Drift compensation for reduced spatial resolution transcoding

This paper discusses the problem of reduced-resolution transcoding of compressed video bitstreams. An analysis of drift errors is provided to identify the sources of quality degradation when transcoding to a lower spatial resolution. Two types of drift error are considered: a reference picture error, which has been identified in previous works, and error due to the noncommutative property of motion compensation and down-sampling, which is unique to this work. To overcome these sources of error, four novel architectures are presented. One architecture attempts to compensate for the reference picture error in the reduced resolution, while another architecture attempts to do the same in the original resolution. We present a third architecture that attempts to eliminate the second type of drift error and a final architecture that relies on an intra block refresh method to compensate all types of errors. In all these architectures, a variety of macroblock level conversions are required, such as motion vector mapping and texture down-sampling. These conversions are discussed in detail. Another important issue for the transcoder is rate control. This is especially important for the intra refresh architecture since it must find a balance between number of intra blocks used to compensate errors and the associated rate-distortion characteristics of the low-resolution signal. The complexity and quality of the architectures are compared. Based on the results, we find that the intra refresh architecture offers the best trade-off between quality and complexity, and is also the most flexible. This work may not be copied or reproduced in whole or in part for any commercial purpose. Permission to copy in whole or in part without payment of fee is granted for nonprofit educational and research purposes provided that all such whole or partial copies include the following: a notice that such copying is by permission of Mitsubishi Electric Information Technology Center America; an acknowledgment of the authors and individual contributions to the work; and all applicable portions of the copyright notice. Copying, reproduction, or republishing for any other purpose shall require a license with payment of fee to Mitsubishi Electric Information Technology Center America. All rights reserved. Copyright c ©Mitsubishi Electric Information Technology Center America, 2002 201 Broadway, Cambridge, Massachusetts 02139 Published in IEEE Transaction on Circuits and Systems for Video Technology, vol. 12, no. 11, pp. 10091020, November 2002. IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. XX, NO. Y, MONTH 2001 1 Drift Compensation for Reduced Spatial Resolution Transcoding Peng Yin, Student Member, IEEE, Anthony Vetro*, Member, IEEE, Bede Liu, Fellow, IEEE, and Huifang Sun, Fellow, IEEE Contact Author Information: Dr. Anthony Vetro, Mitsubishi Electric Research Labs, 558 Central Ave, Murray Hill, NJ 07974. Tel: +1-908-363-0504 Fax: +1-908-363-0550, Email: [email protected]. Manuscript received October 2001; revised June 2002. P. Yin and B. Liu are with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08540. A. Vetro and H. Sun are with are with Mitsubishi Electric Research Laboratories, Murray Hill, NJ 07974. August 1, 2002 DRAFT IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. XX, NO. Y, MONTH 2001 2 Abstract This paper discusses the problem of reduced-resolution transcoding of compressed video bitstreams. An analysis of drift errors is provided to identify the sources of quality degradation when transcoding to a lower spatial resolution. Two types of drift error are considered: a reference picture error, which has been identified in previous works, and error due to the non-commutative property of motion compensation and down-sampling, which is unique to this work. To overcome these sources of error, four novel architectures are presented. One architecture attempts to compensate for the reference picture error in the reduced resolution, while another architecture attempts to do the same in the original resolution. We present a third architecture that attempts to eliminate the second type of drift error and a final architecture that relies on an intra block refresh method to compensate all types of errors. In all these architectures, a variety of macroblock level conversions are required, such as motion vector mapping and texture down-sampling. These conversions are discussed in detail. Another important issue for the transcoder is rate control. This is especially important for the intra refresh architecture since it must find a balance between number of intra blocks used to compensate errors and the associated rate-distortion characteristics of the low-resolution signal. The complexity and quality of the architectures are compared. Based on the results, we find that the intra refresh architecture offers the best trade-off between quality and complexity, and is also the most flexible.This paper discusses the problem of reduced-resolution transcoding of compressed video bitstreams. An analysis of drift errors is provided to identify the sources of quality degradation when transcoding to a lower spatial resolution. Two types of drift error are considered: a reference picture error, which has been identified in previous works, and error due to the non-commutative property of motion compensation and down-sampling, which is unique to this work. To overcome these sources of error, four novel architectures are presented. One architecture attempts to compensate for the reference picture error in the reduced resolution, while another architecture attempts to do the same in the original resolution. We present a third architecture that attempts to eliminate the second type of drift error and a final architecture that relies on an intra block refresh method to compensate all types of errors. In all these architectures, a variety of macroblock level conversions are required, such as motion vector mapping and texture down-sampling. These conversions are discussed in detail. Another important issue for the transcoder is rate control. This is especially important for the intra refresh architecture since it must find a balance between number of intra blocks used to compensate errors and the associated rate-distortion characteristics of the low-resolution signal. The complexity and quality of the architectures are compared. Based on the results, we find that the intra refresh architecture offers the best trade-off between quality and complexity, and is also the most flexible.