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Figure 3.7 Variations of light and shadows increase the “3D effect'' of the two cubes
that the natural vision of human beings is an active vision. The eyes are almost always
moving to observe the environment and the crystalline lenses change their form to
facilitate accommodation. On the other hand, a camera observes the real environment
in a passive vision. The depth of field of any photograph is a depth index that corre-
sponds partially to the accommodation phenomenon. We can thus use these effects of
depth of field in computer-generated images to give a more “3D effect'' to the space
observed.
In monocular vision, the monocular cues that are learnt subconsciously make it
possible to understand the third dimension with one eye, even if the perception of depth
is clearly better quantified with binocular vision. Monocular cues can be categorised
in the following manner:
1 Light and shadows:
The variations in light and shadows on the objects help having a better perception of
the three-dimensional form of these objects (Figure 3.7).
2 Relative dimensions:
Each object sends its image to the eye. The dimensions of this image are proportional
to that of the object and decrease as per the distance with respect to the eye. The
brain knows the “normal'' dimensions of real objects. It can thus understand their
approximate distances. Similarly, estimate of depth is facilitated for a series of real or
imaginary objects of identical shapes: they are basically perceived at the same size and
placed at different distances in a 3D space (Figure 3.8).
3 Interposition or blanking:
An object can partially hide another object placed behind it, which makes it possible to
relatively position these objects in depth (Figure 3.9). The brain interprets this image
through cognitive reasoning: it spontaneously perceives an ellipse in front of a rectangle
and not as two adjacent shapes on the same plane.
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