In order to prevent a safety accident during driving, a forward collision warning system calculates a safety distance from a forward vehicle, and generates an alarm to warn a driver when a collision is likely to occur. The forward collision warning system senses the forward vehicle through a sensor or camera, and calculates a relative distance or relative velocity between the ego vehicle and the forward vehicle. Then, the forward collision warning system calculates a time to collision (TTC) from the relative distance and the relative velocity, and warns of a collision with the forward vehicle using the TTC.
For example, as illustrated in FIG. 1, an image taken by a camera mounted on a vehicle traveling on the road 10 shows a forward situation. When a forward vehicle 12 is present in the image, the forward vehicle 12 may be detected through the following process. First, the forward collision warning system acquires a forward image through the camera, and performs inverse filtering on the acquired image in order to distinguish the forward vehicle 12 from other objects. Then, the forward collision warning system recognizes the shape of the forward vehicle 12, and determines whether the forward vehicle 12 is a vehicle. When it is determined that the forward vehicle 12 is a vehicle, the forward collision warning system recognizes the bottom of a rear wheel or the end of a lower shadow area of the forward vehicle 12 as a distance reference point, and calculates a relative distance to the forward vehicle 12 based on the distance reference point.
In FIG. 2, the left drawing includes a coordinate axis indicating an actual position of the forward vehicle 12 relative to the ego vehicle 14, and the right drawing includes a coordinate axis in an image taken by the camera of the ego vehicle 14. Here, the origin point corresponds to a vanishing point. Since the distance reference point is set to the bottom of the rear wheel or the end of the lower shadow area of the forward vehicle 12 as described above, ‘Y=0’ in the coordinate axis of the forward vehicle 12. Thus, the forward collision warning system can simply calculate the relative distance to the forward vehicle as expressed by Equation 1 below.
                    Z        =                  λ          ⁢                      h            y                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, Z represents the relative distance between the ego vehicle and the forward vehicle, λ represents a focal distance of the camera, h represents the height of the camera, and y represents the height of the distance reference point on the camera.
When the relative distance calculation is repeated in successive images, the relative velocity may be calculated through Equation 2 below.
                    v        =                                            Z                              t                +                                  Δ                  ⁢                                                                          ⁢                  t                                                      -                          Z              t                                            Δ            ⁢                                                  ⁢            t                                              [                  Equation          ⁢                                          ⁢          2                ]            
In Equation 2, v represents the relative velocity of the ego vehicle relative to the forward vehicle, Zt represents a relative distance at time t, and Zt+Δt represents a variation of the relative distance depending on a time variation Δt.
When a TTC obtained by dividing the relative distance by the relative velocity is less than a threshold value as expressed by Equation 3 below, the forward collision warning system warns of a collision risk.
                    TTC        =                              Z            v                    <                      TTC            threshold                                              [                  Equation          ⁢                                          ⁢          3                ]            
In Equation 3, TTC represents a time to collision, and TTCthreshold represents the threshold value of the TTC.
The conventional forward collision warning system can easily acquire the relative distance. As illustrated in FIG. 3, however, when the conventional forward collision warning system cannot detect the rear wheel of the forward vehicle because the forward vehicle is too close to the ego vehicle, the conventional forward collision warning system may not acquire the distance reference point or cause a misrecognition. Such a situation may frequently occur when the vehicle is driven at low speed. Furthermore, since a collision warning is not generated in time, a collision accident may occur.
Korean Patent Publication No. 10-2013-0007243 discloses a forward collision warning method and system capable of successfully recognizing a forward vehicle under various illumination intensities or road conditions. Referring to the related art document, the forward collision warning method and system minimizes the load of a CPU through vehicle recognition and tracking. However, a vehicle tracking unit of the related art document detects the bottom of a forward vehicle recognized by a forward vehicle recognition unit and sets the bottom of the forward vehicle to a feature point of the forward vehicle. Therefore, when the forward vehicle is too close to the ego vehicle as illustrated in FIG. 2, the forward collision warning method and system cannot acquire a feature point, and may fail to generate a collision warning.