Block prediction is done by a weighted sum of blocks from the reference picture. It allows enhanced picture quality in scenes where there are changes of plane, zoom, or when new objects are revealed. Enhanced Direct/Skip Macroblock Skip and Direct Mode are very frequently used, especially with B-frames. They significantly reduce the number of bits to be coded. These modes are referred to when a block is coded without sending residual error or motion vectors. The encoder will only record that it is a Skip Macroblock. The decoder will deduce the motion vector of Direct/Skip Mode coded block from other blocks already decoded. There are two ways to deduce the motion: Temporal It uses the block motion vector from List 1 frame, located at the same position to deduce the motion vector. List 1 block uses a List 0 block as reference. Spatial It predicts the movement from neighbour macroblocks in same frame. A possible criterion could be to copy the motion vector from a neighboring block. These modes are used in uniform zones of the picture where there is not much movement. In the figure above, pink blocks are Direct/Skip Mode coded blocks. As we can see, they are used very frequently, mainly in B-frames. Additional information Although the use of the term "frame" is common in informal usage, in many cases (such as in international standards for video coding by MPEG and VCEG) a more general concept is applied by using the word "picture" rather than "frame", where a picture can either be a complete frame or a single interlaced field. Video codecs such as MPEG-2, H.264 or Ogg Theora reduce the amount of data in a stream by following key frames with one or more inter frames. These frames can typically be encoded using a lower bit rate than is needed for key frames because much of the image is ordinarily similar, so only the changing parts need to be coded. See also Video compression picture types References Software H.264: http://iphome.hhi.de/suehring/tml/download/ T.Wiegand, G.J. Sullivan, G. Bjøntegaard, A.Luthra: Overview of the H.264/AVC Video Coding Standard. IEEE Transactions on Circuits and Systems for Video Technology, Vol. 13, No. 7, July 2003 Film and video technology Compression algorithms