Graphics Reference
In-Depth Information
communication system, such as the loss of some of the data packets that contain
bitstream data. A decoder may or may not attempt to continue decoding when non-
conforming data is encountered. Nevertheless, the output of an HEVC encoder shall
always fully conform to the HEVC specification.
There are also syntax element values that are reserved in the specification. These
are values that are not in use for a particular version of the HEVC specification, but
may be specified and used in future HEVC versions. An encoder is not allowed to
use reserved values for a syntax element. If the entire syntax element is reserved,
the HEVC specification specifies what value a first version encoder may use. The
encoder must obey these rules in order for the output bitstream to be conforming.
A decoder must ignore the reserved syntax element values. If a reserved value
is found in the NAL unit header, for instance in the NAL unit type or layer ID
syntax elements, the decoder must discard the entire NAL unit. This enables legacy
decoders to correctly decode the base layer of any future bitstream that contains
additional extension layers that are unknown to the decoders made for earlier
versions of the standard.
Some syntax element values are unspecified; those values must be also be ignored
by a decoder, as far as their effect on the standard decoding process is concerned.
The difference between an unspecified value and a reserved value is that a reserved
value may be used in future versions of HEVC while an unspecified value is
guaranteed never to be specified in the future and may be used for other purposes
that are not defined in the standard. The main purpose of unspecified values is to
allow external specifications to make use of them. One example is the unspecified
NAL unit type value 48 which is proposed to be used in the HEVC RTP payload
specification [
11
] to signal aggregated packets that contain multiple NAL units. In
the proposed RTP payload specification, the value 48 is used as an escape code to
indicate that data should not be passed to the HEVC decoder as is, but that additional
RTP header data will follow to identify the locations and sizes of the NAL units
in the RTP packet. The RTP payload specification is described in more detail in
Sect.
2.3.5
.
2.2.4
Non-VCL NAL Unit Types
Tab le
2.2
shows all 32 non-VCL NAL unit types and their NAL unit type values in
the NAL unit header.
There are three parameter set types in HEVC, they are explained further in
Sect.
2.3
.
The access unit delimiter NAL unit may optionally be used to indicate the
boundary between access units. If present, the access unit delimiter must signal
the same temporal ID as the associated coded picture and be the first NAL unit in
the access unit. It has only one codeword in its payload; this codeword indicates
what slice types may occur in the access unit.
