Study On Auto-focusing Based On Digital Image Processing Technology

Image is playing very important part in this information-based society,especially for the equipment in military field. Clear image could provide usefulinformation directly for decision-making by headquarters. Actually, there is no kindof optical system can avoid defocus problem because of the imaging principle. Theimage is blur distortion if the optical measurement system is in defocus state. Itcould exert an influence on subsequent processing and transition, especially effectthe decision-making during war time. Consequently, real-time auto-focus is essentialfor optical measure systems performance to avoid disadvantage of blur distortion.Traditional method always solves this problem by distance measurement or phasedetection, which needs extra hardware and increases costs. This paper studies theproject from image processing based on optical measuring equipment. By depthfrom focus, auto-focus can be realized just based on image information without anyother auxiliary tools, which makes the optical measure systems more intelligent.Based on imaging principle, the auto-focus procedure includes four modules:pre-processing, focus window construction, image quality assessment and searchingfor focal position. This paper proposes solution to every module and the overallprogram around this topic with the practical engineering requirements after studyingdevelopment history of this field at home and abroad and comparing advantage anddisadvantage of many methods.Pre-processing is needed to overcome influence of noise and inappropriate exposure during optical measure systems imaging, meanwhile, can enhance imagequality. To solve the inappropriate exposure, a novel two-dimensional histogrammethod with dynamic window is proposed. This enhances contrast and correctexposure.Focus window construction is useful for refraining from affecting of noise andbackground. At the same time, this could reduce the amount of calculation andmakes the following steps just for target region. There is a feature that the positionand size of target is random duiring measurement. This paper presents a novelmethod from biological vision to construct focus window for the feature. Themethod simulates visual perception based on Itti model, extracts feature message andconstructs regular window via extension by cross-shaped template. It can constructfocus window for any focus state. Meanwhile, this paper provide a way by SVMfrom classification to build focus window and invites branch and bound method tooptimize searching process, which can improve location efficiency for target.Image quality assessment（IQA）is an critical step for focusing. And the IQAresult can affect the focusing result. This paper proposes a novel method to getno-reference IQA based on natural scene statistics（NSS）and spatial transition. Thiscan overcome the effect of content and decide the defocus extent just by only oneframe in primary stage of focusing for optical measurement. Then, it can directsearching process and avoid blind seeking. Then, the paper improves self-correlationfunction for IQA by larger scale and more orientation pertinence. It can make theIQA result stable and be sensitive to sharpness, meanwhile, overcome the influenceof atmospheric turbulence and equipment jitter to optical measurement.Searching is the final step for focusing. Search strategy should be made withconsideration of equipment and application. This paper adopts rough focusing andfine focusing based on hill climbing search method to optimize searching step length.In rough process, the method of combining NSS and spatial transition is applied forIQA. In fine process, the method of advanced correlation function is applied for IQA.The first one is used to defocus degree and direction. The second one is used to reduce costs and ensure accuracy.Finally, this paper provides the whole auto-focus program based on imageprocessing for optical measurment. The program includes hardware and softwarestructure. After a lot of experiments, the paper analyses focusing results, accuracy,stability and real time feature, meanwhile, makes statistic dates. The results showthat the program is feasible and useful for engineering.