Graphics Reference
In-Depth Information
a
b
B
2
B
16
B
2
B
14
B
16
RASL
P
0
P
1
I
13
P
15
P
0
P
1
I
13
P
15
CRA
BLA
Fig. 2.9
Bitstream after splicing when discarding RASL pictures (
a
) and keeping RASL pictures
and converting the CRA picture to BLA (
b
)
to decode these RASL pictures would be to discard them during splicing. The result
for splicing the two streams in Fig.
2.8
by discarding RASL pictures is shown in
Fig.
2.9
a. Note that RADL pictures, if present, could either be kept or discarded.
A disadvantage with this method of discarding RASL pictures is that discarding
data in a stream may impact system layer buffers. The splicer may therefore need
to be capable of modifying low-level system parameters. If the RASL pictures are
forwarded, the system layer buffers are not affected.
Another problem is that the POC values that follow the splicing point would
need to indicate the proper output order relationship relative to the pictures that
precede the splicing point, since a CRA picture does not begin a new CVS. This
could require modification of all POC values that follow the splicing point in the
resulting CVS.
An alternative splicing option that is available in HEVC is a “broken link”
which indicates that the POC timeline, and the prediction from preceding pictures
that RASL pictures may depend on, are broken when splicing is done. Unless the
decoder is informed of the broken link, there could be serious visual artifacts if the
decoder tries to decode the RASL pictures or if the POC values after the splice point
are not appropriately aligned. To avoid visual artifacts, a decoder must be informed
when a splicing operation has occurred in order to know whether the associated
RASL pictures (if present) should be decoded or not. In HEVC, a broken link access
(BLA) picture NAL unit type can be used for such spliced CRA pictures.
During splicing, the CRA picture should be re-typed as a BLA picture. The result
of such an operation for the example in Fig.
2.8
is shown in Fig.
2.9
bwherethe
RASL picture is kept and the CRA picture is re-typed as a BLA picture. A decoder
that encounters BLA and CRA pictures will discard any RASL pictures associated
with BLA pictures but decode the RASL pictures associated with CRA pictures. All
RADL pictures are required to be decoded.
Like an IDR picture, a BLA picture starts a new CVS and resets the POC
relationship calculation. However, the POC value assigned to a BLA picture is not
the value 0—instead, the POC value is set equal to the POC value signaled in the
slice header of the BLA picture—which is a necessary adjustment since the POC
value for a CRA picture would likely be non-zero before its conversion to a BLA
