motion estimation and compensation for very low bitrate video coding

20 Chapter 1. Digital Video Coding at Very-Low BitRate
Although the coding technique of the DFD is often similar to intracoding,
Strobach has demonstrated [120] the ine ciency of applying
the same technique to intra and residual coding because of the drastic
reduction of the spatial correlation after motion compensation. This
inadequacy can also be explained by the fact that the information of
an intra image is uniformly spread out, while the contents of residual
images is located in speci c areas of the images (e.g. the borders of the
moving objects). For instance, Matching Pursuits [90] are designed for
residual coding.
1.3 Coding at Very-Low BitRate
Very-Low BitRate transmission can be understood as transmissions across
bandwidths of 5 to 64kbit=s, or sometimes up to 128kbit=s. Some of
the applications which might be addressed are: wired video phones
(28:8kbit=s modems and below), wireless video phones (under 13kbit=s),
Internet video conferencing (28:8kbit=s modems and below), remote
monitoring, tele-operation, tele-working,::: Appendix A presents the
typical VLBR-like video sequences that are used as test sequences in
the present document.
The rst video coding standards considered higher bitrates. For instance
MPEG-1 [62] isintended to ensure domestic use quality (CD-ROM use)
under 1:5Mbit=s and MPEG-2 [62] proposes digital TV broadcasting
below 10Mbit=s or HDTV at 60Mbit=s. So, what constitutes the main
di erence between codecs working at such bitrates and VLBR ones?
The rst di erence was already introduced in table 1.1: in order to
quickly reach low bitrates, lower spatial and temporal resolutions are
used. But another major di erence can be pointed out: to transmit
(moving) pictures across high-bitrate channels, only the irrelevant part
of the image(s) has to be suppressed, while VLBR compression schemes
generally have to suppress more information. Therefore, VLBR video
coding introduces more artifacts. A key-issue for VLBR coding is thus
an e cient management of the visual degradations that have to be accepted.
The in uence of the bitrate mainly arises at the residual level:
very-low bitrates prevent the coder from sending a su cient amount
of residual information. The imperfections of the motion estimation &
compensation phase are not entirely corrected and many artifacts remain
visible.