Image Processing Reference
Figure 12-1 Motion vector analysis. Source: courtesy of Tektronix.
the normal video channel and can be switched on and off. Set-top boxes that only have
MPEG-2 players would not be affected because they would be unable to see the additional
video stream. Special boxes provided for deaf viewers would allow a signing overlay to
be switched on and presented during prime-time viewing for any and all programs.
Currently, signed video is presented only at non-prime-time hours or for a few daytime
programs on special-interest channels with smaller niche audiences.
Applications for Virtual Sets
Programs requiring virtual sets such as weather forecasts could be significantly improved
by presenting them in an MPEG-4 BIFS framework containing several separate streams of
With codecs prior to MPEG-4, the entire picture had to be composited in the studio
and broadcast as a full-frame moving video. Figure 12-2 simulates the kind of output cre-
ated by a weather-forecasting studio. It shows how a presenter can be layered or compos-
ited over any arbitrary background.
This brings out a further issue regarding visual context. In this example, the presen-
ter is dressed more informally than a weather presenter would normally be on a national
network. However, for a program aimed at youth, this might be appropriate. Ensuring
that the layers are coherent and are contextually or culturally relevant is an important
aspect of the creativity in producing video. Sticking to a consistent visual grammar in your
video productions is important. MPEG-4 allows you to support multiple presenter over-
lays if you wish.