Sometimes it may happen that in correspondence to the symmetry maximum no correct bounding boxes exist. Therefore, a backtracking approach is used: the symmetry map is again scanned for the next local maximum and a new search for a bounding box is performed. Figure 40 shows a situation in which the first symmetry maximum, generated by a building, does not lead to a correct bounding box; on the other hand, the second maximum leads to the correct detection of the vehicle.
Figure 40: A case in which the symmetry of the background results higher than the vehicle symmetry. The search for the bounding box reveals that the left smmetry axis does not corresponds to valid vehicles. The backtracking approach removes the peak near the maximum from the symmetry map and spawns a new bounding box search. highly symmetrical background patterns.