Page 62 - Dynamic Vision for Perception and Control of Motion

46 2 Basic Relations: Image Sequences – “the World”

Under certain lighting conditions, due to different reflection angles, the two
sides potentially visible may appear at different intensity values; this allows recog-
nizing the inner edge. However, this is not a stable feature for object recognition in
the general case.
The length of the rod can be Rod length L
recognized only in the image di- Center of gravity (cg)
rectly when the angle between the (a) (b) (c) Enlarged
optical axis and the main axis of cross–sections
the rod is known. In the special Figure 2.12. Rods with special applications in
case where both axes are aligned,
road traffic
only the cross section as shown in
(a) to (c) can be seen and rod
length is not at all observable. When a rod is thrown by a human, usually, it has
both translational and rotational velocity components. The rotation occurs around
the center of gravity (marked in Figure 2.12), and rod length in the image will os-
cillate depending on the plane of rotation. In the special case where the plane of ro-
tation contains the optical axis, just a growing and shrinking line appears. In all
other cases, the tips of the rod describe an ellipse in the image plane (with different
excentricities depending on the aspect conditions on the plane of rotation).

2.2.4.2 Coarse-to-fine 2-D Shape Models
Seen from behind or from the front at a large distance, any road vehicle may be
adequately described by its encasing rectangle. This is convenient since this shape
has just two parameters, width B and height H. Precise absolute values of these pa-
rameters are of no importance at large distances; the proper scale may be inferred
from other objects seen such as the road or lane width at that distance. Trucks (or
buses) and cars can easily be distinguished. Experience in real-world traffic scenes
tells us that even the upper boundary and thus the height of the object may be omit-
ted without loss of functionality. Reflections in this spatially curved region of the
car body together with varying environmental conditions may make reliable track-
ing of the upper boundary of the body very difficult. Thus, a simple U-shape of
unit height (corresponding to about 1 m turned out to be practically viable) seems
to be sufficient until 1 to 2 dozen pixels on a line cover the object in the image.
Depending on the focal length used, this corresponds to different absolute dis-
tances.
Figure 2.13a shows this very simple shape model from straight ahead or exactly
from the rear (no internal details). If
the object in the image is large (a) (b) (c)
enough so that details may be distin-
guished reliably by feature extrac-
tion, a polygonal shape approxima-
tion of the contour as shown in Figure 2.13. Coarse-to-fine shape model of
Figure 2.13b or even with internal a car in rear view: (a) encasing rectangle of
details (Figure 2.13c) may be chosen. width B (U-shape); (b) polygonal silhou-
In the latter case, area-based features ette; (c) silhouette with internal structure
such as the license plate, the dark