Method and apparatus for processing video signal

1. A method of decoding a video, the method comprising:

determining a reference sample line index of a current block;

deriving an intra prediction mode of the current block;

comparing the intra prediction mode of the current block with a threshold value; and

when the intra prediction mode is less than the threshold value and the current block has a non-square shape whose a width is greater than a height, modifying the intra prediction mode of the current block to a wide angle intra prediction mode by adding a first pre-defined value to the intra prediction mode,

wherein the wide angle intra prediction mode has an angle greater than that of a top-right diagonal intra prediction mode,

wherein the threshold value is adaptively determined based on a ratio of the width and the height of the current block,

wherein the first pre-defined value is 65,

wherein the intra prediction mode of the current block is derived based on an MPM (Most Probable Mode) flag signaled via a bitstream,

wherein the intra prediction mode of the current block is derived from MPM candidates or a default mode in response to the MPM flag equal to a first value and the intra prediction mode of the current block is not derived from the MPM candidates and the default mode in response to the MPM flag equal to a second value,

wherein, in response to the MPM flag equal to the first value, a default mode flag indicating whether the intra prediction mode of the current block is identical to the default mode is decoded from the bitstream,

wherein the default mode is representative of a Planar mode, and

wherein when the reference sample line index of the current block is greater than a second pre-defined value, signaling of the MPM flag is omitted and a value of the MPM flag is inferred to the first value.

2. The method of claim 1 , wherein a total number of wide angle intra prediction modes that is applicable to the current block is adaptively determined depending on the ratio of the width and the height of the current block.

3. The method of claim 1 , wherein when the default mode flag indicates that the intra prediction mode of the current block is not identical to the default mode of the current block, the intra prediction mode of the current block is determined as a MPM candidate specified by an MPM index.

4. A method of encoding a video, the method comprising:

determining a reference sample line index of a current block;

determining an intra prediction mode of the current block;

comparing the intra prediction mode of the current block with a threshold value; and

when the intra prediction mode is less than the threshold value and the current block has a non-square shape whose a width is greater than a height, modifying the intra prediction mode of the current block to a wide angle intra prediction mode by adding a first pre-defined value to the intra prediction mode,

wherein the wide angle intra prediction mode has an angle greater than that of a top-right diagonal intra prediction mode,

wherein the threshold value is adaptively determined based on a ratio of the width and the height of the current block,

wherein the first pre-defined value is 65,

wherein whether the intra prediction mode of the current block is derived from MPM candidates is determined to encode an MPM (Most Probable Mode) flag into a bitstream,

wherein the MPM flag equal to a first value is encoded in response to the determination that the intra prediction mode of the current block is derived from the MPM candidates or a default mode, and the MPM flag equal to a second value is encoded in response to the determination that the intra prediction mode of the current block is not derived from the MPM candidates or the default mode,

wherein, in response to the MPM flag equal to the first value, a default mode flag indicating whether the intra prediction mode of the current block is identical to the default mode is encoded into the bitstream,

wherein the default mode is representative of a Planar mode, and

wherein when the reference sample line index of the current block is greater than a second pre-defined value, encoding of the MPM flag is omitted and a value of the MPM flag is inferred to the first value.

5. The method of claim 4 , wherein a total number of wide angle intra prediction modes that is applicable to the current block is adaptively determined depending on the ratio of the width and the height of the current block.

6. A non-transitory computer readable medium having stored thereon a compressed video data, the compressed video data comprising at least one of:

a reference sample line index specifying a reference sample line of a current block,

wherein when the intra prediction mode is less than a threshold value and the current block has a non-square block whose a width is greater than a height, the intra prediction mode of the current block is modified to a wide angle intra prediction mode by adding a first pre-defined value to the intra prediction mode,

wherein the wide angle intra prediction mode has an angle greater than that of a top-right diagonal intra prediction mode,

wherein the threshold value is adaptively determined based on a ratio of the width and the height of the current block,

wherein the first pre-defined value is 65,

wherein an MPM (Most Probable Mode) flag equal to a first value indicates the intra prediction mode of the current block is derived from the MPM candidates or a default mode, and the MPM flag equal to a second value indicates the intra prediction mode of the current block is not derived from the MPM candidates and the default mode,

wherein, in response to the MPM flag equal to the first value, the compressed video data further comprises a default mode flag indicating whether the intra prediction mode is identical to the default mode or not,

wherein the default mode is representative of a Planar mode, and

wherein when the reference sample line index of the current block is greater than a second pre-defined value, the MPM flag is not comprised in the compressed video data and a value of the MPM flag is inferred to the first value.