motion estimation and compensation for very low bitrate video coding

1.2 Video Coding 19
exhaustive use of the two following types of pictures:
Intra-images: When coding an image in intra mode, only the
spatial correlation inherent to the picture itself is exploited. In
fact, an intra-image is an isolated picture and is compressed as
such (cf. the JPEG [135] algorithm for still pictures compression).
Inter-images: These images are speci c to video sequences in
comparison with still pictures. They allow the coder to exploit
the spatio-temporal correlation between the present image and
the previous one(s) within a sequence. The classical tool used to
achieve compression on such images is motion estimation & compensation
which allows one to obtain a prediction of the present
image from the previous one(s).
{ Residues: As the estimation & compensation of an interimage
is not perfect, the di erence between the estimate and
the original image is computed: the so-called Displaced Frame
Di erence (DFD) has to be sent if it is relevant enough. It can
only be transmitted on its own basis, i.e. as an intra-image.
The rst image of a sequence must of course be encoded as an intraimage.
For the subsequent images, a scene-cut detector compares the
new image with the previous one(s) to check if some temporal correlation
still exists. In the a rmative, the new image will be inter-coded.
Otherwise, the coder considers that a scene-cut has occurred: the new
image di ers so much from the previous one(s) that it is better to encode
it as an intra-image. The user may also enforce intra-image coding to
take place every x seconds to o er speci c functions like fast retrieval,
temporal scalability or resynchronization in case of noisy channel.
According to these considerations, one can a rm that the scheme presented
on gure 1.2 (from [13]) is typical of most (VLBR) video coders.
The rst image is intra-coded. Then the second image is motion estimated
on the basis of the rst one (original or reconstructed). The
motion vectors of course need to be sent to the decoder in order to allow
it to behave the same way. The motion vector eld is applied to the
rst image thereby obtaining a prediction of image two, which is used
for the DFD computation. This DFD is the residual image that also
needs to be coded. The process goes on until some scene cut arises or
until a de ned threshold enforces to start again with an intra-image.