Intra prediction mode encoding/decoding method and apparatus
An intra prediction mode encoding and decoding method, an image decoding device, and an image encoding device operate by deriving most probable modes (MPMs) from surrounding prediction units adjacent to a current prediction unit and deriving an intra prediction mode of the current prediction unit on the basis of an MPM flag indicating whether an MPM having the same prediction mode as the intra prediction mode of the current prediction unit exists among the derived MPMs.
1. An intra prediction mode decoding method, with a decoding apparatus, the method comprising:
generating, with the decoding apparatus, an MPM list comprising MPMs (Most Probable Modes) for a current prediction unit,
wherein the MPMs are derived based on intra prediction modes of neighboring prediction units to the current prediction unit, and the neighboring prediction units comprise a left neighboring prediction unit and an upper neighboring prediction unit,
wherein the MPM list comprises three MPMs as candidate intra prediction modes of the current prediction unit, the three MPMs comprising a first MPM, a second MPM and a third MPM, and
wherein in response to the intra prediction modes of the neighboring prediction units being the same, the first MPM is set to be equal to the intra prediction modes of the neighboring prediction units while the second MPM is set to be different from the intra prediction modes of the neighboring prediction units; and
deriving, with the decoding apparatus, an intra prediction mode of the current prediction unit based on the MPM list and an MPM flag indicating whether there is, among the derived MPMs, an MPM having the same prediction mode as the intra prediction mode of the current prediction unit,
wherein in response to the MPM flag indicating that there is, among the derived three MPMs, no MPM having the same prediction mode as the intra prediction mode of the current prediction unit, deriving the intra prediction mode of the current prediction unit comprises
comparing, with the decoding apparatus, the three MPMs in the MPM list,
rearranging, with the decoding apparatus, the three MPMs based on results of the comparison, the rearranging being performed by changing positions of the three MPMs in an ascending order of mode values, the step of rearranging being performed only if the MPM flag indicates that there is, among the derived three MPMs, no MPM having the same prediction mode as the intra prediction mode of the current prediction unit;
setting, with the decoding apparatus, a remaining intra prediction mode as a candidate intra prediction mode of the current prediction unit, the remaining intra prediction mode being determined based on remaining intra prediction modes except the derived three MPMs among all intra prediction modes, and
deriving, with the decoding apparatus, the intra prediction mode of the current prediction unit by comparing the candidate intra prediction mode of the current prediction unit with the rearranged MPMs.
2. The intra prediction mode decoding method of claim 1 , wherein in the step of deriving the intra prediction mode of the current prediction unit, in response to the candidate intra prediction mode of the current prediction unit being equal to or larger than one of the MPMs in the MPM list, a mode value of the candidate intra prediction mode of the current prediction unit is increased by 1 while the candidate intra prediction mode is sequentially compared with the rearranged three or more MPMs, and
wherein in response to the comparison with the rearranged three or more MPMs being terminated, the candidate intra prediction mode of the current prediction unit is derived as the intra prediction mode of the current prediction unit.