Graphics Reference
In-Depth Information
Tabl e 2. 5
RPS syntax for picture B
7
in Fig.
2.15
Part
Syntax element
Picture
Value
Type
Codeword
Short-term
num_negative_pics
P
1
1
uvlc
'010'
num_positive_pics
P
5
,B
6
2
uvlc
'011'
delta_poc_s0_minus1
P
1
0
uvlc
'1'
used_by_curr_pic_s0_flag
Used
flag
'1'
delta_poc_s1_minus1
B
6
0
uvlc
'1'
used_by_curr_pic_s1_flag
Used
flag
'1'
delta_poc_s1_minus1
P
5
1
uvlc
'010'
used_by_curr_pic_s1_flag
Unused
flag
'0'
Long-term
num_long_term_pics
I
0
1
uvlc
'010'
poc_lsb_lt
0
flc
'00000000'
used_by_curr_pic_lt_flag
Unused
flag
'0'
delta_poc_msb_present_flag
0
flag
'0'
two syntax elements for the RPS convey the number of pictures in S0 and S1; this is
encoded by the uvlc codes '010' and '011'. Then the picture in group S0 is signaled.
The POC delta of P
1
is 1 since its POC value is 4 and the current picture has a
POC value equal to 5. POC deltas are subtracted by 1 before encoding, so the final
signaled value is 0 which results in the uvlc code '1'. Picture P
1
is used for reference
for the current picture B
7
; this is signaled by the flag '1'.
The next syntax elements cover the pictures in S1. The current picture has a POC
value equal to 5 so the codeword '1' is used for B
6
since its POC value is equal to
6. Picture B
6
is also used for reference for the current picture which is indicated by
the flag '1'. The second picture in S1 is P
5
which has a POC value of 8. It is coded
relative to the previous picture in S1 which is B
6
with POC value equal to 6. The
delta is 2 which subtracted by one is equal to 1 and encoded as '010'. Picture P
5
is
not used for reference by the current picture B
7
; this is signaled by the flag '0'.
The next codeword in the RPS signals the number of long-term pictures in the
RPS, which is one and uvlc coded using '010'. Then the POC LSB value of the
long-term picture I
0
is signaled. This is by the codeword '00000000' assuming that
8 bits are used for signaling POC LSB values. The long-term picture I
0
is not used
for reference by the current picture B
7
; this is signaled with the flag '0'. Finally, the
POC MSB information is not signaled for this long-term picture since there are no
other pictures in the DPB sharing the same POC LSB value; this is signaled with
the present flag '0'.
2.4.3.1
RPS Signaling in the Slice Header and SPS
The RPS information is signaled in every slice header for resilience reasons, but
repeating the RPS information as is for every slice could cost many bits. Not only
can pictures be split into multiple slices for which the RPS must be repeated, the
pictures in a bitstream are often coded by repeating the same GOP structure, so the
