中文说明：程序主要用于车型识别，利用两幅图像相减进行提取轮廓，然后再通过计算定长和车底的比进行判断车型。
English Description：
The program is mainly used for vehicle type recognition. The contour is extracted by subtracting two images, and then the vehicle type is judged by calculating the ratio of fixed length and vehicle bottom.