HEVC High-Level Syntax - High Efficiency Video Coding (HEVC) - page 38

Tabl e 2. 3 The five RPS lists

List name Long-term or short-term Availability flag POC

RefPicSetStCurrBefore Short-term Available Lower

RefPicSetStCurrAfter Short-term Available Higher

RefPicSetStFoll Short-term Unavailable -

RefPicSetLtCurr Long-term Available -

RefPicSetLtFoll Long-term Unavailable -

2.4.2 Reference Picture Sets

The process of marking pictures as “used for short-term reference”, “used for long-
term reference”, or “unused for reference” is done using the reference picture set
(RPS) concept. An RPS is a set of picture indicators that is signaled in each slice
header and consists of one set of short-term pictures and one set of long-term
pictures. After the first slice header of a picture has been decoded, the pictures in
the DPB are marked as specified by the RPS.
The pictures in the DPB that are indicated in the short-term picture part of the
RPS are kept as short-term pictures. The short-term or long-term pictures in the
DPB that are indicated in the long-term picture part in the RPS are converted to
or kept as long-term pictures. And finally, pictures in the DPB for which there is
no indicator in the RPS are marked as unused for reference. Thus, all pictures that
have been decoded that may be used as references for prediction of any subsequent
pictures in decoding order must be included in the RPS.
An RPS consists of a set of picture order count (POC) values that are used for
identifying the pictures in the DPB. Besides signaling POC information, the RPS
also signals one flag for each picture. Each flag indicates whether the corresponding
picture is available or unavailable for reference for the current picture. Note that
even though a reference picture is signaled as unavailable for the current picture, it
is still kept in the DPB and may be made available for reference later on and used
for decoding future pictures. From the POC information and the availability flag,
five lists of reference pictures as shown in Table 2.3 can be created.
The list RefPicSetStCurrBefore consists of short-term pictures that are available
for reference for the current picture and have POC values that are lower than
the POC value of the current picture. RefPicSetStCurrAfter consist of available
short-term pictures with a POC value that is higher than the POC value of the
current picture. RefPicSetStFoll is a list that contains all short-term pictures that are
made unavailable for the current picture but may be used as reference pictures for
decoding subsequent pictures in decoding order. Finally, the lists RefPicSetLtCurr
and RefPicSetLtFoll contain long-term pictures that are available and unavailable
for reference for the current picture, respectively.
Figure 2.15 and Table 2.4 show an example referencing structure using three
temporal sub-layers and the content of the RPS lists for each picture in decoding
order.