The major objective of image enhancement is to process the image so as to make the original image more suitable to the particular use. The process of enhancing the useful information of the image can be a process of distortion. Its goal is to improve the visual effect of the image through intentionally stressing the overall or the part property of the image. It has the function of making the vague image clear , stressing the characteristics that are interesting to the people or suppressing the unfavorable characteristics and enlarging the difference of characteristics between different objectives, thus to improve the quality of the image, rich the information, enhance the effect of image interpretation and identification and serve the need of some particular analysis. The way of image enhancement is through some particular methods like add some information, change the data , give prominence to some interested information selectively or conceal some unneeded characteristics of the image in order to make the image mach the visual effect. According to the different spaces in the process of image enhancement, the technology of image enhancement can be divided into two categories, one is based on the spatial domain arithmetic and another on the frequency domain arithmetic. The arithmetic bases on the spatial domain directly deal with the grayscale of the image. The arithmetic bases on the frequency domain is an indirect way which revises the value for coefficient of variation of the image in the transform domain of the image.
