Graphics Reference
In-Depth Information
2.2.2.6
Instantaneous Decoding Refresh (IDR) Pictures
The IDR picture is an intra picture that completely refreshes the decoding process
and starts a new CVS (see Sect.
2.2.3
). This means that neither the IDR picture nor
any picture that follows the IDR picture in decoding order can have any dependency
on any picture that precedes the IDR picture in decoding order. There are two sub-
types of IDR pictures, type IDR_W_RADL that may have associated random access
decodable leading (RADL) pictures and type IDR_N_LP that does not have any
leading pictures. Note that it is allowed, but not recommended, for an encoder to
use type IDR_W_RADL even though the IDR picture does not have any leading
pictures. It is however forbidden to use type IDR_N_LP for an IDR that has leading
pictures. The reason for having two different IDR picture types is to enable system
layers to know at random access whether the IDR picture is the first picture to output
or not. The POC value of an IDR picture is always equal to zero. Thus, the leading
pictures associated with an IDR picture, if any, all have negative POC values.
2.2.2.7
Clean Random Access (CRA) Pictures
A CRA picture is an intra picture that, in contrast to an IDR picture, does not refresh
the decoder and does not begin a new CVS. This enables leading pictures of the
CRA picture to depend upon pictures that precede the CRA picture in decoding
order. Allowing such leading pictures typically makes sequences containing CRA
pictures more compression efficient than sequences containing IDR pictures (e.g.,
about 6 %, as reported in [
2
]).
Random access at a CRA picture is done by decoding the CRA picture, its
leading pictures that are not dependent on any picture preceding the CRA picture
in decoding order (see Sect.
2.2.2.8
below), and all pictures that follow the CRA in
both decoding and output order. Note that a CRA picture does not necessarily have
associated leading pictures.
2.2.2.8
Random Access Decodable Leading (RADL) and Random
Access Skipped Leading (RASL) Pictures
The leading pictures must be signaled using either the RADL or RASL NAL unit
type. RADL and RASL pictures can belong to any temporal sub-layer, but they are
not allowed to be referenced by any trailing picture. A RADL picture is a leading
picture that is guaranteed to be decodable when random access is performed at the
associated IRAP picture. Therefore, RADL pictures are only allowed to reference
the associated IRAP picture and other RADL pictures of the same IRAP picture.
A RASL picture is a leading picture that may not be decodable when random
access is performed from the associated IRAP picture. Figure
2.6
shows two RASL
pictures which are both non-decodable since picture P
2
precedes the CRA picture in
decoding order. Because of its position in decoding order, a decoder that performs
