motion estimation and compensation for very low bitrate video coding

Introduction 9
motion eld that is more di cult to encode e ciently. This analysis is
detailed at the beginning of Chapter Four and leads to the hypothesis
that voluntarily simplifying the contents of the images to code could
be pertinent. The motion estimation would then be performed between
two images with the same characteristics in terms of the accuracy of the
contents description, and it is expected that the resulting motion eld
will be easier to encode.
This idea of pre-processing is rst formalized with the help of the Rate-
Distortion theory: both intra-coding and pre-processing are modeled as
the combination of a low-pass lter and additive white noise. Conditions
for improving the coder performances are derived from this model. The
in uence of various types of pre-processing on coding image sequences
with the ITU H.263 standard is then experimentally tested. These preprocessings
are: intra-coding and Gaussian, median or morphological
ltering. Prospects are also formulated for selective pre-processing according
to the contents relevance.
The aim of Chapter Five, which focuses on compensation, is to demonstrate
that it is possible to subjectively improve the result of the motion
compensation stage (at the decoder) without modifying the estimation
(at the encoder) nor the transmission (the bitstream) 1 . This is possible
by taking the spatial contents of the reference image into account in
order to adapt the motion information to it.
The warping image techniques that have been described at the end of
Chapter Two o er such a possibility, namely to easily adapt the motion
information on irregular grid. However, their estimation phase is very
e ort-demanding because of its iterative nature. On the contrary, the
BMA reveals itself a very e cient estimation method while its compensation
stage generates an image that su ers from so-called \blocking
artifacts". We do thereafter suggest to use an asymmetric scheme that
consists in a BMA estimation and a warping compensation. The warping
technique involves meshes made out of triangular patches (that ensures
a direct link with the a ne transform). Moreover, warping techniques
(should they use triangular, quadrilateral or other structures) are very
well-suited for further editing and modi cation of images, manipulations
that an increasing number of users like toachieve.
1 Of course, if one wants to use the proposed reconstruction scheme in the coding
loop, it has to modify the coder so as to include the developed tool, which in turn
will modify the contents (not the structure) of the bitstream.