ISPRS Commission III, Vol.34, Part 3A ,,Photogrammetric Computer Vision“, Graz, 2002
Suppose p, is the kth edge pixel of an edge line patch, and it is
the pn, -th neighbour of its previous pixel p,.,» Where both p,
and p, , have been removed from the binary image of edge
pixels. In order to find the k+1th edge pixel of the edge line
patch, the 8-neighbors of p , are examined, where the
neighbour having a direction near to n , has a higher priority,
so that is examined first. If the &'th neighbour of p, 1$ found an
edge pixel, it is accepted as the &--1th edge pixel of the edge
line patch. On the other hand, if no edge pixel is found in the 8-
neighbours of p, , an examination along the extension of 7 "
from p, is conducted. If the blanks from p, to edge pixel q in
Figure 5(b) is less than a given threshold, q is accepted as the
k+1th edge pixel. If no edge pixel is found in all the above
examinations, the procedure of linking edge pixels of the edge
line patch stops.
rod o«a
[] rj E]
Ne O's OC
Ka SM
m C LU
Figure 5. Searching in 8-neighbors and along the extension of
local direction.
Breaking edge line patches is conducted, where the edge line
patch is broken down at the pixel of the rapid change of
direction at either local or/and global level. For any pixel in the
edge line patch, if the angle from starting pixel to the pixel to
end pixel is smaller than a given threshold 7, then the pixel is
regarded as a broken pixel at global level (see Figure 6(a)). In
addition, for any pixel Æ in the edge line patch, given a value s
for local length, if the angle from pixel Æ-s to pixel Æ to pixel
k+s is smaller than the threshold ¢, then pixel Æ is regarded as a
broken pixel at local level (see Figure 6(b)).
Broken pixel
: r if o<t
Starting pixel End
(a) Broken pixel at global level
Broken pixel
if act
End
(b) Broken pixel at local level
Starting pixel if acf
Figure 6. Breaking edge line patches
3.2 Jointing Edge Line Patches
For any edge line patch e , it is jointed with other edge line
y P
patches in an iterative way as follows. Given two thresholds L
in pixel and X in degree, a searching space is defined at each
terminal point p as shown in Figure 7(a) where ], is the
extensional direction of e, at p. For any edge pixel q in the
A - 408
search space, where q is on an edge line patch e, > à cost is
calculated as follows (see Figure 7(b)).
E(q) = |, |+|o;| (1)
Where, /, is a line passing through p and q, I, is the tangential
line of e at q, ot, is the angle between / and l,, and a, is
the angle between /, and /,. If an edge pixel q yielding the
minimal cost E(q) is found, e is jointed to e, as shown in
Figure 7(c).
Figure 7. Jointing edge line patches
Specify a starting
point by the operator
Y
Automatically trace a
road line by template
matching
md
Assign a directional
point by the operator
T y Y
Guide the road line
tracing using the
directional point
aut 2 nx
Figure 8. Flow of semi-automatic road line extraction
End
4. ROAD LINE EXTRACTION
In this research, road lines are extracted in a semi-automatic
way by matching a road template with both road mask (M) and
road seeds (S). The flow of road line extraction is shown in
Figure 8. At the beginning, a starting point is specified by the
operator. A road line is extended from the starting point in an
iterative way, where in each iteration, a rectangular road