High Efficiency Video Coding (HEVC) is a new video coding standard currently being developed in Joint Collaborative Team-Video Coding (JCT-VC). JCT-VC is a collaborative project between Moving Picture Experts Group (MPEG) and International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). Currently, an HEVC Model (HM) is defined that includes a number of new tools and is considerably more efficient than H.264/Advanced Video Coding (AVC).
A picture in HEVC is partitioned into one or more slices, where each slice is an independently decodable segment of the picture. This means that if a slice is missing, for instance got lost during transmission, the other slices of that picture can still be decoded correctly. In order to make slices independent, they do not depend on each other. No bitstream element of another slice of the same picture is required for decoding any element of a particular slice.
Each slice contains a slice header which independently provides all required data for the slice to be independently decodable. One example of a data element present in the slice header is the slice address, which is used for the decoder to know the spatial location of the slice. Another example is the Buffer Description also referred to as Reference Picture Set which contains information of which reference pictures to be used when decoding a picture. However, these are only examples of data elements in the slice header.
HEVC has mechanisms for handling reference pictures. The reference pictures are previously decoded pictures to be used for decoding of a current picture. A decoded picture buffer (DPB) contains pictures decoded by the decoder. A reference picture in HEVC is a picture in the decoded picture buffer (DPB) that is available for reference by being marked “used for reference”. There may also be pictures in the DPB that are marked “unused for reference”, these pictures are not available for reference and are not reference pictures.
HEVC uses absolute signaling of reference pictures. The absolute signaling is realized by signaling what reference pictures to keep at the decoder. The signalling is done in a Buffer Description also referred to as Reference Picture Set (RPS), for each picture explicitly or by using a reference to a Sequence Parameter Set (SPS). The RPS also contains an indication of which pictures can be used for reference by the current picture. Reference pictures indicated to be used by the current picture are included in reference picture lists in the decoder. The reference picture lists are then used in the decoding process of the current slice of the current picture.
Each reference picture in the RPS is either identified as a short-term picture or as a long-term picture. The information if it is a short-term or a long-term reference picture is signaled in the RPS by sending two separate sets, one with all short-term reference pictures and one with all long-term reference pictures. An alternative design would be to send a single list and for each element indicate with a flag if it is a long-term reference picture or a short-term reference picture.
Picture Order Count (POC) is used in HEVC to define the output order (or display order) of pictures and also to identify reference pictures. Syntax elements used to derive the POC is signaled for each reference picture in the RPS. For short-term reference pictures the values of POC in the RPS must be identical to the values of POC signaled in the slice header of the reference picture to which the values of the POC in the RPS are referring. For long-term reference pictures there are two options; either the values of POC in the RPS are identical to the values of POC signaled in the slice header of the reference picture to which the values of the POC in the RPS are referring or the values of POC in the RPS are a shorter representation of to the values of POC signaled in the slice header (a.k.a. the least significant bits of the picture order count value or the POC_LSB) of the reference picture to which the values of the POC in the RPS are referring such that the reference pictures are uniquely identifiable. The latter is only allowed when there is only one picture in the DPB with a specific POC_LSB. For long-term reference pictures the term “corresponds to” is used to denote the identification of a reference picture using any of the two above mentioned options.
When buffer descriptions were originally proposed for inclusion in HEVC, the proposal included a marking process performed so that reference pictures that are in the DPB but not included in the RPS are marked as “unused for reference” prior to decoding of the current picture. The output process is also performed prior to the decoding of the current picture.