Methods and apparatuses of encoding/decoding intra prediction mode using candidate intra prediction modes

1. A decoding method, the decoding method comprising:

determining whether a first intra prediction mode derived from a first neighboring block located at the left side of a current block is identical to a second intra prediction mode derived from a second neighboring block located at the upper side of the current block,

wherein when the first intra prediction mode is not identical to the second intra prediction mode, the decoding method comprises:

deriving a first candidate intra prediction mode from the first intra prediction mode and a second candidate intra prediction mode from the second intra prediction mode;

setting a third candidate intra prediction mode equal to a planar mode when none of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode;

setting the third candidate intra prediction mode equal to a DC mode when one of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode, and the other is not the DC mode; and

setting the third candidate intra prediction mode equal to a vertical mode when one of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode, and the other is the DC mode, and

wherein when the first intra prediction mode is identical to the second intra prediction mode, the decoding method comprises:

determining whether the first and second intra prediction modes are identical to the DC mode;

setting the first candidate intra prediction mode equal to the planar mode when the first and second intra prediction modes are identical to the DC mode, setting the second candidate intra prediction mode equal to the DC mode, and setting the third candidate intra prediction mode equal to the vertical mode.

2. The method of claim 1 , wherein three candidate intra prediction modes are arranged according to an order of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode.

3. The method of claim 1 , wherein the first neighboring block corresponds to a block including a pixel positioned at (x−1, y) when a position of an upper-left pixel of the current block is (x, y); and the second neighboring block corresponds to a block including a pixel positioned at (x, y−1) when the position of the upper-left pixel of the current block is (x, y), where the x and y are an integer value.

4. The method of claim 1 , further comprising:

decoding information indicating whether one of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode is identical to the intra prediction mode of the current block; and

deriving the intra prediction mode of the current block based on the identical one from among the first, second, and third candidate intra prediction modes when one of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode is identical to the intra prediction mode of the current block.

5. The method of claim 1 , further comprising:

decoding information indicating whether one of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode is identical to the intra prediction mode of the current block; and

deriving the intra prediction mode of the current block based on a remaining intra prediction mode different from the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode when none of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode is identical to the intra prediction mode of the current block.

6. The method of claim 5 , wherein deriving of the intra prediction mode of the current block based on the remaining intra prediction mode different from the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode when none of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode is identical to the intra prediction mode of the current block comprises:

deriving the intra prediction mode of the current block by comparing the remaining intra prediction mode with the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode, respectively.

7. A decoding apparatus, comprising one or more processors configured to execute processor-executable instructions to:

decode information indicating whether one candidate intra prediction mode from among a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode is identical to an intra prediction mode of a current block, the first, second, and third candidate intra prediction modes being three candidate intra prediction modes for the current block; and

derive the intra prediction mode of the current block based on the information, wherein

the first candidate intra prediction mode is derived from the first intra prediction mode, the second candidate intra prediction mode is derived from the second intra prediction mode when a first intra prediction mode derived from a first neighboring block located at the left side of the current block is not identical to a second intra prediction mode derived from a second neighboring block located at the upper side of the current block;

derive the first candidate intra prediction mode from the first intra prediction mode and the second candidate intra prediction mode from the second intra prediction mode;

set the third candidate intra prediction mode equal to a planar mode when none of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode,

set the third candidate intra prediction mode equal to a DC mode when one of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode, and the other is not the DC mode, and

set the third candidate intra prediction mode equal to a vertical mode when one of the first candidate intra prediction mode and the second candidate intra prediction mode is the planar mode, and the other is the DC mode, and

wherein when the first intra prediction mode and the second intra prediction mode are identical to the DC mode, the one or more processors is configured to:

set the first candidate intra prediction mode equal to the planar mode, to set the second candidate intra prediction mode equal to the DC mode, and to set the third candidate intra prediction mode equal to the vertical mode when the first intra prediction mode derived from the first neighboring block located at the left side of the current block is identical to the second intra prediction mode derived from the second neighboring block located at the upper side of the current block.

8. The apparatus of claim 7 , wherein three candidate intra prediction modes are arranged according to an order of the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode.

9. The apparatus of claim 7 , wherein the first neighboring block corresponds to a block including a pixel positioned at (x−1, y) when a position of an upper-left pixel of the current block is (x, y); and the second neighboring block corresponds to a block including a pixel positioned at (x, y−1) when the position of the upper-left pixel of the current block is (x, y), where the x and y are an integer value.

10. The apparatus of claim 7 , wherein the one or more processors is configured to derive the intra prediction mode of the current block based on the identical one from among the first, second, and third candidate intra prediction modes when one of the first, second, and third candidate intra prediction modes is identical to the intra prediction mode of the current block; and

the one or more processors is configured to derive the intra prediction mode of the current block based on a remaining intra prediction mode different from the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode when none of the first, second, and third candidate intra prediction modes is identical to the intra prediction mode of the current block.

11. The apparatus of claim 10 , wherein the one or more processors is configured to derive the intra prediction mode of the current block by comparing the remaining intra prediction mode with the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode, respectively, when none of the first, second, and third candidate intra prediction modes is identical to the intra prediction mode of the current block.