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

1. A decoding method, comprising:

determining whether a first neighboring block located at left side of a current block or a second neighboring block located at upper side of the current block is available for deriving an intra prediction mode of the current block;

deriving a first intra prediction mode based on the determined availability for the first neighboring block or a second intra prediction mode based on the determined availability for the second neighboring block,

wherein the first intra prediction mode is derived as DC mode when the first neighboring block is not available and is derived as an intra prediction mode of the first neighboring block when the first neighboring block is available, and

wherein the second intra prediction mode is derived as DC mode when the second neighboring block is not available and is derived as an intra prediction mode of the second neighboring block when the second neighboring block is available;

deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode based on the first intra prediction mode and the second intra prediction mode;

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

obtaining an intra prediction mode of the current block based on the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode when the information indicates that 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,

wherein when the first intra prediction mode is not identical to the second intra prediction mode, the step of deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode comprises:

deriving 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;

setting 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;

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 and both the first intra prediction mode and the second intra prediction mode are the planar mode or the DC mode, the step of deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode comprises:

setting the first candidate intra prediction mode equal to the planar 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 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.

3. An encoding method, comprising:

determining whether a first neighboring block located at left side of a current block or a second neighboring block located at upper side of the current block is available for deriving an intra prediction mode of the current block;

deriving a first intra prediction mode based on the determined availability for the first neighboring block or a second intra prediction mode based on the determined availability for the second neighboring block,

wherein the first intra prediction mode is derived as DC mode when the first neighboring block is not available and is derived as an intra prediction mode of the first neighboring block when the first neighboring block is available, and

wherein the second intra prediction mode is derived as DC mode when the second neighboring block is not available and is derived as an intra prediction mode of the second neighboring block when the second neighboring block is available;

deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode based on the first intra prediction mode and the second intra prediction mode;

obtaining intra prediction mode of the current block based on the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode; and

encoding 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,

wherein when the first intra prediction mode is not identical to the second intra prediction mode, the step of deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode comprises:

deriving 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;

setting 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;

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 and both the first intra prediction mode and the second intra prediction mode are the planar mode or the DC mode, the step of deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode comprises:

setting the first candidate intra prediction mode equal to the planar 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.

4. The method of claim 3 , 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.

5. A non-transitory computer-readable recording medium storing a bitstream generated by an encoding method, the bitstream including information indicating whether one of a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode is identical to the intra prediction mode of a current block, wherein the encoding method comprising:

determining whether a first neighboring block located at left side of the current block or a second neighboring block located at upper side of the current block is available for deriving an intra prediction mode of the current block;

deriving a first intra prediction mode based on the determined availability for the first neighboring block or a second intra prediction mode based on the determined availability for the second neighboring block,

wherein the first intra prediction mode is derived as DC mode when the first neighboring block is not available and is derived as an intra prediction mode of the first neighboring block when the first neighboring block is available, and

wherein the second intra prediction mode is derived as DC mode when the second neighboring block is not available and is derived as an intra prediction mode of the second neighboring block when the second neighboring block is available;

deriving the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode based on the first intra prediction mode and the second intra prediction mode;

obtaining intra prediction mode of the current block based on the first candidate intra prediction mode, the second candidate intra prediction mode, and the third candidate intra prediction mode; and

encoding the 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,

wherein when the first intra prediction mode is not identical to the second intra prediction mode, the step of deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode comprises:

deriving 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;

setting 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;

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 and both the first intra prediction mode and the second intra prediction mode are the planar mode or the DC mode, the step of deriving a first candidate intra prediction mode, a second candidate intra prediction mode, and a third candidate intra prediction mode comprises:

setting the first candidate intra prediction mode equal to the planar 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.