QuickBird影像城市阴影信息的提取与消除

Study on City Shadow Extraction and Elimination in Quick Bird Images

城市绿地信息在城市研究中的重要作用。但由于各种因素的影响,城市绿地信息提取的精度受到很大的限制,其中,城市中建筑物的阴影是城市绿地信息提取的一个重要限制因素。本研究选取呼和浩特市城区的Quick Bird影像,在获取最佳波段组合的基础上,利用多种方法对遥感影像的阴影信息进行提取和消除,以期获得最佳的阴影消除方法,高效地提取城市绿地信息。首先,通过比值运算、波段重组,增强处理影像阴影信息,用最佳指数法分析Quick Bird影像阴影提取的最佳波段组合;然后,根据阴影在近红外波段的最小亮度值与最大亮度值的范围建立掩膜,成功提取影像的阴影信息;最后,将色彩空间变换分别与同态滤波和Gamma矫正结合以消除影像阴影,并与其他方法进行对比。研究结果表明,Quick Bird影像阴影提取的最佳波段组合为3/4、4、2波段组合,最佳亮度值范围为70-165;色彩空间变换与Gamma矫正相结合的方法可更好地消除阴影,并能较好地保留影像的彩色信息,是消除阴影的最佳方案。

Urban green space, as the main element of urban structure, plays a very important role in urban studies. In recent years,remote sensing technology has been applied in various industries with its rapid development. It has become one of the major techniques for city information（especially, green space） extraction with the emergence of high-resolution remote sensing images.Compared to the low-resolution images, the high-resolution images have many advantages such as clear texture detail and rich information. However, the existence of shadows in the urban district has a great impact on image classification, interpretation,mapping and so on. Therefore, the building shadows in the images have become one of the important limiting factors for green space extraction. In order to effectively extract city（green space） information from remote sensing images, it is necessary to extract and eliminate shadows from remote sensing images. This study is based on the Quick Bird image of Hohhot city. Firstly, by utilizing the methods of band ratio process and band recombination, the image shadow information was enhanced; and the optimal band combinations for shadow extraction were derived using optimum index factor（OIF）. Secondly, the image mask was established and the shadow information was extracted based on shadow threshold values（the minimum values and maximum values） of nearinfrared band. At last, the shadow was removed by combining color space transformation with homomorphic filter and gamma correction; and then the effects were compared with other methods. The results show that, for shadow extraction, the optimal band combination is a combination of band 3/4, 4 and 2; and the best DN range is between 70 and 165. The method combined with gamma correction and color space transformation can effectively eliminate shadows and retain color information of Quick Bird image. It is the best solution for shadow elimination in this study. The results can provide theoretical basis and technical support for efficient city（especially, green space） information extraction based on high-resolution remote sensing images.