Method for coding a digital image, decoding method, devices, terminal equipment and related computer programs

A method for decoding a digital image from a bitstream, said image being divided into a plurality of blocks processed in a defined order, the bitstream comprising, for a block, coded data representative of a vector of transformed coefficients, said method comprising the following acts, implemented by a decoding device for the block, called a current block:

• decoding the amplitudes of the coefficients of the vector of transformed coefficients, extracted from the bitstream;
• selecting a sequence of M signs of coefficients to be predicted in the coefficient vector, with M being an integer greater than or equal to 2;
• predicting values of the signs of the sequence from a plurality of assumptions of combinations and a cost function;
• decoding values of a prediction indicator of the signs selected from coded data extracted from the bitstream, the prediction indicator of a sign being indicative of a difference between its actual value and its predicted value;
• calculating the decoded values of the selected signs from the decoded prediction indicator values and the predicted values;
• reconstructing coefficients of the block from decoded amplitudes and decoded signs;
• wherein the predicting comprises the following sub-acts for at least one second assumption, so-called current assumption:
• obtaining a pixel vector associated with the current assumption, by linear combination of a pixel vector associated with at least a first assumption previously processed and different from the current assumption by the value of at least one sign (sj), with j an integer between 0 and M, and obtained by transforming a coefficient vector comprising the combination of signs of the first assumption and at least one pixel pattern vector obtained by transforming a coefficient vector associated with said at least one sign (sj) of the sequence, of the same dimension as the vector of transformed coefficients, and comprising an amplitude coefficient equal to that of said vector at the position of the coefficient corresponding to the sign (sj) in said vector, all the other coefficients being set to zero;
• calculating the cost function for the current assumption from the pixel vector obtained.

A device for coding a digital image, said image being divided into a plurality of blocks of pixels processed in a defined order, said device comprising:

• a processor;
• and
• a non-transitory computer-readable medium comprising instructions stored thereon, which when executed by the processor configure the device to perform acts comprising:
• processing the current block for providing a vector of transformed coefficients;
• selecting a sequence of N signs of coefficients to be predicted in the coefficient vector, with N being an integer greater than or equal to 2;
• predicting values of the signs of the sequence from a plurality of assumptions of combinations and a cost function;
• for a sign of the sequence, coding an indicator representative of a difference between its actual value and its predicted value;
• wherein the predicting comprises for at least one second assumption called a current assumption:
• obtaining a pixel vector associated with the current assumption, by linear combination of a pixel vector associated with at least a first assumption previously processed and different from the current assumption by the value of at least one sign (sj), with j an integer between 0 and M, and obtained by transforming a coefficient vector comprising the combination of signs of the first assumption and at least one pixel pattern vector obtained by transforming a coefficient vector associated with said at least one sign (sj) of the sequence, of the same dimension as the vector of transformed coefficients, and comprising an amplitude coefficient equal to that of said vector at the position of the coefficient corresponding to the sign (sj) in said vector, all the other coefficients being set to zero;
• calculating the cost function for the current assumption from the pixel vector obtained.

A device for decoding a digital image from a bitstream, said image being divided into a plurality of blocks processed in a defined order, the bitstream comprising, for a block, coded data representative of a vector of transformed coefficients, said device comprising:

