ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision“, Graz, 2002 
  
template (7) of a pre-defined width (W) and length (L) is rotated 
at one of the terminal point of the road line and matched with 
the integrated image of road mask and road seed to find the next 
road line point (see Figure 9). The procedure stops when 
matching result reduces to a level lower than a give threshold. 
At this point, the operator examines whether the road line is 
correct, or run to a wrong direction, or loss its way in a local 
maximum. A control point is assigned whenever necessary by 
the operator to guide the road line extraction. In this research, 
matching cost of the road template at an image point p with a 
rotation angle & is defined as follows. 
fi. py- Y G' je M)|IQ' 7e S) Q) 
(i,j)eT 
A o. —sino Yi ; 
BEY. es 1 i o px 3) 
Jy sin o 
cosa |j PY 
In the followings, we address the algorithms of automatically 
tracing a road line from a starting point and guiding the road 
line tracing using a directional point in details. 
4.1 Tracing a road line from a starting point 
Given a starting point p, —(x,,y,) , road line extraction 
consists of two steps, i.e. finding the starting direction and 
extending the road line in an iterative way. 
Finding the direction that a road is most probably pass through 
Po is conducted as follows. If there are road seeds near Po the 
directions of road seeds are re-sampled using a histogram. The 
most often appeared directions a, €[0,360°) and 
o, ela, —Aa,a, + Aa) are estimated, where A is a pre- 
defined angle based on the maximum permissible curvature of a 
road line. On the other hand, if there is no road seed near b 
matching of road template is conducted in the range of [0, 360?]. 
Angles o, and qv, are estimated subsequently, which yields 
Or) = Max a and 
f, (0. p) „max, f( p) 
a,)= max 
Jr 2) ae [o +180°—Ay,œ, +180+Ay) 
o, are the directions to extend the road line from p to the 
sides. Let 
f(a, p) respectively. o, and 
opposite P, = P, + L*(cosa,,sinæ,)" and 
P» = p, + L*(cosa,,sin œ, )" | and P» with the 
extensional direction X, and o, are added to the road line as 
new road terminal points (see Figure 9(a)(b)). 
Extending the road line from a terminal point p, with the 
extensional direction of . is conducted as follows. If there are 
road seeds near p, and their directions belong to the range of 
[, — ^o, a, * Aa]. the most often appeared direction qx i4 of 
the road seeds is found using histogram. On the other hand, if 
no road seed is found with a direction in a, - Ao, a, + Ac]; 
matching of road template is conducted in the range of 
[a, -Aa.a, +Ac] - An angle si that yields 
fr (0. D) gi is a t (a, p) is 
Pur = p, * L* (cosa, sino)! » p, With the extensional 
is added to the road line as a new road terminal 
estimated. Let 
direction ¢ d 
point (see Figure 9(c)(d)). 
  
  
   
  
  
  
  
     
  
  
  
  
  
  
  
hr iu FH i "OE i 7 
m / ^x " Road k & Ae FA d Fe. f 
AON M c oad mask _! d oad seeds bs 
VS p ; 
2 
* ; 
EM (d 
ir (d) 
Specified by the operator 
o Newly estimated road point 
e Previously estimated road point 
ssn. Road line 
L 
foe d] W Road template 
Figure 9. Tracing a road line, (a)(b) estimation of the starting 
direction by template matching and using road seeds, 
(c)(d) extending a road line by template matching 
and using road seeds. 
4.2 Guiding the road line tracing using a directional point 
A control point is assigned whenever necessary by the operator 
to guide the road line extraction. If the control point is on the 
road line that has been extracted, it means that the road should 
stop at the point, so that the road line is cut down (see Figure 
11(a)). Otherwise the control point works as a directional point, 
where the road line is guided to go pass through the control 
point and extend further away (see Figure 11(b)) In the 
followings, we address the second case in detail. 
  
Find the nearest road point p to q. 
y 
Trace the road line from p to q. 
y 
Extend the road line from q. 
  
  
  
  
  
  
  
Figure 10. Flow of guiding a road line tracing using a 
directional control point q. 
Given a directional control point q, the road line is modified 
and further extended as shown in Figure 10. The distance from 
any road point p, to q is calculated as follows. 
D(p,,q)7 0, * (180? —m) * 9, *|p, — q| (4) 
Where 1| is the angle between p, ,p, and Did: The nearest 
road point p to q is the one that satisfies 
D(p,q)= min D(p, 4) ©) 
A - 409