Image encoding/decoding methods and apparatuses

1. A method of decoding an image, the method comprising:

determining a current block by partitioning a coding block in the image based on a tree-based block division;

determining an intra prediction mode of the current block based on a syntax element obtained from a bitstream; and

performing intra prediction of the current block based on the intra prediction mode,

wherein the syntax element comprises a flag indicating whether a pre-defined default mode is used for the intra prediction of the current block,

wherein, in response to the flag indicating that the default mode is used for the intra prediction of the current block, the intra prediction mode is set to the default mode,

wherein, in response to the flag indicating that the default mode is not used for the intra prediction of the current block, the intra prediction mode is derived from a candidate group including a plurality of most probable mode (MPM) candidates,

wherein the default mode is representative of a Planar mode, and

wherein, in response to a case where the current block is partitioned into a plurality of sub-blocks, the intra prediction is sequentially performed in units of the sub-blocks and the plurality of sub-blocks belonging to the current block uses the same intra prediction mode for the intra prediction.

2. The method of claim 1 , wherein the default mode is not included in the candidate group.

3. The method of claim 2 , wherein the plurality of MPM candidates comprises at least one of an intra prediction mode of a surrounding block (candIntraPredMode), candIntraPredMode−N, or candIntraPredMode+N, and

wherein N is equal to 2.

4. The method of claim 2 , wherein the plurality of MPM candidates comprises at least one of a DC mode, a vertical mode, a horizontal mode, (a vertical mode−M) or (a vertical mode+M), and

wherein M is equal to 4.

5. The method of claim 1 , wherein, in response to the intra prediction mode being a directional mode, the intra prediction mode is modified by adding or subtracting a predetermined offset,

wherein the intra prediction is performed using the modified intra prediction mode, and

wherein the modification of the intra prediction mode is selectively performed based on at least one of a size, a shape, partition information, a value of the intra prediction mode, or a component type of the current block.

6. The method of claim 1 , wherein performing the intra prediction comprises:

partitioning the current block into the plurality of sub-blocks by using a vertical line or a horizontal line,

wherein a number of the sub-blocks belonging to the current block is variably determined based on at least one of a size or a shape of the current block.

7. The method of claim 1 , wherein performing the intra prediction comprising:

modifying a prediction value, generated by performing the intra prediction, based on a predetermined reference sample and a weight.

8. The method of claim 7 , wherein each of the reference sample and the weight is determined based on at least one of a size of the current block, a position of a current sample to be modified, or the intra prediction mode of the current block.

9. The method of claim 1 , wherein, in response to a case where the default mode is used for the intra prediction of the current block, an intra reference line for the current block for the intra prediction is restricted to be determined as only a reference line adjacent to the current block among a plurality of reference lines.

10. The method of claim 1 , wherein the tree-based block division includes at least one of a binary-tree division or a ternary-tree division, and

wherein the binary-tree division is a division type of horizontally or vertically dividing one coding block into two coding blocks and the ternary-tree division is a division type of horizontally or vertically dividing one coding block into three coding blocks.

11. A method of encoding an image, the method comprising:

determining a current block by partitioning a coding block in the image based on a tree-based block division;

determining an intra prediction mode of the current block; and

performing intra prediction of the current block,

wherein a prediction block of the current block is generated by performing the intra prediction based on the intra prediction mode,

wherein a flag indicating whether a pre-defined default mode is used for the intra prediction of the current block is encoded into a bitstream,

wherein, in response to a case where the default mode is used for the intra prediction of the current block, the intra prediction mode is determined to be the default mode,

wherein, in response to a case where the default mode is not used for the intra prediction of the current block, the intra prediction mode is determined from a candidate group including a plurality of most probable mode (MPM) candidates,

wherein the default mode is representative of a Planar mode, and

wherein, in response to a case where the current block is partitioned into a plurality of sub-blocks, the intra prediction is sequentially performed in units of the sub-blocks and the plurality of sub-blocks belonging to the current block uses the same intra prediction mode for the intra prediction.

12. A non-transitory computer-readable storage medium for storing a bitstream generated by an image encoding method,

the image encoding method comprising:

determining a current block by partitioning a coding block based on a tree-based block division;

determining an intra prediction mode of the current block; and

performing intra prediction of the current block,

wherein a prediction block of the current block is generated by performing the intra prediction based on the intra prediction mode,

wherein a flag indicating whether a pre-defined default mode is used for the intra prediction of the current block is encoded into the bitstream,

wherein, in response to a case where the default mode is used for the intra prediction of the current block, the intra prediction mode is determined to be the default mode,

wherein, in response to a case where the default mode is not used for the intra prediction of the current block, the intra prediction mode is determined from a candidate group including a plurality of most probable mode (MPM) candidates,

wherein the default mode is representative of a Planar mode, and

wherein, in response to a case where the current block is partitioned into a plurality of sub-blocks, the intra prediction is sequentially performed in units of the sub-blocks and the plurality of sub-blocks belonging to the current block uses the same intra prediction mode for the intra prediction.