Warped Discrete Cosine Transform-Based Low Bit-Rate Block Coding Using Image Downsampling
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Research Article Warped Discrete Cosine Transform-Based Low Bit-Rate Block Coding Using Image Downsampling ¨ Sarp Erturk Kocaeli University Laboratory of Image and Signal Processing (KULIS), Electronics and Telecommunication Engineering Department, University of Kocaeli, 41040 Kocaeli, Turkey Received 18 May 2006; Revised 30 January 2007; Accepted 6 February 2007 Recommended by Mauro Barni This paper presents warped discrete cosine transform (WDCT)-based low bit-rate block coding using image downsampling. While WDCT aims to improve the performance of conventional DCT by frequency warping, the WDCT has only been applicable to high bit-rate coding applications because of the overhead required to define the parameters of the warping filter. Recently, low bit-rate block coding based on image downsampling prior to block coding followed by upsampling after the decoding process is proposed to improve the compression performance for low bit-rate block coders. This paper demonstrates that a superior performance can be achieved if WDCT is used in conjunction with image downsampling-based block coding for low bit-rate applications. Copyright © 2007 Sarp Ert¨urk. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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INTRODUCTION
Block-based discrete cosine transform (DCT) encoders are incorporated into many image and video coding standards as a result of their high decorrelation performance and the availability of fast DCT algorithms enabling real-time implementation [1]. The DCT is used in the JPEG image coding standard, the MPEG-1 and MPEG-2 video coding standards, as well as the ITU-T H.261 and H.263 recommendations for real-time visual communications. The warped discrete cosine transform (WDCT) is a cascade connection of conventional DCT and all-pass filters whose parameters are adjusted to provide frequency warping and thereby improve the coding performance [2]. WDCTbased compression is shown to outperform conventional DCT-based compression for high bit-rate applications. However, for low bit-rate applications the overhead required to encode the all-pass filter parameters of each block becomes significant in WDCT and the compression performance falls below conventional DCT. Recently, it has been shown that downsampling before DCT coding and upsampling after decoding can improve the objective and subjective performance at low bit-rates [3]. If an image frame is downsampled before compression, the available amount of data per pixel that can be encoded in the transform coding increases for a fixed bit-rate so that the reconstructed image quality can be improved. A standard
anti-aliasing filter has been used as decimation filter, and a linear interpolation kernel has been used after decoding in [3]. It has been demonstrated in [4] that the performance can even be enhanced if optimal or near-optimal decimation and interpolation filters are used within this schem
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