ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

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The
Guide
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Bi
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Wh
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Bi-directional encoding
As discussed in Forward—General concepts, there are three different frame
types available for the DivX encoder to select from. These are intra-frames,
predicted-frames and bi-directional-frames, or
I , P and B frames respectively.
Recall that in an I-frame all blocks are intra-blocks and are encoded as the image
filling the block. In a P-frame blocks can be either intra-blocks or forward
predicted blocks—those described by a vector referencing a matching area of
image in the previous frame (known as forward prediction).
Bi-directional frames can contain blocks that are intra, forwards predicted,
backwards predicted, or both forwards
and backwards predicted. This means
that B-frames reference not only the previous frame but the next frame also.
In the diagram frames 2 and 4 are bi-directional frames and any macroblock may
be predicted from either the previous (forward) or next (backward) frame. If a
block is both forwards and backwards predicted then when motion compensation
takes place the block image will be the result of blending the appropriate areas of
the forwards and backwards frames together.
Notice that frame 4 contains some intra-blocks because in the example these
blocks could not be predicted from either the forwards or backwards frame.
What is bi-directional encoding?
Frame 1 (I) Frame 3 (P) Frame 5 (P) Frame 2 (B) Frame 4 (B)