Method, apparatus and system for encoding and decoding the significance map for residual coefficients of a transform unit

A method of decoding video data from a bitstream of video data, the method being characterized in comprising:

• receiving, from the bitstream, encoded data of residual coefficients for at most four size 4×4 sub-blocks within a size 8×8 square block, encoded data of at most two significant coefficient group flags for indicating whether at least one of the residual coefficients within a corresponding one of the sub-blocks within the square block is significant, and encoded data of a position of a last significant coefficient in the square block;
• determining, in accordance with a backward diagonal scan order, the at most two significant coefficient group flags respectively associated with at most two of the sub-blocks within the size 8×8 square block, the determining of the at most two significant coefficient group flags being based on the encoded data of the at most two significant coefficient group flags from the bitstream, and inferring a respective significant coefficient group flag for each of sub-blocks different from the at most two of the sub-blocks within the size 8×8 square block, wherein a significant coefficient group flag for a sub-block of the four sub-blocks that is located in an upper-left corner of the square block is inferred to a first value, and a significant coefficient group flag for a sub-block of the four sub-blocks that contains the last significant coefficient is inferred to the first value;
• determining, in accordance with a backward diagonal scan order, respectively for at most four sub-blocks in the size 8×8 square block, residual coefficients in a case where the determined or the inferred significant coefficient group flag indicates at least one of the residual coefficients is significant based on the encoded data of residual coefficients respectively for the at most four sub-blocks;
• determining that all residual coefficients for at most two of the sub-blocks are zero in a case where the at most two determined significant coefficient group flags do not indicate at least one of the residual coefficients in the at most two sub-blocks is significant;
• and
• generating the video data based on the determined residual coefficients for the sub-blocks in the square block.