15-01-2013, 02:08 PM
Image compression methods and standards
Image compression.pdf (Size: 2.39 MB / Downloads: 105)
Predictive coding
Predictive coding is based on eliminating the redundancies of closely
spaced pixels – in space and/or in time – by extracting and coding only
the new information in each pixel. The new information is defined as
the difference between the actual and predicted value of the pixel.
Motion compensated prediction residuals
Since successive frames in a video sequence are often quite similar,
coding their differences can reduce temporal redundancy and provide
significant compression. On the other hand, when a frame sequence
contains rapidly moving objects, the similarity between neighboring
frames is reduced. The attempt to use LPC on images with little
temporal redundancy may lead to data expansion. Video
compression systems avoid the problem of data expansion by:
1. Tracking object movement and compensating for it during the
di i d diff i
Spring 2008 ELEN 4304/5365 DIP 14
prediction and differencing process;
2. Switching to an alternative coding method when there is
insufficient inter-frame correlation (similarity between frames) to
make predictive coding advantageous
Values from ±8 to ±64 pixels are common, and the horizontal search
area often is significantly larger than the vertical search area.
Another, more computationally efficient measure is the sum of
absolute distortions (SAD) that omits the 1/mn factor.
Spring 2008 ELEN 4304/5365 DIP 18
For the specified selection criterion (say, MAD), motion estimation
is performed by searching for the dx and dy minimizing MAD(x,y)
over the allowed range of motion vector displacements – block
matching. An exhaustive search is efficient but expensive; there are
fast algorithms that are inexpensive but don’t guarantee optimum.
Motion compensated prediction residual was computed by dividing
the latest figure into 16x16 macroblocks and comparing each
macroblock to all possible 16x16 macroblock in the earlier frame
within ±16 pixels position. The MAD criterion was used. The
resulting standard deviation was 5.62 and the entropy was 3.04
bits/pixel.