Graphics Reference
In-Depth Information
a
b
P
0
P
3
P
2
P
1
P
4
B
0
B
1
B
2
B
3
B
4
Fig. 2.12
Two referencing structures that need a DPB size of at least three pictures
A picture is
decoded
Non-
reference
Short-term
Long-term
Fig. 2.13
Picture marking
but the maximum size may be further limited depending on the combination of the
picture size and the “level” of decoding capability that is used. Note that HEVC
specifies the current picture to be included in the DPB, so a DPB size of one would
not allow for any reference pictures. If the DPB size is one, all pictures must be intra
coded.
The pictures in the DPB are marked to indicate their reference statuses. Each
picture in the DPB is marked as “unused for reference”, “used for short-term
reference”, or “used for long-term reference”. It is common to refer to these types
of pictures as non-reference, short-term, and long-term pictures, respectively.
A reference picture is either a short-term or a long-term picture. The difference
is that a long-term picture can be kept in the DPB much longer than a short-term
picture. There is a rule determining how long a short-term picture can stay in the
DPB. It says that the POC span of the set of pictures consisting of (1) the current
picture, (2) prevTid0Pic, (3) the short-term reference pictures in the DPB, and (4) the
pictures in the DPB that are waiting for output, must be within half of the POC span
covered by POC LSB. This rule guarantees the correctness of POC MSB derivation
and improves error robustness by enabling the decoder to identify lost short-term
pictures.
A non-reference picture is a picture that is not used for reference but may still be
kept in the DPB if it needs to be output later.
Picture markings change for each decoded picture as shown in Fig.
2.13
.After
a picture has been decoded, it is initially always marked as a short-term picture. A
short-term picture may stay as short-term or change to a non-reference or long-term
picture. Long-term pictures may stay as long-term or change into non-reference
pictures, but they can never be made into short-term pictures again. A non-reference
picture can never be made into a reference picture again.
A picture in the DPB may be held for future output regardless of whether it is a
reference or non-reference picture. When a picture has been decoded, it is generally
waiting for output, unless pic_output_flag in the slice header is equal to 0 or the
