Databases Reference
In-Depth Information
19.3 Motion Compensation
In most video sequences there is little change in the contents of the image from one frame to
the next. Even in sequences that depict a great deal of activity, there are significant portions
of the image that do not change from one frame to the next. Most video compression schemes
take advantage of this redundancy by using the previous frame to generate a prediction for
the current frame. We have used prediction previously when we studied differential encoding
schemes. If we try to apply those techniques blindly to video compression by predicting the
value of each pixel by the value of the pixel at the same location in the previous frame, we
will run into trouble because we would not be taking into account the fact that objects tend to
move between frames. Thus, the object in one frame that was providing the pixel at a certain
location
with its intensity value might be providing the same intensity value in the next
frame to a pixel at location
(
i
0
,
j
0
)
. If we don't take this into account, we can actually increase
the amount of information that needs to be transmitted.
(
i
1
,
j
1
)
Example19.3.1:
Consider the two frames of amotion video sequence shown in Figure
19.1
. The only differences
between the two frames are that the devious looking individual has moved slightly downward
and to the right of the frame, while the triangular object has moved to the left. The differences
between the two frames are so slight, you would think that if the first frame was available to
both the transmitter and receiver, not much information would need to be transmitted to the
receiver in order to reconstruct the second frame. However, if we simply take the difference
between the two frames, as shown in Figure
19.2
, the displacement of the objects in the frame
results in an image that contains more detail than the original image. In other words, instead
of the differencing operation reducing the information, there is actually more information that
needs to be transmitted.
F I GU R E 19 . 1
Two frames of a video sequence.
