Method and apparatus for encoding/decoding an image

An image encoding apparatus comprising:

• a sub-group generator processor that divides a group of pictures (GOP) including a plurality of image pictures included in an image sequence into sub-groups according to consecutive bi-directional pictures;
• a correlation calculator that calculates correlations between first bi-directional pictures of a first sub-group and reference pictures referred to by the first bi-directional pictures for each of a plurality of encoding modes including a bi-directional picture non-reference mode wherein the first bi-directional pictures are not used as reference pictures, and a bi-directional picture reference mode wherein at least one of the first bi-directional pictures is used as a reference picture;
• an encoding mode selector that selects one of the plurality of encoding modes based on the correlations;
• and
• an encoding unit that encodes B pictures of each sub-group according to the selected encoding mode.

An image decoding method comprising:

• using a processor which performs the method comprising the steps of:
• reading encoding mode information included in an input bitstream;
• dividing a plurality of encoded image pictures of the bitstream into sub-groups, and determining encoding modes used to encode first bi-directional pictures of a first sub-group based on the encoding mode information which indicates an encoding mode used to encode the first bi-directional pictures of the first sub-group among a plurality of encoding modes which include at least a bi-directional picture non-reference mode wherein the first bi-directional pictures are not used as reference pictures and a bi-directional picture reference mode wherein at least one of the first bi-directional pictures is used as reference picture;
• determining a decoding sequence of the first bi-directional pictures of the first sub-group and reference pictures according to the encoding modes;
• and
• decoding the first bi-directional pictures of the first sub-group using the decoding sequence and the reference pictures.

An image decoding apparatus comprising:

• an encoding mode processor that reads encoding mode information included in an input bitstream, divides a plurality of encoded image pictures of the input bitstream into sub-groups, and determines encoding modes used to encode bi-directional pictures of the sub-groups based on the encoding mode information which indicates an encoding mode used to encode the first bi-directional pictures of the first sub-group among a plurality of encoding modes which include at least a bi-directional picture non-reference mode wherein the first bi-directional pictures are not used as reference pictures and a bi-directional picture reference mode wherein at least one of the first bi-directional pictures is used as reference picture;
• a decoding sequence determiner that determines decoding sequences of the bi-directional pictures of the sub-groups and reference pictures according to the encoding modes;
• and
• a decoding unit that decodes the bi-directional pictures of the sub-groups using the decoding sequences and the reference pictures.