AUD其实就是用来分隔图像的，两个AUD之间是一个编码图像，解析Nal时，如果遇到AUD，说明后面是一个新的图像。
AUD的NAL Unit type 值是9.
nal_ref_idc 需要填0.
nal_ref_idc 需要填0.
后面是AUD的详细说明，
如果只是想知道他的作用，那么到此就可以了，后面的无需再看。
1. 相关术语:
access unit: A set of NAL units always containing exactly one primary coded picture. In addition to the
primary coded picture, an access unit may also contain one or more redundant coded pictures or other NAL
units not containing slices or slice data partitions of a coded picture. The decoding of an access unit always
results in a decoded picture.
primary coded picture, an access unit may also contain one or more redundant coded pictures or other NAL
units not containing slices or slice data partitions of a coded picture. The decoding of an access unit always
results in a decoded picture.
Coded picture: A coded representation of a picture. A coded picture may be either a coded field or a coded
frame. Coded picture is a collective term referring to a primary coded picture or a redundant coded picture, but
not to both together.
frame. Coded picture is a collective term referring to a primary coded picture or a redundant coded picture, but
not to both together.
primary coded picture: The coded representation of a picture to be used by the decoding process for a
bitstream conforming to this Recommendation | International Standard. The primary coded picture contains all
macroblocks of the picture. The only pictures that have a normative effect on the decoding process are primary
coded pictures. See also redundant coded picture.
bitstream conforming to this Recommendation | International Standard. The primary coded picture contains all
macroblocks of the picture. The only pictures that have a normative effect on the decoding process are primary
coded pictures. See also redundant coded picture.
2. AUD 语法定义:
Access unit delimiter RBSP syntax
access_unit_delimiter_rbsp( ) { C Descriptor
primary_pic_type 6 u(3)
rbsp_trailing_bits( ) 6
}
primary_pic_type 6 u(3)
rbsp_trailing_bits( ) 6
}
3. AUD 语义描述:
The access unit delimiter may be used to indicate the type of slices present in a primary coded picture and to simplify the
detection of the boundary between access units. There is no normative decoding process associated with the access unit
delimiter.
primary_pic_type indicates that the slice_type values for all slices of the primary coded picture are members of the set
listed in Table 7-2 for the given value of primary_pic_type.
Table 7-2 – Meaning of primary_pic_type
primary_pic_type slice_type values that may be present in the primary coded picture
0 I
1 I, P
2 I, P, B
3 SI
4 SI, SP
5 I, SI
6 I, SI, P, SP
7 I, SI, P, SP
detection of the boundary between access units. There is no normative decoding process associated with the access unit
delimiter.
primary_pic_type indicates that the slice_type values for all slices of the primary coded picture are members of the set
listed in Table 7-2 for the given value of primary_pic_type.
Table 7-2 – Meaning of primary_pic_type
primary_pic_type slice_type values that may be present in the primary coded picture
0 I
1 I, P
2 I, P, B
3 SI
4 SI, SP
5 I, SI
6 I, SI, P, SP
7 I, SI, P, SP
4. AUD Nal unit type相关信息
nal_unit_type 9 Access unit delimiter access_unit_delimiter_rbsp( )
nal_ref_idc shall be equal to 0 for all NAL units having nal_unit_type equal to 9.
5. NAL 单元的顺序，这里我只保留AUD相关说明
The first of any of the following NAL units after the last VCL NAL unit of a primary coded picture specifies the start of
a new access unit.
- access unit delimiter NAL unit (when present)
a new access unit.
- access unit delimiter NAL unit (when present)
The following constraints shall be obeyed by the order of the coded pictures and non-VCL NAL units within an access
unit.
- When an access unit delimiter NAL unit is present, it shall be the first NAL unit. There shall be at most one access
unit delimiter NAL unit in any access unit.
unit.
- When an access unit delimiter NAL unit is present, it shall be the first NAL unit. There shall be at most one access
unit delimiter NAL unit in any access unit.
6. 图片描述：
