Method for encoding/decoding an intra-picture prediction mode using two intra-prediction mode candidate, and apparatus using such a method

1. An intra-prediction mode decoding method, comprising:

deriving a first candidate intra-prediction mode and a second candidate intra-prediction mode of a current prediction unit (PU);

determining whether an intra-prediction mode of the current PU is identical to the first candidate intra-prediction mode or the second candidate intra-prediction mode based on n-bit information;

when the intra-prediction mode of the current PU is identical to one of the first candidate intra-prediction mode and the second candidate intra-prediction mode, determining which one of the first candidate intra-prediction mode and the second candidate intra-prediction mode is identical to the intra-prediction mode of the current PU, based on additional m-bit information; and

decoding the intra-prediction mode of the current PU,

wherein deriving the first candidate intra-prediction mode and the second candidate intra-prediction mode comprises:

deriving a first neighbor prediction mode based on a first neighbor PU adjacent to the current PU,

wherein when the first neighbor PU is available, the first neighbor prediction mode is set to as an intra prediction mode of the first neighbor PU, and

wherein when the first neighbor PU is unavailable, the first neighbor prediction mode is set to a DC mode;

deriving a second neighbor prediction mode based on a second neighbor PU adjacent to the current PU,

wherein when the second neighbor PU is available, the second neighbor prediction mode is set to as an intra prediction mode of the second neighbor PU, and

wherein when the second neighbor PU is unavailable, the second neighbor prediction mode is set to a DC mode; and

deriving the first candidate intra-prediction mode and the second candidate intra-prediction mode from the first neighbor prediction mode and the second neighbor prediction mode,

wherein when the first neighbor prediction mode and the second neighbor prediction mode are different, the first candidate intra-prediction mode and the second candidate intra-prediction mode are determined without comparing values of the first neighbor prediction mode and the second neighbor prediction mode.

2. The intra-prediction mode decoding method of claim 1 , further comprising:

when the intra-prediction mode of the current PU is identical to none of the first candidate intra-prediction mode and the second candidate intra-prediction mode, decoding the intra-prediction mode of the current PU based on additional bit specifying remaining intra-prediction mode information.

3. The intra-prediction mode decoding method of claim 1 , wherein when the first neighbor prediction mode and the second neighbor prediction mode is identical, the first candidate intra-prediction mode is set as the planar mode and the second candidate intra-prediction mode is set as the DC mode.

4. The intra-prediction mode decoding method of claim 1 , wherein n and m are 1.

5. A decoder comprising:

a processor configured with processor-executable instructions to:

derive a first candidate intra-prediction mode and a second candidate intra-prediction mode of a current prediction unit (PU),

decode information regarding whether an intra-prediction mode of the current PU is identical to a first candidate intra-prediction mode or a second candidate intra-prediction mode based on n-bit information included in a bit stream,

decode information regarding as to which one of the first candidate intra-prediction mode and the second candidate intra-prediction mode, the intra-prediction mode of the current PU is identical to, based on additional m-bit information, when the intra-prediction mode of the current PU is identical to one of the first candidate intra-prediction mode and the second candidate intra-prediction mode, and

decode the intra-prediction mode of the current PU based on an additional bit specifying remaining intra-prediction mode information, when the intra-prediction mode of the current PU is identical to none of the first candidate intra-prediction mode and the second candidate intra-prediction mode; and

perform intra-prediction based on the intra-prediction mode of the current PU decoded,

wherein the first candidate intra-prediction mode and the second candidate intra-prediction mode are determined by:

deriving a first neighbor prediction mode based on a first neighbor PU adjacent to the current PU,

wherein when the first neighbor PU is available, the first neighbor prediction mode is set to as an intra prediction mode of the first neighbor PU, and

wherein when the first neighbor PU is unavailable, the first neighbor prediction mode is set to a DC mode;

deriving a second neighbor prediction mode based on a second neighbor PU adjacent to the current PU,

wherein when the second neighbor PU is available, the second neighbor prediction mode is set to as an intra prediction mode of the second neighbor PU, and

wherein when the second neighbor PU is unavailable, the second neighbor prediction mode is set to a DC mode; and

deriving the first candidate intra-prediction mode and the second candidate intra-prediction mode from the first neighbor prediction mode and the second neighbor prediction mode;

wherein when the first neighbor prediction mode and the second neighbor prediction mode are different, the first candidate intra-prediction mode and the second candidate intra-prediction mode are determined without comparing values of the first neighbor prediction mode and the second neighbor prediction mode.

6. The decoder of claim 5 , wherein n and m are 1.

7. The decoder of claim 5 , wherein when the first neighbor prediction mode and the second neighbor prediction mode is identical, the first candidate intra-prediction mode is set as the planar mode and the second candidate intra-prediction mode is set as the DC mode.