Graphics Reference
In-Depth Information
pictures in the RPS as references for prediction. However, it is required that the
RefPicSetStCurr and RefPicSetLtCurr lists are both empty for CRA and BLA
pictures.
2.4.3
Reference Picture Set Signaling
Three pieces of information are signaled for each picture in the RPS; the POC
information, the availability state, and whether the picture is a short-term or long-
term picture.
The POC information for short-term pictures is signaled in two groups. Group S0
is signaled first and it consists of all short-term pictures with lower POC values than
the current picture. This group is followed by group S1 which contains all short-term
pictures with higher POC values than the current picture. The information for each
group is sent in POC distance order relative to the current picture, starting with the
POC value that is closest to the POC value of the current picture. For each picture, a
POC delta relative to the previous picture is signaled. The current picture acts as the
previous picture for the first picture in each group since there is no previous picture
for the first pictures.
Coding long-term pictures by POC deltas may result in very long codewords
since long-term pictures can stay in the DPB for a very long time. Therefore, long-
term pictures are instead signaled by their POC LSB values. The same number of
bits that is used for the slice header POC LSB values is used also for long-term
pictures. The decoder will match each POC LSB value signaled in the RPS with
the POC LSB values of the pictures in the DPB. Since it is possible to have more
than one picture with the same POC LSB value in the DPB, there is an optional
possibility to also signal POC MSB information for long-term pictures. This MSB
information must be sent when there is a risk that a decoder is unable to correctly
identify the pictures. One way of avoiding this risk is to always signal POC MSB
information for long-term pictures when the corresponding POC LSB value has
been used by two or more different long-term pictures.
The availability state for each picture in the RPS is signaled by a one-bit flag
where '1' indicates that the picture is available for reference for the current picture
and '0' indicates that it is not.
Tab le
2.5
shows example RPS syntax for picture B
7
in Fig.
2.15
.Thefirstcolumn
in the table shows whether the syntax is related to signaling short-term pictures or
long-term pictures. The second column contains the HEVC specification name of
each syntax element. The third column indicates the related pictures in the RPS,
and the fourth column shows the value of the syntax element in the example. The
fifth column shows the type of the syntax element where 'uvlc' denotes a universal
variable-length code, 'flag' is a one-bit binary flag, and 'flc' is a fixed-length code.
The last column shows the resulting bits for encoding the value of each syntax
element using the type.
The POC value of picture B
7
in Fig.
2.15
is 5 and its RPS contains three short-
term pictures; picture P
1
in group S0 and pictures B
6
and P
5
in group S1. The first
