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Flexible Macroblock Ordering or FMO is one of several error resilience tools defined in the Baseline profile of the H.264/MPEG-4 AVC video compression standard. == Description == One of the characteristics of the H.264/AVC standard is the possibility of dividing an image into regions called slices, each of which contains a sequence of macroblocks and can be decoded independently of other slices. These macroblocks are processed in a scan order, normally left to right, beginning at the top. A frame can be composed of a single slice, or multiple slices for parallel processing and error-resilience, because errors in a slice only propagate within that slice. ''Flexible Macroblock Ordering'' enhances this by allowing macroblocks to be grouped and sent in any direction and order, and can be used to create shaped and non-contiguous slice groups. This way, FMO allows more flexibly deciding what slice macroblocks belong to, in order to spread out errors and keep errors in one part of the frame from compromising another part of the frame. FMO builds on top of another error-resilience tool, Arbitrary slice ordering, because each slice group can be sent in any order and can optionally be decoded in order of receipt, instead of in the usual scan order. Individual slices still have to be continuous horizontal regions of macroblocks, but with FMO's slice groups, motion compensation can take place within any contiguous macroblocks through the entire group; effectively, each slice group is treated as one or more contiguous shaped slices for the purposes of motion compensation. Nearly all video codecs allow ''Region of Interest'' coding, in which specific macroblocks are targeted to receive more or less quality, the canonical example being a newscaster's head being given a higher ratio of bits than the background. FMO's primary benefit when combined with RoI coding is the ability to prevent errors in one region from propagating into another region. For example, if a background slice is lost, the background may be corrupted for some time but the newscaster's face will not be affected, and it becomes simpler to send regular refreshes of the most important slice to make up for any errors there. Slices used with FMO are not static, and can change as circumstances change, such as tracking a moving object. A structure called the ''MBAmap'' maps each macroblock to a slice group, and can be updated at any time, with a few default patterns defined, such as Slice Interleaving (groups alternate every scanline) or Scattered Slices (groups alternate every block). With these patterns, FMO allows one retain a better localized visual context so that error-concealment algorithms can reconstruct missing content.〔 Certain advanced encoding techniques can simulate some of FMO's benefits. In H.264/AVC, P (predicted) and B (bipredicted) frames may contain I (intra) blocks, which store independent picture. Rather than create a slice in order to periodically refresh entirely with I or IDR frames, I-blocks can be sent in any desired pattern while predicted blocks make up the rest of the picture. Although errors will still propagate horizontally, I-blocks can be sent in patterns, such as favoring a region of interest or a scattered checkerboard, to simulate shaped slice refreshes. With bidirectional communication to the client, lost slices can be refreshed as soon as detected, but this is not feasible for wider broadcast. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「flexible macroblock ordering」の詳細全文を読む スポンサード リンク
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