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