Page 81 - Video Coding for Mobile Communications Efficiency, Complexity, and Resilience

58 Chapter 3. Video Coding: Standards

motion vector mode, and 12 new optional modes (annexes I–T). This is brieJy
described in what follows.

3.4.6.1 New types of pictures

Version 2 de/nes three new types of pictures:
1. Scalability pictures: Three types of scalability pictures were added, one
that provides temporal scalability and two that provide SNR or spatial
scalability:
(a) B: a picture having two reference pictures, one of which temporally
precedes the B picture and one of which is temporally subsequent
to the B picture.
(b) EI: a picture having a temporally simultaneous reference picture.
(c) EP: a picture having two reference pictures, one of which tem-
porally precedes the EP picture and one of which is temporally
simultaneous.
These pictures are described in more detail in Section 3.4.6.9 in the
discussion of the new optional scalability mode (annex O).

2. Improved PB-frames: Recent investigations have indicated that the cur-
rent PB-frames utilized by version 1 are not su,ciently robust for
continual use. Encoders implementing the PB-frames mode are lim-
ited to use only bidirectional prediction. In some situations, this results
in a lack of usefulness of the PB-frames mode. An improved, more
robust type of PB-frames has been added to enable heavier, higher-
performance use of the PB-frames mode. This is described in more de-
tail in Section 3.4.6.7 in the discussion of the new optional improved
PB-frames mode (annex M).

3. Custom source formats: As already discussed, version 1 allows only
/ve video source formats (CIF family) with de/ned picture size, picture
shape, and picture clock frequency. Version 2, however, allows a wide
range of optional custom source formats in order to make the standard
apply to a much wider class of video scenes and applications, such as
resizable computer window-based displays, high refresh rates, and wide-
format viewing screens.

3.4.6.2 Modi>ed Unrestricted Motion Vector Mode (modi>ed Annex D)
The optional unrestricted motion vector mode (annex D) of version 1 has been
modi/ed in version 2. Version 2 de/nes a new data /eld called PLUSPTYPE.
When using the unrestricted motion vector mode, if PLUSPTYPE is present