motion estimation and compensation for very low bitrate video coding

Abstract
Motion estimation is a key issue in the eld of moving images analysis.
In the framework of video compression, it is combined with motion
compensation in order to exploit the spatio-temporal correlation of image
sequences along the motion trajectory. It then achieves one of the
most important compression factor of a video coder. The research presented
in this thesis is mainly concerned with improvements of classical
motion estimation and compensation techniques in the context of verylow
bitrate transmissions thanks to contents-adapted information that
can be extracted from images. This concept is consecutively, but independently,
applied to the various steps of exploiting motion in a video
coding scheme: estimation, transmission and compensation.
The manuscript starts with a brief overview of the \state of the arts" in
video compression, introducing underlying concepts while putting some
emphasis on very-low bitrate conditions. Motion estimation is further
detailed in the following chapter, which includes a description of motion
representation and disturbing phenomena, as well as a review of the
most commonly used estimation techniques. The contributions of the
thesis are then presented.
First, a reliable motion estimation technique based on multiscale algorithms
is introduced: it outputs segmented and more coherent motion
elds that are adapted to the spatial contents of the images and which
can be more e ciently coded. A model of distribution of the engendered
computational burden is also proposed and demonstrates a linear speedup.
Secondly, the transmission of moving images is analyzed in the light
of the Rate-Distortion theory. Because of the very-low bitrate constraint,
some spatial pre-processing is performed prior to motion estimation in
order to raise the correlation between the already encoded images and
the new ones. Prospects are also formulated for selective pre-processing
according to the contents relevance. Thirdly, the reconstruction (compensation)
of block-based motion elds is subjectively improved by using
image warping techniques. A new corner detector is set up so as to automatically
design an active mesh, thanks to the Delaunay triangulation,
on the reference image. Inverse kriging interpolation then allows one
to determine the motion of the mesh vertices from the original vector
eld. It results in the suppression of the blocking artefacts while o ering
the possibility to further edit and modify the images thanks to the mesh
structure. The present thesis analyses thus three prospects of improving
the quality of existing video schemes.