http://hdl.handle.net/10397/198 Search the Channel search http://repository.lib.polyu.edu.hk/jspui/simple-search http://hdl.handle.net/10397/4701 Title: Wavelet packet based approach for image retrieval in compressed domains<br/><br/>Authors: Cheng, Kin-on; Law, Ngai-fong Bonnie; Siu, Wan-chi<br/><br/>Abstract: Images are often compressed to reduce storage. In order to avoid full decoding in image retrieval, features extracted directly from transformed data have been investigated. The popularity of JPEG and JPEG2000 motivates us to investigate a fast conversion scheme from block-based discrete cosine transform (BDCT) to wavelet domain so that similar features can be extracted from the two different domains. In the proposed scheme, BDCT is first approximated to wavelet packet (WP) transform and then the synthesis WP filter bank is applied to construct wavelet subbands. Studies found that the proposed conversion scheme has a low computational complexity as compared to the baseline approach. Two algorithms are then developed for extracting features in the wavelet subbands for retrieval. Our experimental results show that the proposed algorithms provide a good retrieval performance while the complexity is kept to be low. http://hdl.handle.net/10397/4425 Title: A fast approach for identifying similar features in retrieval of JPEG and JPEG2000 images<br/><br/>Authors: Cheng, Kin-on; Law, Ngai-fong Bonnie; Siu, Wan-chi<br/><br/>Abstract: As digital images are often in compressed forms, image retrieval involves full decoding of images prior to feature extraction. The decoding process can be computation-expensive so feature extraction in compressed domain is desired. In this work, wavelet-based features are extracted as unified features for retrieval of JPEG and JPEG2000 images. A fast algorithm is proposed to approximately transform a JPEG image in the block-based discrete cosine transform (BDCT) domain to wavelet domain so that wavelet-based features can be extracted directly from JPEG images. Our proposed algorithm consists of a multiresolution reordering and a filter bank structure. The former is used to provide a rough approximation of wavelet subbands from BDCT coefficients in bandpass subbands in fine scales while the latter is used to provide an accurate approximation in bandpass subbands in coarse scales. Our theoretical analysis shows that the proposed algorithm can reduce the complexity by at least 79% when comparing with the straight forward approach that uses an inverse BDCT followed by wavelet transform. Besides the reduction in computational complexity, the experimental results demonstrate that our proposed conversion approach has higher retrieval performance than the pure multiresolution reordering approach. http://hdl.handle.net/10397/1944 Title: Reverse-play technique on compressed video across GOP boundaries<br/><br/>Authors: Fu, Chang-hong; Chan, Yui-lam<br/><br/>Abstract: In modern video coding standards, they are mainly designed for normal forward playback. Recently, we have developed a macroblock-based reverse-play scheme for compressed video bitstreams. By exploiting the motionrelationship between two adjacent frames, this scheme can significantly reduce the requirements on the decoder complexity and network bandwidth during reverse playback.However, the MB-based technique does not works well on traversing GOP boundaries reversely since no inter-frame prediction takes place between the last frame of one GOPand the first frame of its succeeded GOP. In this paper, by utilizing a new SP picture type in the H.264, we can solve the GOP discontinuity problem by building linkages across GOP boundaries. Instead of arranging the primary SP-frame before the I-frame, a novel scheme is proposed to allocate various macroblocks within the GOP to be encodedas the SP type. This new arrangement for SP coding is specially designed for our macroblock-based reverse-play scheme and can be proved to eliminate the possible mismatch effect for reverse playback across GOPboundaries. With this allocation strategy, results from our experimental work confirm that the inherent GOP discontinuity problem can be avoided without introducing any mismatch between forward and reverse playback.<br/><br/>Description: DOI: 10.1109/ICNNSP.2008.4590440 http://hdl.handle.net/10397/1943 Title: Efficient motion estimation in H.264 reverse transcoding<br/><br/>Authors: Fu, Chang-hong; Chan, Yui-lam; Siu, Wan-chi<br/><br/>Abstract: In this paper, we propose a fast reverse motion estimation algorithm with efficient mode decision for reverse transcoding of H.264 bitstream. By analyzing the motionvectors and modes decoded from the forward bitstream, the best mode and motion vector for each backward transcoded macroblock are estimated. A remarkable reduction ofcomputational complexity involved in reverse motion estimation can be achieved by the proposed algorithm with only negligible impact on the rate-distortion performance.<br/><br/>Description: DOI: 10.1109/ICIP.2007.4379829