| Summary: | 碩士 === 國立中央大學 === 土木工程研究所 === 98 === The main purpose of image enhancement is to increase the contrast in order to bring out hidden details of an image. Therefore, the image enhancement generally is an important process to have a better image quality for visual applications. In the global approach, the enhancement methods generally use a single mapping function to enhance the whole image. However, a single enhancement mapping function can not improve image contrast satisfactorily since the contrast of an object is interfered by the whole image. Naturally, it is difficult to find a good mapping function to enhance the whole image. In this thesis, we proposed a new contrast enhancement technique which stretches the local contrast to improve the visibility of satellite images, while preserving its natural looks. The proposed method is based on segmentation of an input image that divided into small individual patches. Adjustable histogram equalization with dynamic threshold is applied for every single patch with the consideration of the gap problem appearing between patches. The threshold is based on an exponential function under the relationship between Shannon entropy index and adjustable histogram equalization weighting parameter. The larger entropy index on a segment, the smaller parameter value is used, and vice versa.
The results show that visibility improvement of specific objects is successfully enhanced using the proposed method. This thesis provided a new enhancement algorithm in enhancing contrast and characteristics of an image that could not be enhanced by global contrast enhancement or conventional method.
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