Graphics Reference
In-Depth Information
P
6
P
5
RASL
RADL
P
4
RASL
P
0
I
1
P
2
I
3
P
7
IRAP
CRA
TRAIL
Fig. 2.6
RADL and RASL pictures
random access at the position of the CRA picture will not decode the P
2
picture, and
therefore cannot decode these RASL pictures and will discard them. Even though it
is not forbidden to use the RASL type for decodable leading pictures, such as the
RADL picture in Fig.
2.6
, it is recommended to use the RADL type when possible
to be more network friendly. Only other RASL pictures are allowed to be dependent
on a RASL picture. This means that every picture that depends on a RASL picture
must also be a RASL picture. RADL and RASL pictures may be mixed in decoding
order, but not in output order. RASL pictures must precede RADL pictures in output
order.
All leading pictures of an IDR_W_RADL picture must be decodable and use the
RADL type. RASL pictures are not allowed to be associated with any IDR picture.
A CRA picture may have both associated RADL and RASL pictures, as shown
in Fig.
2.6
. RASL pictures are allowed to reference the IRAP picture preceding
the associated IRAP picture and may also reference other pictures that follow that
IRAP picture in decoding order, but cannot reference earlier pictures in decoding
order—e.g., the RASL pictures in Fig.
2.6
cannot reference the picture P
0
.
There are three constraints in HEVC that aim to eliminate uneven output
of pictures when performing random access. Two of the constraints depend on
the variable PicOutputFlag which is set for each picture and indicates whether
the picture is to be output or not. This variable is set to 0 when a flag called
pic_output_flag is present in the slice header and is equal to 0, or when the current
picture is a RASL picture and the associated IRAP picture is the first picture in the
CVS (see Sect.
2.2.3
). Otherwise PicOutputFlag is set equal to 1.
The first constraint is that any picture that has PicOutputFlag equal to 1 that
precedes an IRAP picture in decoding order must precede the IRAP picture in
output order. The structure in Fig.
2.7
a is forbidden by this constraint, since picture
P
1
precedes the CRA in decoding order but follows it in output order. If this was
allowed and random access was made at the CRA picture, picture P
1
would be
missing, resulting in uneven output.
The second constraint is that any picture that has PicOutputFlag equal to 1
that precedes an IRAP picture in decoding order must precede any RADL picture
associated with the IRAP picture in output order. A referencing structure that is
