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Title: On transcoding a B-frame to a P-frame in the compressed domain
Authors: Siu, WC 
Chan, YL 
Fung, KT
Issue Date: Oct-2007
Source: IEEE transactions on multimedia, Oct. 2007, v. 9, no. 6, p.1093-1102
Abstract: Only a limited number of methods have been proposed to realize heterogeneous transcoding, for example from MPEG-2 to H.263, or from H.264 to H.263. The major difficulties of transcoding a B-picture to a P-picture are that the incoming discrete cosine transform (DCT) coefficients of the B-frame are prediction errors arising from both forward and backward predictions, whilst the prediction errors in the DCT domain arising from the prediction using the previous frame alone are not available. The required new prediction errors need to be re-estimated in the pixel domain. This process involves highly complex computation and introduces re-encoding errors. We propose a new approach to convert a B-picture into a P-picture by making use of some properties of motion compensation in the DCT domain and the direct addition of DCT coefficients. We derive a set of equations and formulate the problem of how to obtain the DCT coefficients. One difficulty is that the last P-frame inside a GOP with an IBBP structure, for example, needs to be transcoded to become the last P-frame in the IPPP structure, and it has to be linked to the previous reconstructed P-frame instead of to the I-frame. We increased the speed of the transcoding process by making use of the motion activity which is expressed in terms of the correlation between pictures. The whole transcoding process is done in the transform domain, hence re-encoding errors are completely avoided. Results from our experimental work show that the proposed video transcoder not only achieves a speed-up of two to six times that of the conventional video transcoder, but it also substantially improves the quality of the video.
Keywords: B-picture and P-picture
Compressed domain processing
Heterogeneous transcoding
Video coding and transcoding
Publisher: Institute of Electrical and Electronics Engineers
Journal: IEEE transactions on multimedia 
ISSN: 1520-9210
EISSN: 1941-0077
DOI: 10.1109/TMM.2007.902895
Rights: © 2007 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.
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