ADS Technologies USBAV-708 INSTANT VIDEOMPX, Руководство пользователя

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As described in Forward—Predicted-frames and
Intra-frames , each frame in a video can be one
of three types: intra, predicted or bi-directional. It
follows that there must be some logic controlling frame
type selection—the encoder is not human and can’t
actually recognize where one scene changes into the
next.
Recall that when encoding predicted or bi-directional
frames the encoder performs the motion search for each
block composing the frame into one or more reference
frames. For P-frames the reference frame is the previous
frame in the video, for B-frames the reference frames
are both the previous and next frames.
Where a block can’t be matched in a reference frame it
will be encoded as an intra-block and instead of
prediction via a motion vector its image will be stored. One way to select where
to place intra-frames is therefore to set some threshold level specifying the
percentage of blocks that must be intra-coded in order for the frame to be
encoded as an intra-frame. This is the
scene-change threshold .
The scene-change threshold defines the percentage of blocks not tracked from
the reference frames by the motion search required to trigger a scene-change
detection. When a scene-change is detected a key-frame (intra-frame) will be
used in place of a predicted or bi-directional frame.
Here if the threshold were 13% frame 5 would become a key-frame (intra-frame).
QUICK GUIDE
Scene-change threshold
Advanced
Frame 1 (I) Frame 3 (P) Frame 5 (P) Frame 2 (B) Frame 4 (B)
100% untracked 0% untracked 9% untracked 4% untracked 14% untracked