ernate rows of a frame. A frame is composed of two fields, a top field and a bottom field. field macroblock A macroblock containing samples from a single field. All macroblocks of a coded field are field macroblocks. When macroblock-adaptive frame/field decoding is in use, some macroblocks of a coded frame may be field macroblocks. field macroblock pair: A macroblock pair decoded as two field macroblocks. field scan: A specific sequential ordering of transform coefficients that differs from the zig-zag scan by scanning columns more rapidly than rows. Field scan is used for transform coefficients in field macroblocks. flag: A variable that can take one of the two possible values 0 and 1. frame: A frame contains an array of luma samples in monochrome format or an array of luma samples and two corresponding arrays of chroma samples in 4:2:0, 4:2:2, and 4:4:4 colour format. A frame consists of two fields, a top field and a bottom field. frame macroblock: A macroblock representing samples from the two fields of a coded frame. When macroblock-adaptive frame/field decoding is not in use, all macroblocks of a coded frame are frame macroblocks. When macroblock-adaptive frame/field decoding is in use, some macroblocks of a coded frame may be frame macroblocks. frame macroblock pair: A macroblock pair decoded as two frame macroblocks. frequency index: A one-dimensional or two-dimensional index associated with a transform coefficient prior to an inverse transform part of the decoding process. hypothetical reference decoder (HRD): A hypothetical decoder model that specifies constraints on the variability of conforming NAL unit streams or conforming byte streams that an encoding process may produce. hypothetical stream scheduler (HSS): A hypothetical delivery mechanism for the timing and data flow of the input of a bitstream into the hypothetical reference decoder. The HSS is used for checking the conformance of a bitstream or a decoder. I slice: A slice that is not an SI slice that is decoded using intra prediction only. info