Advanced digital VCR for compressed videos

Abstract

With the establishment of MPEG video coding standards, many video sequences for modern streaming applications are encoded in MPEG formats. However, the MPEG standards employ motion-compensated prediction in which the compressed data is not invariant to changes in frame order. This hinders users from browsing video in a more interactive way. To enrich the user's viewing experience, it is desirable to perform various video cassette recording (VCR) functionality such as backward, fast-forward/backward, random access, etc. in digital video. Therefore, in this thesis, some novel techniques are suggested for the efficient implementation of VCR functionality in a digital video streaming system with minimum requirements on the decoder complexity and the network traffic. Backward playback is one of the most common VCR functions. One popular approach is to use a reverse transcoder in the server which converts I-P frames into another I-P bitstream in reverse frame order. When a playback device decodes this reverse-encoded bitstream, backward playback can be achieved. To expedite the transcoding process, we propose a fast reverse motion estimation algorithm with smart mode decision for H.264 reverse transcoding. By analyzing the motion vectors and modes decoded from the forward bitstream, the best mode and motion vector for each reverse transcoded macroblock are estimated. A remarkable reduction of computational complexity involved in reverse motion estimation can be achieved by the proposed algorithm with only negligible impact on the rate-distortion performance. Afterwards, we propose a compressed-domain approach for an efficient implementation of the MPEG video streaming system to provide backward playback over a network. In the proposed video streaming server, according to the motion information, macroblocks in the requested frame are classified into two categories - backward macroblocks (BMBs) and forward macroblock (FMBs). Two novel macroblock-based techniques are used to manipulate the necessary macroblocks in the compressed domain and the server then sends the processed macroblocks to the client machine. For BMBs, we propose a sign inversion technique, which is operated in the variable length coding (VLC) domain, to reduce the number of macroblocks to be decoded by the decoder and the number of bits to be sent over the network in the backward-play operation. By identifying the related macroblocks of FMBs in their reference frame, a direct addition technique for discrete cosine transform (DCT) coefficients is designed to further reduce the computational complexity of the decoder. We also contrive a mixed VLC and DCT domain technique for the FMBs to offer better performance of the proposed system. With these compressed-domain techniques, the proposed architecture manipulates macroblocks in the VLC and DCT domain only to achieve a server with low complexity. Experimental results show that, as compared to the conventional system, the new streaming system reduces the required network bandwidth and the decoder complexity significantly. Although the new scheme exhibits promising results for backward playback, it encounters a problem when backward playback traverses the group-of-pictures (GOP) boundary since the proposed sign inversion technique makes use of the motion relationship between two adjacent frames. But, no inter-frame prediction takes place between the last frame of one GOP and the first frame of the successive GOP. In this thesis, we also provide a novel solution to cope with the GOP discontinuity problem of the video bitstream by re-building the motion linkages across GOP boundaries. By employing the compressed-domain techniques, the work in this thesis shows significant improvements in terms of the server complexity, the network traffic, and the quality of reconstructed video during backward playback. Undoubtedly, the results of our work will certainly be useful for the future development of digital VCR.