ADS Technologies USBAV-708 INSTANT VIDEOMPX, User Manual

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When the encoder finds a matching area for a block
in the previous frame its position is recorded by use
of a
vector . A vector is simply a numerical
representation of direction and magnitude. For
example the vector
(4, -7) might mean right 4 units,
down 7 units.
Because these vectors happen to represent the
movement of a macroblock they are called
motion vectors . In the
illustrations the red arrows represent the motion vectors.
Storing the motion vector of each block from frame-to-frame allows
DivX to recreate much of each new frame from the last using far less
data than if each block were stored as a complete image. The process
is similar to cutting up one frame into little pieces, then maneuvering
them like a make-shift jigsaw puzzle using motion vectors in order to
recreate the next frame. Of course the resulting picture is not perfect
and DivX has to store some additional data to correct for the difference
(known as the
residual ). Even so, recreating the picture using motion
vectors and residual consumes far less storage than recording the
entire image for every single frame.
The process of tracking blocks back to matching areas in the previous
frame (known as the
reference frame) is called the motion search, or
motion estimation . The frame recreated from a reference frame by
application of motion estimation is known as the compensated frame.
You can see the motion search
in action using the encoder feedback
window.
While encoding set
Show picture to
Compensated frame and turn on
Show Motion Vectors .
The white lines overlaid on the
compensated frame represent motion
vectors returned by the motion search,
tracking each block back to a
matching area in the reference frame.
Where blocks are grey in the compensated frame the motion search
failed to find a suitable match in the reference frame. These blocks will
be encoded as image data as opposed to motion vectors.
Vector (4,-7)