Decoder and encoder and methods for coding of a video sequence

A method for decoding a bitstream to obtain a video sequence, wherein the method comprises, when decoding a current picture from the bitstream:

• performing, by at least one processor of a compliant decoder of a specified video coding standard:
• obtaining, from the bitstream, least significant bits of a picture order count for a long term reference picture included in a reference picture set of the current picture;
• obtaining a flag for said long term reference picture, wherein the flag indicates whether or not to use a picture order count cycle;
• and
• based on the flag indicating that the picture order count cycle is not used and the least significant bits match more than one reference picture in a decoded picture buffer of the decoder, determining that the bitstream is a non-compliant bitstream of the specified video coding standard.

An encoder configured to encode a video sequence into a bitstream, wherein the encoder comprises:

• at least one processor;
• and
• a memory coupled to the at least one processor and storing computer readable program code that is executable by the at least one processor to perform, when encoding a current picture of the video sequence:
• calculate an indication to a previous reference picture, wherein the previous reference picture is located before the current picture in decoding order, wherein the previous reference picture is, relatively to the current picture, a closest previous reference picture, in decoding order, and is not a RASL (random access skipped leading) picture, a RADL (random access decodable leading) picture, or a SLNR (sub-layer non-reference picture);
• create a first set of indicators to previous pictures, wherein the first set comprises:
• the indication to the previous reference picture,
• a second set of indicators to all reference pictures included in a first reference picture set, “RPS”, of the previous reference picture, and
• a third set of indicators to all pictures that follow the previous reference picture in decoding order and precede the current picture in decoding order;
• and
• based on a long term reference picture in a second RPS of the current picture having least significant bits of a picture order count, for which more than one picture indicated in the first set share the same value of the least significant bits of picture order count as the long term reference picture, setting a flag for the long term reference picture, wherein the flag indicates use of a picture order count cycle and least significant bits of the picture order count to indicate the long term reference picture in the second RPS.

A method for decoding a specified video coding standard compliant bitstream to obtain a video sequence, the method comprising:

• while decoding a current picture from the bitstream:
• obtaining, from the bitstream, least significant bits of a picture order count for a long term reference picture included in a reference picture set of the current picture;
• obtaining a flag for said long term reference picture, wherein the flag indicates whether or not to use a picture order count cycle;
• and
• based on the flag indicating that the picture order count cycle is not used and the least significant bits match more than one reference picture in a decoded picture buffer of the decoder, determining that the bitstream is a non-compliant bitstream of the specified video coding standard.