Image coding method using lookup table for intra prediction mode and apparatus therefor
A method for decoding an image by a decoding apparatus according to the present disclosure comprises the steps of: acquiring intra prediction mode information of a current block; rearranging the remaining intra prediction modes on the basis of a lookup table; deriving a subgroup of the current block on the basis of subgroup information; decoding remaining intra prediction mode information on the basis of a binarization method for the subgroup; deriving, as an intra prediction mode of the current block, an intra prediction mode indicated by the decoded remaining intra prediction mode information among the intra prediction modes included in the subgroup; and generating a prediction sample of the current block on the basis of the intra prediction mode.
1. An image decoding method performed by a decoding apparatus, the method comprising:
obtaining intra prediction mode information on a current block, wherein the intra prediction mode information includes subgroup information and remaining intra prediction mode information;
rearranging remaining intra prediction modes based on a lookup table;
deriving a plurality of subgroups for the rearranged remaining intra prediction modes;
deriving a subgroup for the current block based on the subgroup information;
decoding the remaining intra prediction mode information based on a binarization method for the subgroup;
deriving an intra prediction mode indicated by the decoded remaining intra prediction mode information among intra prediction modes included in the subgroup as an intra prediction mode for the current block; and
generating a prediction sample for the current block based on the intra prediction mode,
wherein rearranging of the remaining intra prediction modes based on the lookup table includes:
obtaining an index indicating one of a plurality of lookup tables, and
deriving the lookup table based on the index.
2. The image decoding method of claim 1 , wherein the rearranging of the remaining intra prediction modes based on the lookup table includes:
constructing a most probable mode (MPM) list including MPM candidates for the current block; and
rearranging the remaining intra prediction modes excluding the MPM candidates from among intra prediction modes in priority order indicated in the lookup table.
3. The image decoding method of claim 1 , wherein the plurality of lookup tables are predetermined.
4. The image decoding method of claim 1 , wherein:
the subgroups include subgroup 1 and subgroup 2;
the subgroup 1 includes K1 intra prediction modes preceding in a rearranged order; and
the subgroup 2 includes K2 intra prediction modes following the intra prediction modes included in the subgroup 1 in a rearranged order.
5. The image decoding method of claim 1 , wherein:
a binarization method for the subgroup 1 is a truncated binary (TB) code; and
a binarization method for the subgroup 2 is a fixed length (FL) code.
6. An image encoding method performed by an encoding apparatus, the method comprising:
determining an intra prediction mode for a current block;
generating a prediction sample for the current block based on the intra prediction mode;
rearranging remaining intra prediction modes based on a lookup table;
deriving a plurality of subgroups for the rearranged remaining intra prediction modes; and
encoding remaining intra prediction mode information based on a binarization method for a subgroup including the intra prediction mode for the current block,
wherein rearranging of the remaining intra prediction modes based on the lookup table includes:
deriving one of a plurality of lookup tables as the lookup table, and
generating an index indicating the derived lookup table among the plurality of lookup tables.
7. The image encoding method of claim 6 , wherein the rearranging of the remaining intra prediction modes based on the lookup table includes:
constructing a most probable mode (MPM) list including MPM candidates for the current block; and
rearranging the remaining intra prediction modes excluding the MPM candidates from among intra prediction modes in priority order indicated in the lookup table.
8. The image encoding method of claim 6 , further comprising encoding subgroup information indicating the subgroup for the current block.
9. The image encoding method of claim 6 , wherein the plurality of lookup tables are predetermined.
10. The image encoding method of claim 6 , wherein:
the subgroups include subgroup 1 and subgroup 2;
a binarization method for the subgroup 1 is a truncated binary (TB) code; and
a binarization method for the subgroup 2 is a fixed length (FL) code.
11. A non-transitory computer-readable storage medium storing a bitstream generated by a method, the method comprising:
determining an intra prediction mode for a current block;
generating a prediction sample for the current block based on the intra prediction mode;
rearranging remaining intra prediction modes based on a lookup table;
deriving a plurality of subgroups for the rearranged remaining intra prediction modes; and
encoding remaining intra prediction mode information based on a binarization method for a subgroup including the intra prediction mode for the current block; and
generating the bitstream including the remaining intra prediction mode information,
wherein rearranging of the remaining intra prediction modes based on the lookup table includes:
deriving one of a plurality of lookup tables as the lookup table, and
generating an index indicating the derived lookup table among the plurality of lookup tables.