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d 0 =
0 . 7).
But for highmagnifications of zooms ( f > 32mm), the images are perceived as flattened
in depth. The distance d 0 =
0,8m (for f
=
16mm: ρ
=
1 . 4
for f
=
22mm: ρ
=
1
for f
=
32mm: ρ
=
0 . 8m implies a correct field of vision facing the screen
without perceiving its lines.
Constraint A : the natural relation between accommodation and convergence of
the eyes is slightly disrupted if the main object is observed at zero parallax z p 0 =
0. As
a result, the lag D h of the images should be adjusted:
f
·
M
·
e c
z p 0 =
(13.2)
2
·
D h
Constraint B1 :
the horizontal parallax should be limited experimentally.
P h < 1 . 5
=
0 . 026 rd
1
z p 0
z
1
f
·
M
·
e c ·
P h =
< 0,026
(13.3)
d 0
It is necessary to take the smallest possible distance e c (distance between the 2 cameras)
mechanically, which in our case is equal to 65mm, so that the possible variation of z
around z p 0 is the greatest.
If z > z p 0 , then it is necessary that z < 1 / (1 /z p 0
0 . 026
·
d 0 /f
·
M
·
e c )
If z < z p 0 , then it is necessary that z > 1 / (1 /z p 0 +
·
·
·
0 . 026
d 0 /f
M
e c ) (to be avoided
on a small screen)
Constraint B2 : the vertical parallax is zero in case of cameras with parallel axes.
Constraint B3 : two neighbouring planes at the level of the display, observed simul-
taneously, should not have a big difference in depth. The threshold S that is not to be
exceeded is defined by the proximities, inverse of the distances z 1 and z 2 of the two
planes:
1
z 1
1
z 2
< S
(13.4)
with z expressed in metres, the limit of S is 1m 1 and it is preferred that S is equal to
0.5m 1 .
13.1.3.3 Deductions of constraints
The constraints that limit the viewing area depth-wise are No. 2 and 4. The first two
inequations are derived depending on the different values of the focal length f (16, 22,
32 and 48mm) and z 1 or z 2 =
z p 0 for the third inequation (Figure 13.11).
13.1.3.4 Limitation of stereoscopic vision
For f
16mm, constraint B3 is always verified if constraint B1 is verified, which
means that when the horizontal parallaxes are limited, the differences in depth between
the objects are also sufficiently limited to provide a vision without strain. For focal
lengths < 16mm that is best avoided (depth distortion is very important), ρ> 1 . 4, it is
constraint B3 that limits the zone of observation. Vision is no problem for focal lengths
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