| With the development of computer technology, multimedia technology and increase of the demand of information, multimedia information has become the main resource data of all sorts of information systems. The range of information media expands rapidly which is disposed by computers, not only we require the databases and other information systems can deal with storage and key frames retrieval to the images, video and audio, but also we need to analyze the semanteme of multimedia data to reach deeper retrieval levels. So content-based multimedia information retrieval comes forth.Content-based multimedia information retrieval includes images, video and audio retrieval, however, content-based video retrieval is one of the most important research fields. Content-based video retrieval is a process which bases on scenes, shots, frames, moving objects and color, texture, shape characters in the image data in order to find out images that satisfy with given visual characterizations from a huge video database. At present, the major work of content-based video retrieval is concentrating on shot segmentation, key frames extraction and scenario reconstruction through identifying and describing color, texture, shape, spatial relation of image. Content-based video retrieval breaks through the localization of traditional retrieval based on expressions and picks up content information as clues to establish indexes. It is a kind of approximate matching. During the process of retrieval, it adopts the method of resembling matching and decreasing the range of query results to get the more accurate retrieval result. This paper is mostly aiming at the technology of key frames extraction and makes deep research into these aspects. It investigates domestic and overseas research status in correlative fields, and makes an aborative research and analysis of the existing methods of content-based video processing and retrieval. Taking example for compressed video file MPEG-2, the paper analyzes detailedly video sequence and layer structure of the video data in the international standard, and dose lots of work on the acquirement of compressed image sequence MPEG-2. At last, the paper puts forward an improved method compared with the traditional methods of key frames extraction. The method is an improved block-based IDC key frame extraction method.This paper has several parts as follow:Analysis of video sequenceAcquirement of image sequence Luminance histogram acquirement and analysis Traditional key frames extraction methods analysis, compare, test Putting forward an improved key frames extraction method and relevant experimental data Acquirement of image sequence is the data source of analysis and process of key frames. So analysis of MPEG-2 compressed video stream and acquirement of image sequence is the basis of subsequent work. Luminance histogram reflects luminance statistical information of images. So we pick up the value of Y, which stands for luminance from the YUV color model to get the luminance histogram of images and make qualitative analysis work. The detection and retrieval of key frames is where the shoe pinches in the paper. At last, the paper makes an experimental analysis and capability comparison aiming at the traditional key frames extraction methods, such as Absolute Distance method, Euclidean Distance method, X Square method, Double Threshold Value method, Block method, I Frame/DC Coefficient method and so on. The paper puts forward an improved block-based IDC key frame extraction method in compressed video sequence and achieves good retrieval effect.In order to test the retrieval effect of the improved key frames extraction algorithm, we adopt a testing model with the parameters of Recall,Precision and Retrieval Time. Considering universality and catholicity of video data,we select several video such as cartoon, film snippet, advertisement , science and educational film to check the retrieval result of key frames.The experiment proves that we make use of key frame extraction methods in pels fi...
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