Implementation
of Block matching algorithm for motion estimation:
With the advent of the multimedia age
and the spread of Internet, video storage on CD/DVD and streaming video has
been gaining a lot of popularity. The ISO Moving Picture Experts Group (MPEG)
video coding standards pertain towards compressed video storage on physical
media like CD/DVD, where as the International Telecommunications Union (ITU)
addresses real-time point-to-point or multi-point communications over a
network. The former has the advantage of having higher bandwidth for data
transmission. In either standard the basic flow of the entire compression
decompression process is largely the same. The encoding side estimates the
motion in the current frame with respect to a previous frame. A motion
compensated image for the current frame is then created that is built of blocks
of image from the previous frame. The motion vectors for blocks used for motion
estimation are transmitted, as well as the difference of the compensated image
with the current frame is also JPEG encoded and sent. The encoded image that is
sent is then decoded at the encoder and used as a reference frame for the
subsequent frames. The decoder reverses the process and creates a full frame.
The whole idea behind motion estimation
based video compression is to save on bits by sending JPEG encoded difference
images which inherently have less energy and can be highly compressed as
compared to sending a full frame that is JPEG encoded. The underlying
supposition behind motion estimation is that the patterns corresponding to
objects and background in a frame of video sequence move within the frame to
form corresponding objects on the subsequent frame. The idea behind block
matching is to divide the current frame into a matrix of ‘macro blocks’ that
are then compared with corresponding block and its adjacent neighbors in the
previous frame to create a vector that stipulates the movement of a macro block
from one location to another in the previous frame. This movement calculated
for all the macro blocks comprising a frame, constitutes the motion estimated
in the current frame. The search area for a good macro block match is
constrained up to p pixels on all fours sides of the corresponding macro
block in previous frame. This ‘p’ is called as the search parameter.
In the entire motion based video compression
process, motion estimation is the most computationally expensive and
time-consuming process. The research in the past decade has focused on reducing
both of these side effects of motion estimation. Block matching techniques are
the most popular and efficient of the various motion estimation techniques.
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