Page 63 - Dynamic Vision for Perception and Control of Motion

2.2 Objects 47

tires, or the groups of signal lights (usually in orange or reddish color) may allow
more robust recognition and tracking.

2.2.4.3 Coarse-to-fine 3-D Shape Models

If multifocal vision allows tracking the silhouette of the entire object (e.g., a vehi-
cle) and of certain parts, a detailed measurement of tangent directions and curves
may allow determining the curved contour. Modeling with Ferguson curves [Shirai
1987], “snakes” [Blake 1992], or linear curvature models easily derived from tangent
directions at two points relative to the chord direction between those points [Dick-
manns 1985] allows efficient piecewise representation. For vehicle guidance tasks,
however, this will not add new functionality.
If the view onto the other car is from an oblique direction, the depth dimension
(length of the vehicle) comes into play. Even with viewing conditions slightly off
the axis of symmetry of the vehicle observed, the width of the car in the image will
start increasing rapidly because of the larger length L of the body and due to the
sine-effect in mapping.
Usually, it is very hard to determine the lateral aspect angle, body width B and
length L simultaneously from visual measure-
ments. Therefore, switching to the body diago- Diagonal D
nal D as a shape representation parameter has L
x f
proven to be much more robust and reliable in
real-world scenes [Schmid 1993]. Figure 2.14
L/2
shows the generic description for all types of O B/2
rectangular boxes. For real objects with H y f
-L/2
rounded shapes such as road vehicles, the en- B
casing rectangle often is a sufficiently precise
description for many purposes. More detailed Figure 2.14. Object-centered re-
shape descriptions with sub–objects (such as presentation of a generic box
wheels, bumper, light groups, and license with dimension L, B, H; origin in
plate) and their appearance in the image due to center of ground plane
specific aspect conditions will be discussed in
connection with applications.
3-D models with different degrees of detail: Just for tracking and relative state
estimation of cars, taking one of the vertical edges of the lower body and the lower
bound of the object into account has proven sufficient in many cases [Thomanek
1992, 1994, 1996]. This, of course, is domain specific knowledge, which has to be
introduced when specifying the features for measurement in the shape model. In
general, modeling of highly measurable features for object recognition has to de-
pend on aspect conditions.
Similar to the 2-D rear silhouette, different models may also be used for 3-D
shape. Figure 2.13a corresponds directly to Figure 2.14 when seen from behind.
The encasing box is a coarse generic model for objects with mainly perpendicular
surfaces. If these surfaces can be easily distinguished in the image and their separa-
tion line may be measured precisely, good estimates of the overall body dimen-