motion estimation and compensation for very low bitrate video coding

28 Chapter 1. Digital Video Coding at Very-Low BitRate
Figure 1.8: Result of the tree decomposition (B) with coarser thresholds
-\Foreman" (left) and an arti cial image (right)
One of its advantages is the use it can make of the correlation between
luminance and chrominance components: for color images, the same tree
is used to describe the three components.
If the reader has a close look at the \Foreman" decomposition (image
1.7(center)), he/she will notice that the image is rather complicated.
This complexity has to be reduced to transmit such a picture
over a VLBR channel. A rst way toachieve such a reduction of the
complexity is to use coarser thresholds (Figure 1.8).
The visual result is not particularly pleasant. The solution adopted here
is to interpret the pictures on a region basis: after a segmentation step,
all regions are classi ed according to their subjective relevance, and the
algorithm tries to keep a good quality on the interesting regions (e.g.
speaker's head,:::) while coarsely coding the other regions.
Interesting regions determination. For the purpose of region selection,
an interest criterion is de ned [72, 20, 19]. It can automatically
distinguish between what is important and what is not. Three criteria
are combined so as to provide the nal classi cation:
the image border criterion: it is based on the fact that the
most important part of an image in a video sequence is correctly
centered and that the eye precision is the best in the center of the