapter on human vision (see the chapter “The senses of man''
of volume 1 of this treatise), for numerical values). The problem is more crucial for
low-width screens and the operator must move away from them sufficiently to obtain
an adequately small parallax angle, but his field of vision decreases relatively. Increase
in visual immersion created by 3D vision can be negated by decreasing the field of
vision.
We have mentioned that the vergence-accommodation relation can artificially be
made incorrect when we look at stereoscopic images on a screen: a virtual object placed
behind the screen makes the optical axes converge on it while the eyes adapt to the
screen (Figure 13.5). This modification of the vergence-accommodation relation may
create visual difficulties and strain to the user, except when the object is at screen level.
To conclude, it is necessary to restrict positioning the main object, should it exist, close
to the plane of the screen.
Cognitive considerations signify another problem. A computer or television screen
is perceived by the brain as a window bordered by the edges of the screen. These edges
can be a source of confusion. The objects displayed in negative parallax and partially
blanked by the edges of the screen (mentally behind the window) and therefore located
in front of the screen by binocular vision create a conflict in their placement. This
phenomenon can prevent fusion of images and disturb the observer. It is important
to make the inference that it is rarely desirable to create objects “emerging from the
screen'', in spite of the appealing aspect that it offers. This technique is to be used
only for very large screens of spectacular demonstrations of 3D films. You may notice
that advertisements of stereoscopic systems often make claims about this phenomenon
through images, which in fact is impossible to reproduce on these devices!
If the observer is translated parallel to the screen, he sees the objects move
abnormally: he perceives pseudoscopic movements (Figure 13.6).
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