Methods for encoding and decoding images, corresponding encoding and decoding devices and computer programs

A method for decoding a stream representing at least one encoded image, comprising the following steps:

• obtaining at least one sub-sequence comprising a predetermined grouping of symbols;
• and
• for at least one sub-sequence thus obtained and for at least one context, wherein at least one step is performed by a processor device:
• initializing at least one probability of appearance of a symbol associated with said context, called a current probability of appearance, taking account of at least two probabilities of appearance of said symbol, called known probabilities of appearance, pertaining to two previously decoded sub-sequences of a current decoded neighboring group, a decoded neighboring group comprising at least two previously decoded sub-sequences, amongst which at least one belongs to a decoded neighboring group associated with a sub-sequence distinct from said current sub-sequence;
• successive entropy decoding of said symbols according to an encoding scan order implemented during an encoding of said image and, for each symbol, updating of said current probability of appearance for the context associated with said symbol;
• storing said current probability of appearance of said decoded symbols in the current sub-sequence in a computer-readable memory, after decoding the last symbol of said current sub-sequence.

A device for encoding at least one image represented by a sequence of symbols generated according to a predetermined encoding scan order, wherein the device comprises:

• means for determining sub-sequences in each sequence, a sub-sequence comprising a predetermined grouping of symbols;
• and
• means for scanning determined sub-sequences comprising, for at least one current sub-sequence and for at least one context:
• means for determining a neighboring group comprising a least two previously encoded sub-sequences, called a current encoded neighboring group, amongst which at least one belongs to an encoded neighboring group associated with a sub-sequence, distinct from said current sub-sequence, said distinct sub-sequence not belonging to said encoded neighboring group of the current sub-sequence, and said current sub-sequence not belonging to the encoded neighboring group of said distinct sub-sequence;
• means for initializing at least one probability of appearance of a symbol associated with said context, called a current probability of appearance, taking account of at least two probabilities of appearance of said symbol, called known probabilities of appearance, relative to two previously encoded sub-sequences of said current encoded neighboring group;
• means for successive entropy encoding of said symbols according to said predetermined encoding scan order, and for each symbol, means for updating said current probability of appearance for the context associated with said symbol;
• and
• means for storing said current probability of appearance of said encoded symbols in the current sub-sequence, after encoding of the last symbol of said current sub-sequence.

A device for decoding a stream representing at least one encoded image, wherein the device comprises:

• means for obtaining at least one sub-sequence comprising a predetermined grouping of symbols;
• and
• means for performing, for at least one sub-sequence thus obtained and for at least one context:
• initializing at least one probability of appearance of a symbol associated with said context, called a current probability of appearance, taking account of at least two probabilities of appearance of said symbol, called known probabilities of appearance, pertaining to two previously decoded sub-sequences of a current decoded neighboring group, a decoded neighboring group comprising at least two previously decoded sub-sequences, amongst which at least one belongs to a decoded neighboring group associated with a sub-sequence distinct from said current sub-sequence, said distinct sub-sequence not belonging to said encoded neighboring group of the current sub-sequence and said current sub-sequence not belonging to the encoded neighboring group of said distinct sub-sequence;
• successive entropy decoding of said symbols according to an encoding scan order implemented during an encoding of said image and, for each symbol, means for updating of said current probability of appearance for the context associated with said symbol;
• and
• storing said current probability of appearance of said decoded symbols in the current sub-sequence, after decoding of the last symbol of said current sub-sequence.

A non-transitory computer-readable memory comprising instructions stored thereon to implement a method for encoding at least one image represented by a sequence of symbols generated according to a predetermined encoding scan order, when said instructions are executed by a processor, wherein the instructions comprise:

• instructions for determining sub-sequences in each sequence, a sub-sequence comprising a predetermined grouping of symbols;
• and
• instructions for scanning determined sub-sequences comprising the following sub-steps for at least one current sub-sequence and for at least one context:
• determining a neighboring group comprising a least two previously encoded sub-sequences, called a current encoded neighboring group, amongst which at least one belongs to an encoded neighboring group associated with a sub-sequence, distinct from said current sub-sequence, said distinct sub-sequence not belonging to said encoded neighboring group of the current sub-sequence, and said current sub-sequence not belonging to the encoded neighboring group of said distinct sub-sequence;
• initializing at least one probability of appearance of a symbol associated with the context, called a current probability of appearance, taking account of at least two probabilities of appearance of said symbol, called known probabilities of appearance, relative to two previously encoded sub-sequences of said current encoded neighboring group;
• successive entropy encoding of said symbols according to said predetermined encoding scan order and, for each symbol, updating of said current probability of appearance for the context associated with said symbol;
• and
• storing said current probability of appearance of said encoded symbols in the current sub-sequence, after encoding of the last symbol of said current sub-sequence.