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different parameters of stimuli displayed on a cathode screen with active eyeglasses:
the colour, the display time, the vertical position on the screen and the positive or
negative parallax. The results imply that the limit of merging (given in angle of
parallax) is very low for a brief stimulus of 0.2 seconds: either 27minutes of arc
(negative parallax), or 24minutes of arc (positive parallax) compared with a stim-
ulus of 2 seconds: either 5 degrees (parallax < 0), or 1.5 degrees (parallax > 0). It is
therefore more difficult to observe rapidly moving 3D images. Red is easier to merge
than white (for stimuli of 2 seconds: 6.2 degrees for red and 3.7 degrees for white in
parallax < 0). Errors in estimating the depth are determined by experiments: the error
(of the parallax angle) is 2.2 minutes for images with parallax angles between 0 and
30minutes.
The studies conducted at IRBA (Army Biomedical Research Institute) of the
Defence Ministry, while being more accurate, corroborate the previous results. The
limit of merging also depends on the spatial frequency of images, which is not studied
in the previously mentioned works. On the basis of their studies, we can conclude
that greater horizontal disparities can be merged when visual simulations have low
spatial frequencies. Their studies have highlighted two mechanisms involved in the
fusion depending on the time of stimulus: immediate merging for smaller disparities
and non-immediate merging for greater disparities, putting the reflex vergence of eyes
into play. The results also show that: at spatial frequency of 4.8 cycles/degree, imme-
diate merging up to 20' of arc and maximum merging at about 52' of arc. At a spatial
frequency of 0.22 cycles/degree, immediate merging up to 80' of arc and maximum
merging at about 176' of arc (Perrin et al., 1998).
The images to be displayed for each eye should have sufficient resolution and fre-
quency, whether on one or on two screens: it is recommended to catch at least 25 images
per second for each eye, and hence a frame frequency of 50Hz (60Hz American stan-
dard). According to the techniques used, it is necessary in some cases to have 120Hz
displays for stereoscopic screens and 180Hz monochrome displays for head-mounted
displays using a cathode ray tube. It is necessary to note that some manufacturers
recommend screens with such frequencies, but at the expense of the resolution, which
is reduced in relation to lower frequencies. The higher the resolution, the better is the
quality of images. The threshold of satisfaction depends on the operations that you
wish to perform with the display. The resolution should be better if you wish to read or
write a text on the screen (in the case of computers) or display aesthetic images (in the
case of films).
13.3 CONCLUSION
The user of a stereoscopic vision system should remember that restitution of vision
is similar to natural vision, but not identical. This restitution depends on geometric
and psychophysical criteria. Let's not forget that each human being reacts differently
in the presence of stereoscopic vision. Visual immersion theoretically appears to be an
additional advantage in virtual reality, but the technical and ergonomic problems that
it raises are to be taken into account for frequent and prolonged use.
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