Page 82 - Dynamic Vision for Perception and Control of Motion

3 Subjects and Subject Classes
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Since most of the developments of vision systems for road vehicles are using the
simple approach of mounting cameras directly on the vehicle body, some of the
implications will be discussed first so that the limitations of this type of visual sen-
sor are fully understood. Then, the more general and much more powerful verte-
brate-type active vision capabilities will be discussed.

3.3.2.1 Eyes Mounted Directly on the Body

Since spatial resolution capabilities improve with elevation above the ground, most
visual sensors are mounted at the top of the front windshield. Figure 3.3 shows an
example of stereovision. A single camera
or two cameras with different focal
lengths, not requiring a large stereo base,
can be hidden nicely behind the rear-view
mirror inside the vehicle. The type of vi-
Figure 3.3. Two cameras mounted fix sion system may thus be discovered by
on vehicle body visual inspection when the underlying
principles are known.
Pitch effects: When driving on smooth surfaces, pitch perturbations on the body
are small (less than 1°), usually. Strong braking actions and accelerations may lead
to pitch angle changes of up to 3 or 4°. Pitch angles have an influence on the verti-
cal range of rows to be evaluated when searching for objects on a planar ground at
a given distance in the 3-D world. For a camera with a strong telelens (f.o.v. of ~
same size as the perturbation, 3 to 4°), this means that an object of interest previ-
ously tracked may no longer be visible at all in a future image! In a camera with a
normal lens of ~ 35° vertical f.o.v., this corresponds to a shift of only ~ 10 % of the
total number of rows (~ 50 rows in absolute terms). This clearly indicates that
body-fixed vision sensors are limited to cameras with small focal lengths. They
may be manageable for seeing large objects in the near range; however, they are
unacceptable for tracking objects of the same size further away.
When the front wheels of a car drive over an obstacle on the ground of about 10
cm height with the rear wheels on the flat ground (at a typical axle distance ~ 2.5
m), an oscillatory perturbation in pitch with amplitude of about 2° will result. At 10
meters distance, this perturbation will shift the point, where an optical ray through
a fixed pixel hits an object with a vertical surface, by almost half a meter up and
down. However, at 200 meters distance, the vertical shift will correspond to plus or
minus 10 meters! Assuming that the highly visible car body height is ~ 1 m, this
perturbation in pitch (min. to max.) will lead to a shift in the vertical direction of 1
unit (object size) at 10 m distance, while at 200 m distance, this will be 20 units.
This shows that object-oriented feature extraction under perturbations requires a
much larger search range further away for this type of vision system. Looking al-
most parallel to a flat ground, the shift in look-ahead distance L for a given image
line z is much greater. To be precise, for a camera elevation of 1.5 m above the
ground, a perturbation of 50 mrad (~ 3°) upward shifts the look-ahead distance
from 30 m to infinity (to the horizon).
If the pitch rate could be measured inertially, the gaze-controllable eye would
allow commanding the vertical gaze control by the negative value of the pitch rate