A Linear Model-based Sub-pixel Motion Estimation Method for H.264/avc Standard

H.264/AVC [1], which is a recently published video coding standard, was developed and has been updated by Joint Video Team (JVT) of ISO/IEC MPEG and ITU-T VCEG. The H.264 encoder, similar to that of the existing video standards such as ISO/IEC MPEG-1, MPEG-2, MPEG-4, ITU-T H.261, H.263 [2], is composed of a temporal model, a spatial model, and an entropy encoder. The runtime of the temporal model, which is named motion estimation (ME) part to eliminate the temporal redundancy between adjacent frames, occupies more than 80% of the whole runtime of the H.264 encoder [3]. Furthermore, due to various advanced encoding technologies such as variable block-size ME, multiple reference frames, and quarter-pixel ME and compensation, the computational complexity was increased dramatically. Although the advanced encoding technologies have significantly improved the quality performance of the H.264 standard, the high complexity imposes restrictions on real time video applications, especially for mobile devices. The motion estimator in the H.264 encoder conducts the integer-pixel ME (IME), followed by the fractional-pixel ME (FME). Particularly, the runtime of IME is about half of the total encoding time [3]. In which, many fast IME algorithms including the unsymmetrical-cross multihexagon-grid search (UMHexagonS) [4] have been developed up to now to reduce integer search points. The proposed IME algorithms have different search strategies to satisfy the accuracy of motion estimation and the search speed, respectively. On the other hand, the FME part also takes up over 40% of the total ME runtime [3]. After performing the FME process, about 1-3 PSNR improvement can be expected. The FME part, therefore, has been recently regarded as an important research topic in H.264/AVC. The conventional full fractional-pixel search always requires a fixed number of search points and consumes a large amount of memory due to the interpolation process for fractional-pixel search. Thus, in this paper, a linear model-based sub-pixel motion estimation method for H.264/AVC standard is proposed to efficiently reduce the computational complexity and maintain the quality performance without the interpolation process. 2. PREVIOUS FME ALGORITHM