TAILIEUCHUNG - H.264 and MPEG-4 Video Compression phần 6

tiếp theo là cấu hình cho mã hóa khả năng mở rộng, mã hóa kết cấu vẫn còn và chất lượng cao ('phòng') mã hóa video. Ngoài ra các công cụ mã hóa của "tự nhiên" (thực tế) vật liệu video, MPEG-4 Visual định nghĩa một tập hợp các hồ sơ để mã hóa của 'tổng hợp | CODING ARBITRARY-SHAPED REGIONS 131 Figure Boundary MB Figure Boundary MB after horizontal padding 132 MPEG-4 VISUAL edge pixel. Transparent MBs are always padded after all boundary MBs have been fully padded. If a transparent MB has more than one neighbouring boundary MB one of its neighbours is chosen for extrapolation according to the following rule. If the left-hand MB is a boundary MB it is chosen else if the top MB is a boundary MB it is chosen else if the right-hand MB is a boundary MB it is chosen else the lower MB is chosen. Transparent MBs with no nontransparent neighbours are filled with the pixel value 2N-1 where N is the number of bits per pixel. If N is 8 the usual case these MBs are filled with the pixel value 128. Texture Coding in Boundary Macroblocks The texture in an opaque MB the pixel values in an intra-coded MB or the motion compensated residual in an inter-coded MB is coded by the usual process of 8 X 8 DCT quantisation runlevel encoding and entropy encoding see Section . A boundary MB consists partly of texture pixels inside the boundary and partly of undefined transparent pixels outside the boundary . In a core profile object each 8 X 8 texture block within a boundary MB is coded using an 8 X 8 DCT followed by quantisation run-level coding and entropy coding as usual see Section for an example . The Shape-Adaptive DCT part of the Advanced Coding Efficiency Profile and described in Section provides a more efficient method of coding boundary texture. CODING ARBITRARY-SHAPED REGIONS 133 The Main Profile A Main Profile CODEC supports Simple and Core objects plus Scalable Texture objects see Section and Main objects. The Main object adds the following tools interlace described in Section object-based coding with grey alpha plane shape Sprite coding. In the Core Profile object shape is specified by a binary alpha mask such that each pixel position is marked as opaque or transparent . The Main .

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