• a processor;
• and
• a non-transitory computer-readable medium comprising instructions stored thereon, which when executed by the processor configure the device to perform acts comprising:
• decoding the amplitudes of the coefficients of the vector of transformed coefficients, extracted from the bitstream;
• selecting a sequence of N signs of coefficients to be predicted in the coefficient vector, with N being an integer greater than or equal to 2;
• predicting values of the signs of the sequence from a plurality of assumptions of combinations and a cost function;
• decoding values of a prediction indicator of the signs selected from coded data extracted from the bitstream, the prediction indicator of a sign being indicative of a difference between its actual value and its predicted value;
• calculating the decoded values of the selected signs from the decoded prediction indicator values and the predicted values;
• wherein the predicting comprises, for at least one second assumption called a current assumption:
• obtaining a pixel vector associated with the current assumption, by linear combination of a pixel vector associated with at least a first assumption previously processed and different from the current assumption by the value of at least one sign (sj), with j an integer between 0 and M, and obtained by transforming a coefficient vector comprising the combination of signs of the first assumption and at least one pixel pattern vector obtained by transforming a coefficient vector associated with said at least one sign (sj) of the sequence, of the same dimension as the vector of transformed coefficients, and comprising an amplitude coefficient equal to that of said vector at the position of the coefficient corresponding to the sign sj in said vector, all the other coefficients being set to zero;
• and
• calculating the cost function for the current assumption from the pixel vector obtained.

A non-transitory computer-readable medium comprising instruction stored thereon, which when executed by a processor of a coding device, configure the coding device to perform a method of coding a digital image, said image being divided into a plurality of blocks of pixels processed in a defined order, said method comprising the following acts, implemented for a current block:

• processing the current block to provide a vector of transformed coefficients;
• selecting a sequence of M signs of coefficients to be predicted in the coefficient vector, with M being an integer greater than or equal to 2;
• predicting values of the signs of the sequence from a plurality of assumptions of combinations and a cost function;
• for a sign of the sequence, coding an indicator representative of a difference between its actual value and its predicted value;
• wherein the predicting comprises the following sub-acts for at least one second assumption, called a current assumption:
• obtaining a pixel vector associated with the current assumption, by linear combination of a pixel vector associated with at least a first assumption previously processed and different from the current assumption by the value of at least one sign (sj), with j an integer between 0 and M, and obtained by transforming a coefficient vector comprising the combination of signs of the first assumption, and at least one pixel pattern vector obtained by transforming a coefficient vector associated with said at least one sign (sj) of the sequence, of the same dimension as the vector of transformed coefficients, and comprising an amplitude coefficient equal to that of said vector at the position of the coefficient corresponding to the sign (sj) in said vector, all the other coefficients being set to zero;
• calculating the cost function for the current assumption from the pixel vector obtained.

A non-transitory computer-readable medium comprising instruction stored thereon, which when executed by a processor of a decoding device, configure the decoding device to perform a method of decoding a digital image from a bitstream, said image being divided into a plurality of blocks processed in a defined order, the bitstream comprising, for a block, coded data representative of a vector of transformed coefficients, said method comprising the following acts, implemented for the block, called a current block:

• decoding the amplitudes of the coefficients of the vector of transformed coefficients, extracted from the bitstream;
• selecting a sequence of M signs of coefficients to be predicted in the coefficient vector, with M being an integer greater than or equal to 2;
• predicting values of the signs of the sequence from a plurality of assumptions of combinations and a cost function;
• decoding values of a prediction indicator of the signs selected from coded data extracted from the bitstream, the prediction indicator of a sign being indicative of a difference between its actual value and its predicted value;
• calculating the decoded values of the selected signs from the decoded prediction indicator values and the predicted values;
• reconstructing coefficients of the block from decoded amplitudes and decoded signs;
• wherein the predicting comprises the following sub-acts for at least one second assumption, so-called current assumption:
• obtaining a pixel vector associated with the current assumption, by linear combination of a pixel vector associated with at least a first assumption previously processed and different from the current assumption by the value of at least one sign (sj), with j an integer between 0 and M, and obtained by transforming a coefficient vector comprising the combination of signs of the first assumption and at least one pixel pattern vector obtained by transforming a coefficient vector associated with said at least one sign (sj) of the sequence, of the same dimension as the vector of transformed coefficients, and comprising an amplitude coefficient equal to that of said vector at the position of the coefficient corresponding to the sign (sj) in said vector, all the other coefficients being set to zero;
• calculating the cost function for the current assumption from the pixel vector obtained.