Abstract: A new algorithm-flexible relaxation bag method for vehicles to the most notable features-car length extracted.Flexible use of relaxation algorithm models feature extraction process,but also the key image frames selective extraction process.At the same time,the process is to determine the existence of vehicles.In this paper,the flexible relaxation bag algorithm is updated.A updated algorithm called elastic collision algorithm has further precision in some situation.The experiments show that the flexibility relaxation method has better statistics feature,to extract effectively video key frame.The method extract features accurately,is able to effectively meet the demands of the real-time system.It is also applied in other photograph fields.