motion estimation and compensation for very low bitrate video coding

134 Chapter 5. Mesh-Based Motion Compensation
Figure 5.6 demonstrates all the intermediate results of this rst step
with respect to original image 5.2(a). Comparing gure (b) with gure
(c), one can see that the selection of candidate points already rejects
several points as background ones.
5.2.1.2 Following Half Boundaries
Tracking the half-boundaries is then the next stage of the algorithm.
This procedure involves two steps: initialization and half-boundary following.
The initialization stage consists in selecting candidate points that are
relevant enough so as to start a tracking procedure. These points are
selected according to the following criteria:
the residual value of the point isabove a threshold T 2 (of course
T 2 >T 1);
the point is connected to at least one other candidate point of the
same type (`p' or `n') which has not yet been traced before and
which also has a residue greater than T 2.
Whereas Noble uses 4-connection for the second criterion, we still use 8connection
as it will be explained later. Noble also adds a third condition
which is that none of the neighbor points can have already been traced.
The reason for this is to avoid beginning a contour in a junction region
(where many edges meet). As this condition makes the whole process
more complex and does not bring a real improvement, we suppressed it
in our implementation.
Noble then proposes to perform an oriented tracking of half-boundaries.
The contour following direction is de ned so that boundaries are traversed
in the forward direction keeping the darker side of intensity to
the right (i.e. negative labels are tracked in the forward direction keeping
the positive labels on the left). Starting from a valid initial candidate
point, contour tracking is rst performed forwards and then backwards.
Noble uses a set of twelve 3 3 templates ( gure 5.7) allowing to follow
straight lines and 90 right or left angles. In the meantime, end of
contours (gaps) as well as multiple junctions (when the next point has
already been traced) are detected.
Moreover, in order to avoid the problems of wrong association in case
of contours passing through an arrow or `W'-junction, highly curved