Page 85 - Dynamic Vision for Perception and Control of Motion

3.3 Perceptual Capabilities 69

ing in the actual simultaneous field of view can be detected and can attract atten-
tion (if there is no other sensory modality like hearing in animals, calling for atten-
tion in a certain direction). If the entire potential field of view has to be covered for
detecting other objects, this can be achieved only by time-slicing attention with the
wide field of view through sequences of viewing direction changes (scans). Usu-
ally, in most applications there are mission elements and maneuvers for which the
viewing area of interest can be determined from the mission plan for the task to be
solved next. For example, turning off onto a crossroad to the right or left automati-
cally requires shifting the field of view in this direction (Chapter 10 and Section
14.6.5).
The request for economy in vision data leads to foveal-peripheral differentia-
tion, as mentioned above. The size and the increase in resolution of the foveal
f.o.v. are interesting design parameters to be discussed in Chapter 12. They should
be selected such that several seconds of reaction time for avoiding accidents can be
guaranteed. The human fovea has a f.o.v. from 1 to 2°. For road vehicle applica-
tions, a ratio of focal lengths from 3 to 10 as compared to wide-angle cameras has
proven sufficient for the same size of imaging chips in all cameras.
Once gaze control is given, the modes of operation available are characteristic
of the system. Being able to perform very fast gaze direction changes reduces the
time delays in saccadic vision. In order to achieve this, usually, nonlinear control
modes taking advantage of the maximal power available are required. Maximum
angular speeds of several hundred degrees per second are achievable in both bio-
logical and technical systems. This allows reducing the duration of saccades to a
small fraction of a second even for large amplitudes.
For visual tracking of certain objects, keeping their image centered in the field
of view by visual feedback reduces motion blur (at least for this object of special
interest). With only small perturbations remaining, the relative direction to this ob-
ject can be read directly from the angle encoders for the pointing platform (solving
part of the so-called “where”-problem by conventional measurements). The overall
vision process will consist of sequences of saccades and smooth pursuit phases.
Search behavior for surveillance of a certain area in the outside world (3-D
space) is another mode of operation for task performance. For optimal results, the
parameters for search should depend on the distance to be covered.
When the images of the camera system (vehicle eye) are analyzed by several de-
tection and recognition processes, there may be contradictory requirements for
gaze control from these specialists for certain object classes. Therefore, there has to
be an expert for the optimization of viewing behavior taking the information gain
for mission performance of the overall system into account. If the requirements of
the specialist processes cannot be satisfied by a single viewing direction, sequential
phases of attention with intermediate saccadic gaze shifts have to be chosen
[Pellkofer 2003]; more details will be discussed in Chapter 14. It is well known that
the human vision system can perform up to five saccades per second. In road traf-
fic environments, about one to two saccades per second may be sufficient; coarse
tracking of the object not viewed by the telecamera may be done by one of the
wide-angle cameras meanwhile.
If the active vision system is not able to satisfy the needs of the specialists for
visual interpretation, it has to notify the central decision process to adjust mission