The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B3b. Beijing 2008 
c) A road segment has a constant width. 
2) Radiometry 
d) The road surface usually is smooth and homogeneous; 
e) The road has a good contrast with its adjacent areas. 
3) Topology 
f) The road will continue and do not stop without a 
reason; 
g) The roads intersect and form a network. 
4) Function 
h) The roads connect human settlements. 
5) Context 
i) Overpasses, higher roads, adjacent buildings and tress 
may cast a shadow; 
j) Roads may be occluded by vehicles and other obstacles. 
The operator use the above characters and prior knowledge to 
detect and identify a road segment, and the proposed tracker 
works based on a), b), c), d), e) properties 
2.2 Principles of angular texture signature 
A texture measure is described in (Haverkamp, 2002) and 
defined and extended by us as follows. At each pixel p of a 
grey image, angular texture signature (ATS) T(a, w, h, p) is 
defined as the mean, standard deviation, variance or entropy for 
a rectangular set of pixels of width w and height h around the 
potential road pixel p whose principal axis lies at an angle of 
a from the potential road direction. This measure is computed 
for a set of angles (2 0 ,..., Cl n at pixel p .At the point p , the 
ATS is defined as the set of values 
{T(a 0 ,w,h,p),T(a { ,w,h,p),...,T(a n ,w,h,p)} . 
The graph and values of an ATS for a single point p are 
shown in Fig. 1. The local limits on this graph correspond to the 
most likely directions of the road at point p (e.g. directions 3, 
10, 20, 29). At each pixel p , the number k and location of the 
strong local limits are computed from the ATS. For example, 
the signature shown in Fig. 1(a) has 4 limits that are significant. 
We refer to the number k of limit as the degree of the pixel. 
The texture measures that are used in road detection are: the 
degree of the pixel and the direction of the limit. In our 
approach, on the assumption that the roads have the above a), b), 
c), d), e) properties, a rectangular template is extended from and 
rotated 180 deg from the perpendicular of the potential road 
direction about each road pixel. At discrete intervals about the 
pixel, the ATS is calculated; and the direction of the limit is 
regarded as the road direction. If the ATS takes the variance or 
entropy as measure, the direction of local minimum is taken; 
while if the ATS takes the mean as measure, the direction of 
local minimum is takes for bright roads and the direction of 
local maximum is taken for darker road as shown in Fig. 2. 
Once the road direction is given, move on one step along the 
direction and iterate the above process until the tracker fails or 
reaches to another tracked road trajectory or reaches to the 
boundary of the image. 
The parabola in the X _L y plane can have arbitrary 
orientation, having an equation of the form: 
Ax + Bxy + Cy + Dx + Ey + F — 0 (l) 
where X and y are coordinates of point on the parabola, 
A,B,C,D,E,F are parameters. 
100 
Fig.l Angular texture signature (a) The effect of rotating 
templates in 36 discrete angles (the rectangles whose indexes 
are odd is invisible for convenience) (b) The standard 
deviations of 36 templates (c) The means of 36 templates