D.-C. Tseng, C.-C. Huang
10.4236/wjet.2017.53B010 83 World Journal of Engineering and Technology
2. Vehicle Detection
Edge information is an important feature for detecting vehicles in an image, es-
pecially the horizontal edges of underneath shadows. In the detection stage, ve-
hicle candidates are generated based on local edge features. Firstly, the appropri-
ate edge points are extracted in the region of interest
(ROI) defined by lane
marks as shown in
Figure 1. Then, the significant horizontal edges indicating
vehicle locations are refined. The negative horizontal edges (NHE) are thought
belonging to underneath shadow. The positive horizontal edges (PHE) are
mostly formed by bumper, windshield and roof of vehicles. For each kind of ho-
rizontal edges with different property, a specific procedure to find vertical bor-
ders of vehicles is applied around each horizontal edge. Finally, the horizontal
edges and vertical borders are used to find the bounding boxes of vehicle candi-
dates.
The horizontal edges are expected belonging to underneath shadows or ve-
hicle body. Considering the sunlight influences, the horizontal edges belonging
to underneath shadows are further divided into the moderate or the long by edge
widths. The procedures for searching the paired vertical borders are designed
according to the specific properties of different kinds of horizontal edges. In the
proposed vehicle detection, three detecting methods (Case-1, Case-2, and Case-
3) are respectively designed to generate vehicle candidates based on the hori-
zontal edges of underneath shadows, long underneath shadows, and vehicle bo-
dies. In the situations of clear weather with less influence from sunlight, the un-
derneath shadows are distinct and the widths of shadows are similar to vehicle
width; Case-1 method proposed based on horizontal edges of moderate under-
neath shadows is adequately used to generate candidates. In the situations of
sunny weather with lengthened shadow, there is the strong possibility that only
one vertical border is searched by using Case-1 method; Case-2 method is pro-
posed based on horizontal edge of long underneath shadows to search two cor-
responding vertical borders in a larger region, as shown in
Figure 2. In the situ-
ations of bad weather disturbed by water spray and reflection, the underneath
shadows are not reliable and are not observed at the worst; Case-3 method base
on vehicle bodies is applied in a different way to retrieve the missing cases of
using Case-1 and 2 methods.
If the only one vertical edge of the NHE is at the right side, it is taken as the
Figure 1. Two examples of
ROI
setting based on the detected lane marks.