Graphics Reference
In-Depth Information
I
0
P
2
P
3
P
4
P
5
P
1
P
6
Fig. 2.14
A referencing structure with DPB size equal to 3, NumReorderPics equal to 1, and
MaxLatencyPictures equal to 4
picture is a RASL picture that is associated with the first picture in a CVS. If that is
the case, the picture will not be output.
The picture marking and picture output are done in separate processes, but when a
picture is both a non-reference picture and not waiting for output, the picture storage
buffer in the DPB is emptied and can be used to store future decoded pictures.
The encoder is responsible to manage picture marking and picture output such that
the number of pictures in the DPB does not exceed the DPB size as indicated by
sps_max_dec_pic_buffering_minus1.
Two other codewords in the SPS that are related to picture output are
sps_max_num_reorder_pics and sps_max_latency_increase_plus1, which both can
be sent for each temporal sub-layer. sps_max_num_reorder_pics, here denoted as
NumReorderPics, indicates the maximum number of pictures that can precede
any picture in decoding order and follow it in output order. The value of
NumReorderPics for the referencing structure in Fig.
2.12
a is two since picture
P
3
has two pictures that precede it in decoding order but follow it in output order.
NumReorderPics for Fig.
2.12
b is zero since no picture is sent out-of-order.
sps_max_latency_increase_plus1 is used to signal MaxLatencyPictures, which
indicates the maximum number of pictures that can precede any picture in output
order and follow that picture in decoding order. Figure
2.14
shows the difference
between NumReorderPics and MaxLatencyPictures. NumReorderPics is here equal
to one since P
1
is the only picture that is sent out-of-order. MaxLatencyPictures is
set to four since pictures P
2
,P
3
,P
4
,andP
5
all precedes P
1
in output order. The
minimum DPB size for this referencing structure is three.
One can say that NumReorderPics denotes the minimum number of picture stores
in the DPB that are necessary for taking care of out-of-order pictures, and that
MaxLatencyPictures denotes the minimum amount of coding delay, measured in
pictures, that is caused by out-of-order pictures.
For low-delay applications it is recommended to use referencing structures that
have no coding delay caused by out-of-order output. This is achieved by ensuring
that the decoding order and output order is the same which can be expressed by
signaling either NumReorderPics or MaxLatencyPictures, or both, equal to zero.
