Image encoding/decoding method and device using palette mode, and method for transmitting bitstream

1. An image decoding method performed by an image decoding apparatus, the image decoding method comprising:

obtaining palette information and palette index prediction information of a current block from a bitstream, based on a palette mode applying for the current block;

constructing a palette predictor for the current block based on the palette information and constructing a palette table for the current block based on the palette predictor;

generating a palette index map for the current block based on the palette index prediction information; and

decoding the current block based on the palette table and the palette index map,

wherein whether the palette mode applies for the current block is determined based on a split structure of the current block being a local dual tree structure,

wherein whether the split structure of the current block is the local dual tree structure is determined based on at least one of a tree type of the current block and a split structure of a coding tree unit (CTU) including the current block,

wherein the local dual tree structure is a split structure in which additional splitting is performed only for a luma block within a CTU that has been split into a single tree structure.

2. The image decoding method of claim 1 , wherein, based on the current block being a chroma block and the split structure of the current block being the local dual tree structure, the palette predictor is not updated.

3. The image decoding method of claim 1 , wherein, based on the current block being a chroma block and the split structure of the current block being not the local dual tree structure, the palette predictor is updated based on the palette table.

4. The image decoding method of claim 3 , wherein the updated palette predictor comprises at least one palette entry included in the palette table.

5. The image decoding method of claim 3 , wherein the updated palette predictor comprises at least one new palette entry not included in the palette table within a predetermined maximum size range.

6. The image decoding method of claim 1 , wherein, based on the split structure of the current block being the local dual tree structure and a treeType of the current block being a dual tree chroma, it is determined that the palette mode does not apply for the current block.

7. The image decoding method of claim 1 , wherein whether the split structure of the current block is the local dual tree structure is determined based on a prediction mode type of the current block and a tree type of the current block.

8. The image decoding method of claim 7 , wherein the split structure of the current block is determined to be the local dual tree structure based on the tree type of the current block being dual tree chroma and the prediction mode type of the current block being a mode type for which intra prediction, intra block copy (IBC) and palette mode are all applicable.

9. An image encoding method performed by an image encoding apparatus, the image encoding method comprising:

based on a palette mode applying for a current block, constructing a palette predictor for the current block and constructing a palette table for the current block based on the palette predictor;

generating a palette index map for the current block based on the palette table; and

encoding the current block based on the palette index map,

wherein whether the palette mode applies for the current block is determined based on a split structure of the current block being a local dual tree structure,

wherein whether the split structure of the current block is the local dual tree structure is determined based on at least one of a tree type of the current block and a split structure of a coding tree unit (CTU) including the current block,

wherein the local dual tree structure is a split structure in which additional splitting is performed only for a luma block within a CTU that has been split into a single tree structure.

10. The image encoding method of claim 9 , wherein, based on the current block being a chroma block and the split structure of the current block being the local dual tree structure, the palette predictor is not updated.

11. The image encoding method of claim 9 , wherein, based on the current block being a chroma block and the split structure of the current block being not the local dual tree structure, the palette predictor is updated based on the palette table.

12. The image encoding method of claim 9 , wherein, based on the split structure of the current block being the local dual tree structure and a treeType of the current block being a dual tree chroma, it is determined that the palette mode does not apply for the current block.

13. A method for transmitting a bitstream generated by an image encoding method, the image encoding method comprising:

based on a palette mode applying for a current block, constructing a palette predictor for the current block and constructing a palette table for the current block based on the palette predictor;

generating a palette index map for the current block based on the palette table; and

encoding the current block based on the palette index map,

whether the palette mode applies for the current block is determined based on a split structure of the current block being a local dual tree structure,

wherein whether the split structure of the current block is the local dual tree structure is determined based on at least one of a tree type of the current block and a split structure of a coding tree unit (CTU) including the current block,

wherein the local dual tree structure is a split structure in which additional splitting is performed only for a luma block within a CTU that has been split into a single tree structure.