ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

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Intra-frames and Predicted-frames
The DivX encoder makes use of three frame types during encoding.
These are:
Intra-frames , Predicted-frames and Bi-directional-frames .
More commonly these are known as I , P and B frames. B-frames are
discussed later in the guide as an advanced topic.
An intra-frame is one in which all of the macroblocks
are stored as images rather than as motion vectors.
As noted previously, recording the image of a block is
the most expensive method in terms of storage and
hence I-frames are the most expensive type of frame.
Fewer I-frames in the video generally translates to
better compression and I-frames are normally used
by the encoder only when too few blocks can be
tracked from the reference frame by the motion search algorithm.
I-frames serve a very important purpose: All of the blocks in an I frame
are stored as images, thus decoding an I frame reveals a complete
picture without dependency on reference frames. For this reason I
frames are also known as
key-frames , and they are the only type of
frame completely independent of all others.
A predicted frame is one that can contain forward-predicted blocks—in
other words blocks that have been predicted via a motion vector from
the previous frame by the motion search. It is normally unlikely that
all
of the blocks in a P-frame can be predicted, and where blocks can not
be tracked from the previous frame an intra-block is used in its place,
similar to those found in an I-frame. Because P-frames reconstruct
much of the frame by applying motion vectors to the previous frame
(
motion compensation ) they are far less expensive in terms of storage
than I-frames.
One or more P-frames may follow an I-frame:
A greater ratio of P-frames to I-frames leads to a higher compression
ratio.
An intra-frame
Frame 1 (I) Frame 2 (P) Frame 3 (P)