Method and apparatus of history-based intra mode coding

1. A method to derive candidate intra prediction modes, comprising:

recording an intra prediction mode of at least one previous coding unit, wherein the previous coding unit is not adjacent to a to-be-processed coding unit; and

adding at least one recorded intra prediction mode into a set of candidate intra prediction modes based on a determination that a number of the candidate intra prediction modes included in the set of candidate intra prediction modes is less than a preset threshold,

wherein recording the intra prediction mode of the at least one previous coding unit comprises:

adding the intra prediction mode of the at least one previous coding unit into a historical table according to a processing order of a plurality of previous coding units comprising the at least one previous coding unit.

2. The method of claim 1 , wherein the set of candidate intra prediction modes is a most probable mode (MPM) list.

3. The method of claim 2 , further comprising coding, wherein for coding of the modes from the MPM list, a truncated unary binarization is used.

4. The method of claim 1 , wherein the set of candidate intra prediction modes includes intra prediction modes that are not included in a most probable mode list.

5. The method of claim 4 , wherein

the candidate intra prediction modes that are not included in the most probable mode list are first divided into a set of selected modes and a set of non-selected modes, and

the candidate intra prediction modes in the set of selected modes are signaled with a fixed-length code and the candidate intra prediction modes in the set of non-selected modes are signaled with a truncated binary code.

6. The method of claim 1 , wherein the historical table is a first-in-first-out table.

7. The method of claim 1 , wherein at least two entries in the historical table are different from one another.

8. The method of claim 1 , wherein

a position of an entry in the historical table is based on a frequency of the entry, and

adding the intra prediction mode of the at least one previous coding unit into the historical table, comprises:

determining whether the intra prediction mode is in the historical table;

adding the frequency of the intra prediction mode by one, when determining that the intra prediction mode is in the historical table;

adjusting the position of the intra prediction mode, when the frequency of the intra prediction mode is higher than both of frequencies of intra prediction modes on adjacent positions;

determining whether the historical table is full, when determining that the intra prediction mode is not in the historical table;

adding the intra prediction mode after a least frequency position of the historical table and setting the frequency of the intra prediction mode as one, when determining that the historical table is not full; and

replacing the entry on the least frequency position of the historical table with the intra prediction mode and setting the frequency of the intra prediction mode as one, when determining that the historical table is full.

9. The method of claim 8 , wherein in the adjusting, the position of the intra mode in the historical table and an adjacent entry is exchanged.

10. The method of claim 1 , wherein adding the at least one recorded intra prediction mode into the set of candidate intra prediction modes, comprises:

adding the at least one recorded intra prediction mode after the intra prediction mode of at least one adjacent coding unit of the to-be-processed coding unit so that a bit for coding indicator of the intra prediction mode of the at least one adjacent coding unit is not more than the bit for coding indicator of the at least one recorded intra prediction mode.

11. A method for encoding a current block of a picture, the method comprising:

providing a predicted block for the current block including the method for deriving candidate intra prediction modes according to claim 1 as samples of the current block and

encoding the current block based on the predicted block.

12. A method for decoding a current block of a picture, the method comprising:

providing a predicted block for the current block including the method for deriving candidate intra prediction modes according to claim 1 as samples of the current block and

reconstructing the current block based on the predicted block.

13. A candidate list device for deriving candidate intra prediction modes, comprising processing circuitry configured to

record an intra prediction mode of at least one previous coding unit into a historical table according to the processing order of a plurality of previous coding units comprising the at least one previous coding unit, wherein the previous coding unit is not adjacent to a to-be-processed coding unit; and

add at least one recorded intra prediction mode into a set of candidate intra prediction modes based on a determination that a number of the candidate intra prediction modes included in the set of candidate intra prediction modes is less than a preset threshold.

14. The device of claim 13 , wherein the set of candidate intra prediction modes is a most probable mode (MPM) list, or the set of candidate intra prediction modes includes intra prediction modes that are not included in a most probable mode list.

15. The device of claim 13 , wherein the historical table is a first-in-first-out table.

16. The device of claim 13 , wherein at least two entries in the historical table are different from one another.

17. The device of claim 13 , wherein

a position of an entry in the historical table is based on a frequency of the entry, and

add the intra prediction mode of the at least one previous coding unit into the historical table, comprises:

determine whether the intra prediction mode is in the historical table;

add the frequency of the intra prediction mode by one, when determining that the intra prediction mode is in the historical table;

adjust the position of the intra prediction mode, when the frequency of the intra prediction mode is higher than both of frequencies of intra prediction modes on adjacent positions;

determine whether the historical table is full, when determining that the intra prediction mode is not in the historical table;

add the intra prediction mode after a least frequency position of the historical table and setting the frequency of the intra prediction mode as one, when the historical table is not full;

replace the entry on the least frequency position of the historical table with the intra prediction mode and setting the frequency of the intra prediction mode as one, when determining that the historical table is full.

18. A decoding apparatus for decoding a current encoded block of a picture, comprising

the candidate list device according to claim 13 for deriving candidate intra prediction modes, and

circuitry configured to:

provide a predicted block as the encoded block using the derived intra prediction modes, and

reconstruct the current block based on the encoded block and the predicted block.