The grey scales of the current image shown as Figure 3(a) is presented as Figure 3(b). The high frequency part shown between point 1 and point 2 means that the scanning line find a vehicle. The vehicle’s shadow and bumper would cause shaking becoming smoothly as the part from point 2 to point 3 in Figure 3(b). If vehicle’s shadow extends to road, the trend of grey scales will tend to a low frequency signal with a positive slope.
Due to recognize the vehicle’s location more clearly, here use difference equation algorithm and one order average filter shown as Eq. (2) for smoothing the grey scale signal.