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 generating a prediction block, 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 a current prediction unit,
deriving an intra prediction mode of the current prediction unit using the mode group indicator and the prediction mode index;
adaptively filtering reference pixels of a current block according to a size of a transform unit and the intra prediction mode of the current prediction unit; and
generating the prediction block according to the intra prediction mode of the current prediction unit,
wherein if the mode group indicator indicates the MPM group, the intra prediction mode of the current prediction unit is set to an intra prediction mode of the MPM group specified by the prediction mode index, and
if the mode group indicator does not indicate the MPM group, the intra prediction mode of the current prediction unit is determined by performing 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 candidate 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 candidate 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 candidate 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 the first candidate is an intra prediction mode having lowest mode number, the second candidate is an intra prediction mode having middle mode number and the third candidate is an intra prediction mode having highest mode number.
3. The method of claim 1 , wherein the three intra prediction modes are reordered in the order of mode number to determine the first candidate, the second candidate and the third candidate.
4. The method of claim 1 , wherein when the left intra prediction mode is not equal to the above intra prediction mode and at least one of the left intra prediction mode and the above intra prediction mode is a non-directional intra prediction mode, the MPM group includes both the non-directional intra prediction modes.
5. The method of claim 4 , wherein when the left intra prediction mode and the above intra prediction mode are non-directional intra prediction modes, the MPM group includes both the non-directional intra prediction modes and a vertical mode.
6. The method of claim 1 , wherein a size of the current block is equal to the size of a transform unit.