Methods of reference picture indication for video coding and related apparatuses

A decoder for decoding a current picture from a bitstream, the decoder comprising:

• a processor;
• and
• a memory coupled to the processor, wherein the memory stores a decoded picture buffer, DPB, and instructions that when executed by the processor causes the processor to perform operations comprising:
• decoding one or more code words in a loop that is run N times by:
• decoding an indication of whether a picture is a long-term picture or not;
• and
• deriving a positive integer K[i]>0 for each long-term picture, where i is the index in the loop, wherein the positive integer K[i] is derived by two code words A[i] and B[i], where A[i] and B[i] are both integers and K[i] is equal to A[i]*x+B[i]+k, where B[i] is signaled using a fixed length code word of size n, A[i] is signaled using a variable length code, x is equal to 2{circumflex over ( )}n where {circumflex over ( )} is the power operator and k is a constant value equal to 0 or 1;
• deriving an indicator value V[i] for each long-term picture by subtracting the value of the positive integer K[i] from a picture indicator value P of the current picture;
• and
• for each of the indicator values V[i], comparing the indicator value V[i] with values associated with pictures stored in the DPB,
• keeping the pictures stored in the DPB that are marked as short-term or long-term for which there is a match with one of the indicator values V[i] in the DPB as long-term pictures;
• and
• marking long-term pictures for which there is no match with any indicator value V[i] as unused for reference in the DPB,
• wherein N is the number of long-term and short-term pictures to keep in the DPB for the current picture.