Why was the 16×16 prediction size chosen?
The 16×16 area corresponds to the Least Common Multiple (LCM) of 8×8 blocks, given the normative 4:2:0 chroma ratio. Starting with medium size images, the 16×16 area provides a good balance between side information overhead & complexity and motion compensated prediction accuracy. In gist, experiments showed that the 16×16 was a good trade-off between complexity and coding efficiency. What do B-pictures buy you? A. Since bi-directional macroblock predictions are an average of two macroblock areas, noise is reduced at low bit rates (like a 3-D filter, if you will). At nominal MPEG-1 video (352 x 240 x 30, 1.15 Mbit/sec) rates, it is said that B-frames improves SNR by as much as 2 dB. (0.5 dB gain is usually considered worth-while in MPEG). However, at higher bit rates, B- frames become less useful since they inherently do not contribute to the progressive refinement of an image sequence (i.e. not used as prediction by subsequent coded frames). Regardless, B-frames are still politically c
