Method for encoding a digital image and verifying a transform applied to a current residual block, corresponding decoding method, devices, and associated computer programs

A device for encoding a digital image to be decoded by a decoding device, 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 for encoding to perform the following acts, implemented for a current block, with set size:
• predicting the values of the current block from at least one block previously processed according to a mode of prediction selected from a plurality of preset modes;
• calculating a residual block by subtracting the predicted values from the original values of the current block,
• obtaining a residual block transformed by applying a transform to pixels of the residual block, said transformed residual block comprising coefficients, the transform being part of a list of preset transforms;
• encoding the transformed residual block;
• calculating a value of at least one characteristic representative of at least one transformed residual coefficient of the current block;
• determining initial identification data representative of a sub-list of at least one transform associated with said calculated value of the at least one characteristic, wherein knowledge of said sub-list is shared with the decoding device;
• and
• in response to the transform applied to the current block being part of the sub-list identified by the initial data, but not directly obtainable from said sub-list by the decoding device, encoding data complementary to the initial identification data allowing to identify said applied transform in the sub-list.

A method for decoding a digital image from a bit stream comprising encoded data representative of said image, said image being divided into a plurality of blocks of pixels processed in a defined order, said method comprising the following, implemented by a decoding device for a current block:

• predicting the current block from at least one previously processed block;
• decoding encoded values of coefficients of a transformed residual block extracted from the bitstream;
• calculating a value of at least one characteristic representative of at least one coefficient of the transformed residual block;
• decoding data representative of a transform in a known sub-list of at least one transform associated with said calculated value of the at least one characteristic;
• decoding an identifier of the transform to be applied to the transformed residual block at least from the decoded data;
• obtaining a residual block by applying to the transformed residual block the inverse of the transform identified by the identifier;
• reconstructing a decoded block from the residual block and from the prediction of the current block.

A device for decoding a digital image from a bitstream comprising encoded data representative of said image, said image being divided into a plurality of blocks of pixels processed in a defined order, wherein the device comprises:

• a processor;
• and
• a non-transitory computer-readable medium comprising instructions stored thereon, which when executed by the processor configure the device for decoding to perform the following acts implemented for a current block:
• predicting the current block from at least one previously processed block;
• decoding encoded values of coefficients of a transformed residual block extracted from the bitstream;
• calculating a value of at least one characteristic representative of at least one coefficient of the transformed residual block;
• decoding data representative of a transform in a known sub-list of at least one transform associated with said calculated value of the at least one characteristic;
• decoding an identifier of the transform to be applied to the transformed residual block at least from the decoded data;
• obtaining a residual block by applying to the transformed residual block the inverse transform identified by the identifier;
• reconstructing a decoded block from the residual block and from the prediction of the current block.

A non-transitory computer-readable medium comprising a computer program stored thereon comprising instructions for implementing a method for encoding a digital image to be decoded by a decoding device, when executed by a processor of an encoding device, 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, with preset size:

• predicting values of the current block from at least one block previously processed in a mode of prediction selected from a plurality of preset modes,
• calculating a residual block by subtracting the predicted values from the original values of the current block,
• obtaining a residual block transformed by applying a transform to pixels of the residual block, said transformed residual block comprising coefficients, the transform being part of a preset list of transforms;
• encoding the transformed residual block;
• calculating a value of at least one characteristic representative of at least one transformed residual coefficient of the current block;
• determining an initial identification data representative of a sub-list of at least one transform associated with said calculated value of the at least one characteristic, wherein knowledge of said sub-list is shared with the decoding device;
• and
• in response to the transform applied to the current block being part of the sub-list identified by the initial data, but not directly obtainable from said sub-list by the decoding device, encoding data complementary to the initial identification data allowing to identify said applied transform in the sub-list.

A non-transitory computer-readable medium comprising a computer program stored thereon comprising instructions for implementing a method for decoding a digital image from a bit stream, when executed by a processor of a decoding device, said bit stream comprising encoded data representative of said image, said image being divided into a plurality of blocks of pixels processed in a defined order, said method comprising the following, implemented for a current block:

• predicting the current block from at least one previously processed block;
• decoding encoded values of coefficients of a transformed residual block extracted from the bitstream;
• calculating a value of at least one characteristic representative of at least one coefficient of the transformed residual block;
• decoding data representative of a transform in a known sub-list of at least one transform associated with said calculated value of the at least one characteristic;
• decoding an identifier of the transform to be applied to the transformed residual block at least from the decoded data;
• obtaining a residual block by applying to the transformed residual block the inverse transform identified by the identifier;
• and
• reconstructing a decoded block from the residual block and from the prediction of the current block.