Axis Network video Manual page 57

VIDEO COMPRESSION - CHAPTER 7 57
Figure 7.1b With difference coding, only the first image (I-frame) is coded in its entirety. In the two following
images (P-frames), references are made to the first picture for the static elements, i.e. the house. Only the moving
parts, i.e. the running man, are coded using motion vectors, thus reducing the amount of information that is sent and
stored.
Other techniques such as block-based motion compensation can be applied to further reduce
the data. Block-based motion compensation takes into account that much of what makes up a
new frame in a video sequence can be found in an earlier frame, but perhaps in a different loca-
tion. This technique divides a frame into a series of macroblocks (blocks of pixels). Block by
block, a new frame can be composed or 'predicted' by looking for a matching block in a reference
frame. If a match is found, the encoder codes the position where the matching block is to be
found in the reference frame. Coding the motion vector, as it is called, takes up fewer bits than
if the actual content of a block were to be coded.
Search window
Matching block
Target block
Motion vector
P-frame
Earlier reference frame
Figure 7.1c Illustration of block-based motion compensation.
With interframe prediction, each frame in a sequence of images is classified as a certain type of
frame, such as an I-frame, P-frame or B-frame.