Method and apparatus of deriving intra prediction mode using most probable mode group
Provided is a method that constructs an MPM group including three intra prediction modes, determines the intra prediction mode of the MPM group specified by the prediction mode index as the intra prediction mode of the current prediction unit if the mode group indicator indicates the MPM group, and derives the intra prediction mode of the current prediction unit using the prediction mode index and the three prediction modes of the MPM group if the mode group indicator does not indicate the MPM group. Accordingly, additional bits resulted from increase of a number of intra prediction mode are effectively reduced. Also, an image compression ratio can be improved by generating a prediction block similar to an original block.
1. A method of deriving an intra prediction mode of a current prediction unit, comprising:
entropy-decoding a mode group indicator and a prediction mode index;
constructing a Most Probable Mode (MPM) group including three intra prediction modes using a left intra prediction mode and an above intra prediction mode of the current prediction unit;
if the mode group indicator indicates the MPM group, determining an intra prediction mode of the MPM group specified by the prediction mode index as the intra prediction mode of the current prediction unit; and
if the mode group indicator does not indicate the MPM group, deriving the intra prediction mode of the current prediction unit using the prediction mode index and the three intra prediction modes of the MPM group,
wherein when the left intra prediction mode is equal to the above intra prediction mode and is one of two non-directional intra prediction modes, the MPM group includes two non-directional intra prediction modes and a vertical mode,
wherein deriving the prediction mode of the current prediction unit using the prediction mode index and the three intra prediction modes of the MPM group comprises ordered sub-steps of:
increasing a value of the prediction mode index by one if the prediction mode index is equal to or greater than a first intra prediction mode of the MPM group;
increasing the value of the prediction mode index by one if the prediction mode index is equal to or greater than a second intra prediction mode of the MPM group;
increasing the value of the prediction mode index by one if the prediction mode index is equal to or greater than a third intra prediction mode of the MPM group; and
determining the value of the prediction mode index as the mode number of the intra prediction mode of the current prediction unit.
2. The method of claim 1 , wherein when the left intra prediction mode is a DC mode, the MPM group includes the DC mode, a planar mode and a vertical mode.
3. The method of claim 1 , wherein when the left intra prediction mode is a planar mode, the MPM group includes the planar mode, a DC mode and a vertical mode.
4. The method of claim 1 , wherein the first intra prediction mode is an intra prediction mode having a lowest mode number, the second intra prediction mode is an intra prediction mode having a middle mode number and the third intra prediction mode is an intra prediction mode having a highest mode number.
5. The method of claim 1 , wherein the three intra prediction modes are reordered in the order of mode number to determine the first intra prediction mode, the second intra prediction mode and the third intra prediction mode.