motion estimation and compensation for very low bitrate video coding

Introduction 7
MPEG-4 standard which adds a new dimension to video coding as it
o ers the possibility to separately encode the di erent objects of a scene
and therefore interact with these objects at the decoder end.
Time-varying image sequences can be compressed by independently coding
each frame (intra-frame image coding) or by extending spatial coding
techniques to the time dimension (e.g. 3D transform coding). However,
the main characteristic of a video sequence is precisely its spatiotemporal
component: most of the information in an image sequence is
the result of motion. A lot of e ort has therefore been put into motion
analysis of video sources. The range of applications of such an analysis
includes, but is not limited to, automatic tracking of targets, piloting
of robots, events detection for surveillance, tridimensional reconstruction
of objects, image restoration... In a video coding context, motion
analysis is mainly used to reduce the inter-image redundancy: instead
of coding every new frame on its own basis, references are searched for
in the previously coded image. On a practical point of view, it means
that one searches for parts of the new picture which are already present
in the previous frame and which have just undergone some movement.
Once the motion parameters have been estimated and transmitted to
the decoder, their application provides a very good prediction of the
new image. This technique, referred to as \motion estimation and compensation",
achieves one of the most important compression factor in
a video coder thanks to its radical reduction of the spatio-temporal redundancy.
Since understanding image formation is a prerequisite for fully grasping
the methods to recover motion information from images, Chapter
Two starts with a short description on how images are generated and
how the real tridimensional motion of objects results in motion on the
bidimensional picture plane. Chapter Two simultaneously presents the
phenomena that can disturb or prevent a correct motion estimation,
and stresses the ill-posed nature of the problem. The chapter provides
a Rate-Distortion justi cation of the use of motion estimation in video
coders and introduces the various models and methodologies that can
constitute the basis of di erent motion algorithms. Classical and emerging
motion estimation techniques developed for image coding purposes
are detailed and the two of them (namely the Block-Matching Algorithm,
BMA, and Image Warping technique) which are used in the present work
conclude the chapter.
More largely, the present thesis mainly deals with motion estimation and