Graphics Reference
In-Depth Information
either (1) keep decoding temporal sub-layer 0 only, (2) decide to start decoding
temporal sub-layer 1 as well as sub-layer 0, or (3) start to decode all three sub-layers.
A similar action is possible for a network node that is forwarding only the lowest
temporal sub-layer, for example due to a previous network congestion situation.
The network node can inspect the NAL unit type of incoming pictures that have
a temporal ID equal to 1. This does not require a lot of computational resources
since the NAL unit type and the temporal ID are found in the NAL unit header and
are easy to parse. When a TSA picture of temporal sub-layer 1 is encountered, the
network node can switch to forward any temporal sub-layer pictures succeeding the
TSA picture in decoding order without any risk of the decoder not being able to
properly decode them as a result of not having all necessary reference pictures that
they depend on.
2.2.2.4
Step-wise Temporal Sub-layer Access (STSA) Pictures
The STSA picture type is similar to the TSA picture type, but it only guarantees that
the STSA picture itself and pictures of the same temporal ID as the STSA picture
that follow it in decoding order do not reference pictures of the same temporal ID
that precede the STSA picture in decoding order. The STSA pictures can therefore
be used to mark positions in the bitstream where it is possible to switch to the sub-
layer with the same temporal ID as the STSA picture, while the TSA pictures can
mark positions in the bitstream where it is possible to switch up to any higher sub-
layer. One example of an STSA picture in Fig.
2.5
is picture P
2
. This picture cannot
be a TSA picture since P
3
references P
1
. However, picture P
2
can be an STSA picture
because P
2
does not reference any picture of sub-layer 1, nor does any sub-layer
1 picture that follows P
2
in decoding order reference any sub-layer 1 picture that
precedes P
2
in decoding order. Both TSA and STSA pictures must have a temporal
ID higher than 0.
Note also that since prediction from a higher to a lower temporal sub-layer is
forbidden in HEVC, it is always possible at any picture to down-switch to a lower
temporal sub-layer, regardless of the picture type or temporal sub-layer.
2.2.2.5
Ordinary Trailing (TRAIL) Pictures
Ordinary trailing pictures are denoted with the enumeration type TRAIL. Trailing
pictures may belong to any temporal sub-layer. They may reference the associated
IRAP picture and other trailing pictures associated with the same IRAP picture, but
they cannot reference leading pictures (or any other pictures that are not trailing
pictures associated with the same IRAP picture). They also cannot be output after
the next IRAP picture in decoding order is output. Note that all TSA and STSA
pictures could instead be marked as TRAIL pictures, and that all TSA pictures could
be marked as STSA pictures. It is, however, recommended that trailing pictures
should use the most restrictive type in order to indicate all possible temporal sub-
layer switching points that exist in the bitstream.
