Graphics Reference
In-Depth Information
The final lists L0 and L1 are used for motion compensation. For uni-prediction,
one motion vector and one reference picture index is indicated for a block. For
instance, the first picture in L0 is the picture to use for motion compensation if the
signaled index for a block is equal to 0.
2.5
Video Usability Information (VUI) and Supplemental
Enhancement Information (SEI)
HEVC VUI and SEI are similar in behavior to the VUI and SEI of AVC. The video
usability information (VUI) is an optional section of syntax within the SPS. VUI
data do not directly impact the decoding process, but provides useful information
in two main categories. The first category is related to display of the decoded
pictures, including information such as aspect ratio, overscan, and color primaries.
The category also includes timing information which is used by the hypothetical
reference decoder (HRD), as described in Sect.
2.6
. The second category of VUI
data is for bitstream restrictions, to provide information to the decoder indicating
that the encoder did not exercise the full flexibilities of the standard. The restrictions
signaled relate to tiles, motion vectors, reference picture lists, and bytes per coded
picture.
SEI messages provide metadata and are generally optional. SEI messages are
carried within SEI NAL units. In AVC, all SEI messages are considered to be of
a prefix type, which means that the SEI message is required to precede all VCL
NAL units of an access unit. HEVC introduces the concept of a suffix SEI message,
which follows a VCL NAL unit of an access unit. Table
2.6
lists all of the HEVC SEI
messages, and indicates whether they are of prefix or suffix type. Some messages
may be used in either prefix or suffix mode.
SEI messages have different rules for persistence. Some SEI messages apply only
to the current access unit, while others persist until another SEI message cancels or
replaces it or until a new CVS begins. The scalable nesting SEI message can be used
to indicate if an SEI message applies to particular temporal sub-layers, and in the
future layered extensions, to which particular layers.
Many of the SEI messages are very similar to related messages in AVC, although
they do not always follow the exact same syntax. Buffering period, picture timing,
and decoding unit info messages are used in the HRD operation, as described
in Sect.
2.6
. The following SEI messages are carried over from AVC: pan-scan
rectangle, filler payload user data registered, user data unregistered, recovery point,
scene information, progressive refinement segment start, progressive refinement
segment end, film grain characteristics, post filter hint, tone mapping information,
and frame packing arrangement.
In AVC, the scalable nesting SEI message is part of the SVC extension in Annex
G, but in HEVC it is included in version 1, because of version 1 support for temporal
scalability. The HEVC temporal sub-layer zero information SEI message is similar
