The invention relates to a front, side or rear view monitoring method for a motor vehicle (such as an automobile) which can warn a driver by using images taken by a video camera installed on the front, side or rear part of the motor vehicle and sensing a motor vehicle or an obstacle ahead of the driver's own motor vehicle during a drive.
A conventional method of this type is disclosed in a publication of Japanese Unexamined Patent Publication No. 241855/1990. The method disclosed in the publication comprises the steps of: picturing a front view from a driver's own motor vehicle that is moving; recognizing a movement of a single point in the pictured front view as an optical flow at predetermined times; gathering information, i.e., the position of a preceding motor vehicle relative to the driver's own motor vehicle and the relative speed, based on the optical flow as well as on the distance of the preceding motor vehicle as measured by a range sensor; and warning the driver of danger when the presence of danger is judged.
To calculate the optical flow, a method called a matching method has here ore been adopted. The matching method detects a corresponding point in two images. The matching method comprises the steps of: setting a window W1 for a target pixel P in an image at a time t, as shown in FIG. 10 (a); calculating a correlative value while moving the window over the entire part of the image or a neighboring area; and finding a corresponding point in a window W2 defined at the time the correlative value is maximized, i.e., a pixel Q, as shown in FIG. 10 (b). An arrow PQ indicates an optical flow. To calculate the above-mentioned correlative value, the following equation is used. EQU .SIGMA.(W1.sub.(x, y) x W2.sub.(x, y))/(.SIGMA.W1.sup.2.sub.(x, y) x .SIGMA.W2.sup.2.sub.(x,y)).sup.1/2 ( 1)
where W1.sub.(x, y), W2.sub.(x, y) are the outputs of coordinates (x, y) within the windows W1, W2.
However, in order to judge danger, the conventional method requires a range sensor for measuring the distance of a preceding motor vehicle to identify the position of the preceding motor vehicle relative to the driver's own motor vehicle, in addition to a video camera for picturing the front view.
Further, in order to calculate the optical flow, the operation of detecting the corresponding point in an image must be performed. As described above, such operation comprises detection of all the pixels in the entire part or a neighboring area of the image relative to a single pixel within the image to find a single corresponding point. To detect the corresponding point over the entire part of the image, this operation must be performed on all the pixels. In addition, a great number of calculations are required to be performed to find correlative values that are to be used as indexes for search.
Therefore, an increased volume of calculation makes realtime processing impossible. For achievement of the realtime processing, a high-speed processor is required, which elevates hardware costs. Further, it is not easy to distinguish the source of an optical flow; e.g., whether the optical flow is derived from the preceding motor vehicle or from scenes outside the road, or characters, symbols, or white lines on the road surface, or the like.