Compensation of Requantization and Interpolation Errors in MPEG-2 to H.264 Transcoding
Implementing MPEG-2 to H.264 transcoding schemes in the pixel domain introduces a high degree of computational complexity. In the transform domain, this transcoding is more computationally efficient, and several methods have been developed to address that approach. However, incompatibilities between the two standards, such as the mismatches between the MPEG-2 and H.264 motion compensation processes, cause several distortions that may affect the overall picture quality. In this study, we address the main distortions that result from requantization errors: luminance half-pixel and chrominance quarter/three-quarter interpolation errors. Then, we propose algorithms that compensate for these errors. The traditional requantization error compensation algorithm for DCT coefficients is updated so that it can be applied to the H.264 integer transform coefficients. Equations that compensate for the luminance half-pixel and chrominance quarter/three-quarter pixel interpolation errors are derived. To remove the interpolation errors, the previous H.264 frame is needed. Thus, the compensation scheme includes a closed-loop H.264 motion compensation process, which is implemented in the pixel domain. To evaluate the performance of the proposed compensation algorithms in terms of picture quality, our scheme is compared with two different cascaded pixel-domain transcoding structures. The first structure reuses the MPEG-2 motion vectors, and the other implements plusmn2 pixels motion vector refinement, but each one has an H.264 deblocking filter. The experimental results show that the proposed compensation algorithms achieve 5-dB quality improvement over the open-loop transform-domain-based transcoding and almost the same picture quality (0.3-0.6 dB) as the cascaded structures. An additional advantage is the reduction in computational complexity that ranges from 13% to 69% compared with the two cascaded methods.