Nowadays, large amount of paper-based topographic maps are still existed in many government department. The scanned
maps of them are very useful for research on city history migration, city planning and so on. However, the brightness of
these maps is not uniform, and creases are existed, so uniform color process is always needed. If the classical Retinex
algorithm is used, the map would have a low brightness and contrast ratio. Therefore, a normal intercepting SSR
algorithm of linear extending is presented in this paper. This algorithm first uses the classical SSR algorithm to process
the data, and then the average value of image and variance are introduced to do normal intercepting linear extending on
the map. Experiment results show that, the improved SSR can not only efficiently eliminate creases and uniform the map
brightness, but also increase the global brightness and contrast ratio. Moreover, this algorithm can also be used in the
pretreatment of grid image registration, thus to enhance the precision, velocity and accuracy of registration.
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