An extension of direct macroblock coding in Predictive (P) slices of the H.264 standard

The consideration of better motion compensation techniques for inter-frame prediction is one of the key reasons why the new H.264 (MPEG-4 AVC) video coding standard can achieve considerably better coding efficiency compared to older standards such as MPEG-2/4 and H.263. These include the use of multiple references and block sizes, a better interpolation filter for subpixel motion compensation, and more efficient exploitation of the spatio-temporal correlation between motion vectors of adjacent blocks through the consideration of SKIP and DIRECT modes. In this paper, we introduce additional methods into H.264 that further enhance motion compensation and can lead to additional improvements in coding efficiency. This is achieved by further exploiting motion vector temporal correlation through the introduction of a new DIRECT macroblock type and an enhancement to the existing Skip Macroblock type within Predictive (P) slices. These new macroblock types can lead to a considerable reduction in the bits required for encoding motion information, while retaining or even improving quality under a Rate Distortion Optimization Framework. Our simulation results suggest that the proposed improvements can lead up to 7.6% average bitrate reduction or equivalently 0.39 dB quality improvement over the current H.264 standard.