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