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The three major picture types found in typical video compression designs are I(ntra) pictures, P(redicted) pictures, and B(i-predictive) pictures (or B(i-directional) pictures). more...
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They are also commonly referred to as I frames, P frames, and B frames. In older material, the term "bi-directional" rather than "bi-predictive" is dominant.
In video compression formats, such as in ITU-T VCEG or ISO/IEC MPEG video standards, often only the differences between pictures will be encoded. For example, in a scene in which a person walks past a stationary background, only the moving region will need to be represented (either using motion compensation or as image data or as a combination of the two, depending on which representation requires fewer bits to adequately represent the picture). The parts of the scene that are not changing need not be sent repeatedly.
Introduction to frame types
Strictly speaking, the term picture is a more general term than frame, as a picture can be either a frame or a field, where a frame is essentially an image captured at some instant in time and a field is the set of every-other line that would form an image at some instant in time. When sending video in interlaced-scan format, the coding of pictures as individual fields is often used rather than the coding of complete frames. Informally, the term "frame" is often used when the actual intent is the more general term "picture".
Typically, pictures are segmented into macroblocks, and individual prediction types can be selected on a macroblock basis rather than being the same for the entire picture, as follows:
I pictures can contain only intra macroblocks;
P pictures can contain either intra macroblocks or predicted macroblocks;
B pictures can contain intra, predicted, or bi-predicted macroblocks;
Furthermore, in the most recent video codec standard H.264, the picture can be segmented into smaller regions called slices and instead of using I, B and P picture type selections, the encoder can choose the prediction style distinctly on each individual slice. Also in H.264 suggested:
SI-frames/slices (Switching I); Facilitates switching between coded streams; contains SI macroblocks (a special type of intra coded macroblock).;
SP-frames/slices (Switching P); Facilitates switching between coded streams; contains P and/or I macroblocks;
multi-frame motion estimation (up to 32 reference frames);
Multi-frame motion estimation will allow to increase quality with the same compression ratio and SI- SP-frames (defined for Extended profile) will allow to increase error resistance. In case of such frames usage and smart decoder it is possible to recover broadcast stream of damaged DVD.
Read more at Wikipedia.org
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